src/hb-raster-draw.cc

/*
 * Copyright © 2026  Behdad Esfahbod
 *
 *  This is part of HarfBuzz, a text shaping library.
 *
 * Permission is hereby granted, without written agreement and without
 * license or royalty fees, to use, copy, modify, and distribute this
 * software and its documentation for any purpose, provided that the
 * above copyright notice and the following two paragraphs appear in
 * all copies of this software.
 *
 * IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
 * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
 * ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
 * IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
 * DAMAGE.
 *
 * THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
 * BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
 * ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
 * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
 *
 * Author(s): Behdad Esfahbod
 */

#include "hb.hh"

#include "hb-raster-image.hh"
#include "hb-geometry.hh"
#include "hb-machinery.hh"

#if defined(__aarch64__) || defined(_M_ARM64)
#include <arm_neon.h>
#define HB_RASTER_NEON 1
#elif defined(__SSE2__) || defined(_M_X64) || defined(_M_AMD64)
#include <emmintrin.h>
#define HB_RASTER_SSE2 1
#endif


/* Fixed-point precision for sub-pixel coordinates.
   8 bits = 24.8: 256 sub-pixel units per pixel. */
#define HB_RASTER_PIXEL_BITS 8
#define HB_RASTER_ONE_PIXEL  (1 << HB_RASTER_PIXEL_BITS)
#define HB_RASTER_PIXEL_MASK (HB_RASTER_ONE_PIXEL - 1)
/* Full-coverage alpha = 2 * ONE_PIXEL^2 */
#define HB_RASTER_FULL_COVERAGE (2 * HB_RASTER_ONE_PIXEL * HB_RASTER_ONE_PIXEL)
/* Flatness threshold for Bézier flattening: max deviation in pixels */
#define HB_RASTER_FLAT_THRESH 0.25f


/* Normalized edge: yH > yL always */
struct hb_raster_edge_t
{
  int32_t xL, yL;   /* lower endpoint (fixed-point) */
  int32_t xH, yH;   /* upper endpoint (fixed-point) */
  int64_t slope;    /* dx/dy in 16.16 fixed point: ((int64_t)dx << 16) / dy */
  int32_t wind;     /* +1 or -1 */
};

/* hb_raster_draw_t — outline rasterizer */
struct hb_raster_draw_t
{
  hb_object_header_t header;

  /* Configuration */
  hb_transform_t<>    transform         = {1, 0, 0, 1, 0, 0};
  float               x_scale_factor    = 1.f;
  float               y_scale_factor    = 1.f;
  hb_raster_extents_t fixed_extents     = {};
  bool                has_extents = false;

  /* Visibility clip box for curve flattening (device pixels); set
     internally by raster-paint so invisible curves collapse to their
     chord.  See hb_raster_draw_set_clip_box(). */
  bool  has_clip_box = false;
  float clip_x0 = 0.f, clip_y0 = 0.f, clip_x1 = 0.f, clip_y1 = 0.f;

  /* Flattening clip resolved from the above (or from fixed extents),
     expanded by one pixel; kept in sync by hb_raster_draw_update_flatten_clip(). */
  bool  flatten_clip_active = false;
  float flatten_clip_x0 = 0.f, flatten_clip_y0 = 0.f;
  float flatten_clip_x1 = 0.f, flatten_clip_y1 = 0.f;

  /* Curve-flattening work, charged one unit per Bézier subdivision.
     When external_work is set (by raster-paint), that session budget
     is charged instead of the standalone per-session one. */
  int64_t  flatten_work_left = HB_RASTER_MAX_DRAW_WORK;
  int64_t *external_work = nullptr;

  /* Accumulated geometry */
  int64_t edges_left = HB_RASTER_MAX_DRAW_EDGES;
  hb_vector_t<hb_raster_edge_t> edges;

  /* Scratch — reused across render() calls */
  hb_vector_t<int32_t> row_area;
  hb_vector_t<int16_t> row_cover;
  hb_vector_t<hb_vector_t<unsigned>> edge_buckets;
  hb_vector_t<unsigned> active_edges;

  /* Recycled image for zero-malloc render */
  hb_raster_image_t *recycled_image = nullptr;
};

static HB_ALWAYS_INLINE void
hb_raster_draw_transform_point (const hb_raster_draw_t *draw,
				float x, float y,
				float &tx, float &ty)
{
  tx = x; ty = y;
  draw->transform.transform_point (tx, ty);
  tx /= draw->x_scale_factor;
  ty /= draw->y_scale_factor;
}


/* hb_raster_draw_t */

/**
 * hb_raster_draw_create_or_fail:
 *
 * Creates a new rasterizer object.
 *
 * Return value: (transfer full):
 * A newly allocated #hb_raster_draw_t with a reference count of 1. The
 * initial reference count should be released with hb_raster_draw_destroy()
 * when you are done using the #hb_raster_draw_t, or `NULL` on
 * allocation failure.
 *
 * Since: 13.0.0
 **/
hb_raster_draw_t *
hb_raster_draw_create_or_fail (void)
{
  hb_raster_draw_t *draw = hb_object_create<hb_raster_draw_t> ();
  return draw;
}

/**
 * hb_raster_draw_reference: (skip)
 * @draw: a rasterizer
 *
 * Increases the reference count on @draw by one.
 *
 * This prevents @draw from being destroyed until a matching
 * call to hb_raster_draw_destroy() is made.
 *
 * Return value: (transfer full):
 * The referenced #hb_raster_draw_t.
 *
 * Since: 13.0.0
 **/
hb_raster_draw_t *
hb_raster_draw_reference (hb_raster_draw_t *draw)
{
  return hb_object_reference (draw);
}

/**
 * hb_raster_draw_destroy: (skip)
 * @draw: a rasterizer
 *
 * Decreases the reference count on @draw by one. When the
 * reference count reaches zero, the rasterizer is freed.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_destroy (hb_raster_draw_t *draw)
{
  if (!hb_object_should_destroy (draw))
    return;

  hb_raster_image_destroy (draw->recycled_image);
  hb_object_actually_destroy (draw);
  hb_free (draw);
}

/**
 * hb_raster_draw_set_user_data: (skip)
 * @draw: a rasterizer
 * @key: the user-data key
 * @data: a pointer to the user data
 * @destroy: (nullable): a callback to call when @data is not needed anymore
 * @replace: whether to replace an existing data with the same key
 *
 * Attaches a user-data key/data pair to the specified rasterizer.
 *
 * Return value: `true` if success, `false` otherwise
 *
 * Since: 13.0.0
 **/
hb_bool_t
hb_raster_draw_set_user_data (hb_raster_draw_t   *draw,
			      hb_user_data_key_t *key,
			      void               *data,
			      hb_destroy_func_t   destroy,
			      hb_bool_t           replace)
{
  return hb_object_set_user_data (draw, key, data, destroy, replace);
}

/**
 * hb_raster_draw_get_user_data: (skip)
 * @draw: a rasterizer
 * @key: the user-data key
 *
 * Fetches the user-data associated with the specified key,
 * attached to the specified rasterizer.
 *
 * Return value: (transfer none):
 * A pointer to the user data
 *
 * Since: 13.0.0
 **/
void *
hb_raster_draw_get_user_data (const hb_raster_draw_t   *draw,
			      hb_user_data_key_t *key)
{
  return hb_object_get_user_data (draw, key);
}

/**
 * hb_raster_draw_set_transform:
 * @draw: a rasterizer
 * @xx: xx component of the transform matrix
 * @yx: yx component of the transform matrix
 * @xy: xy component of the transform matrix
 * @yy: yy component of the transform matrix
 * @dx: x translation
 * @dy: y translation
 *
 * Sets a 2×3 affine transform applied to all incoming draw
 * coordinates before rasterization.  The default is the identity.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_set_transform (hb_raster_draw_t *draw,
			      float xx, float yx,
			      float xy, float yy,
			      float dx, float dy)
{
  draw->transform = {xx, yx, xy, yy, dx, dy};
}

/**
 * hb_raster_draw_set_scale_factor:
 * @draw: a rasterizer
 * @x_scale_factor: x-axis minification factor
 * @y_scale_factor: y-axis minification factor
 *
 * Sets post-transform minification factors applied during rasterization.
 * Factors larger than 1 shrink the output in pixels. The default is 1.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_set_scale_factor (hb_raster_draw_t *draw,
				 float x_scale_factor,
				 float y_scale_factor)
{
  draw->x_scale_factor = x_scale_factor > 0.f ? x_scale_factor : 1.f;
  draw->y_scale_factor = y_scale_factor > 0.f ? y_scale_factor : 1.f;
}

/**
 * hb_raster_draw_get_scale_factor:
 * @draw: a rasterizer
 * @x_scale_factor: (out) (nullable): x-axis minification factor
 * @y_scale_factor: (out) (nullable): y-axis minification factor
 *
 * Fetches the current post-transform minification factors.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_get_scale_factor (const hb_raster_draw_t *draw,
				 float *x_scale_factor,
				 float *y_scale_factor)
{
  if (x_scale_factor) *x_scale_factor = draw->x_scale_factor;
  if (y_scale_factor) *y_scale_factor = draw->y_scale_factor;
}

/**
 * hb_raster_draw_get_transform:
 * @draw: a rasterizer
 * @xx: (out) (nullable): xx component of the transform matrix
 * @yx: (out) (nullable): yx component of the transform matrix
 * @xy: (out) (nullable): xy component of the transform matrix
 * @yy: (out) (nullable): yy component of the transform matrix
 * @dx: (out) (nullable): x translation
 * @dy: (out) (nullable): y translation
 *
 * Fetches the current affine transform of the rasterizer.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_get_transform (const hb_raster_draw_t *draw,
			      float *xx, float *yx,
			      float *xy, float *yy,
			      float *dx, float *dy)
{
  if (xx) *xx = draw->transform.xx;
  if (yx) *yx = draw->transform.yx;
  if (xy) *xy = draw->transform.xy;
  if (yy) *yy = draw->transform.yy;
  if (dx) *dx = draw->transform.x0;
  if (dy) *dy = draw->transform.y0;
}

/* Recompute the resolved flattening clip box: the internal clip box
   when set, otherwise the fixed output extents when known.  Expanded
   by one pixel so fixed-point rounding at the boundary stays safe.
   Called whenever the clip box or extents change.  A curve whose
   control-point bounding box misses the clip box entirely is replaced
   by its chord: coverage inside the box then depends only on the
   curve's endpoints (intermediate scanline crossings cancel in pairs,
   and crossings right of the box never reach visible pixels), so the
   replacement is exact for rendering. */
static void
hb_raster_draw_update_flatten_clip (hb_raster_draw_t *draw)
{
  if (draw->has_clip_box)
  {
    draw->flatten_clip_active = true;
    draw->flatten_clip_x0 = draw->clip_x0 - 1.f;
    draw->flatten_clip_y0 = draw->clip_y0 - 1.f;
    draw->flatten_clip_x1 = draw->clip_x1 + 1.f;
    draw->flatten_clip_y1 = draw->clip_y1 + 1.f;
  }
  else if (draw->has_extents)
  {
    draw->flatten_clip_active = true;
    draw->flatten_clip_x0 = (float) draw->fixed_extents.x_origin - 1.f;
    draw->flatten_clip_y0 = (float) draw->fixed_extents.y_origin - 1.f;
    draw->flatten_clip_x1 = (float) draw->fixed_extents.x_origin + (float) draw->fixed_extents.width + 1.f;
    draw->flatten_clip_y1 = (float) draw->fixed_extents.y_origin + (float) draw->fixed_extents.height + 1.f;
  }
  else
    draw->flatten_clip_active = false;
}

/**
 * hb_raster_draw_set_extents:
 * @draw: a rasterizer
 * @extents: the desired output extents
 *
 * Overrides the output image extents for the next render.  When set,
 * hb_raster_draw_render() uses the given extents instead of
 * auto-computing them from the accumulated geometry.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_set_extents (hb_raster_draw_t          *draw,
			    const hb_raster_extents_t *extents)
{
  draw->fixed_extents     = *extents;
  draw->has_extents = true;
  hb_raster_draw_update_flatten_clip (draw);
}

/**
 * hb_raster_draw_get_extents:
 * @draw: a rasterizer
 * @extents: (out) (nullable): where to write current extents
 *
 * Gets currently configured output extents.
 *
 * Return value: `true` if extents are set, `false` otherwise.
 *
 * Since: 13.0.0
 **/
hb_bool_t
hb_raster_draw_get_extents (const hb_raster_draw_t    *draw,
			    hb_raster_extents_t *extents)
{
  if (!draw->has_extents)
    return false;

  if (extents)
    *extents = draw->fixed_extents;
  return true;
}

/**
 * hb_raster_draw_set_glyph_extents:
 * @draw: a rasterizer
 * @glyph_extents: glyph extents from hb_font_get_glyph_extents()
 *
 * Transforms @glyph_extents with the rasterizer's current transform and
 * sets the resulting pixel extents for the next render.
 *
 * This is equivalent to computing a transformed bounding box in pixel
 * space and calling hb_raster_draw_set_extents().
 *
 * The resulting dimensions are capped at 4096 pixels per side.
 *
 * Return value: `true` if transformed extents are non-empty and set;
 * `false` otherwise.
 *
 * Since: 13.0.0
 **/
hb_bool_t
hb_raster_draw_set_glyph_extents (hb_raster_draw_t         *draw,
				  const hb_glyph_extents_t *glyph_extents)
{
  float x0 = (float) glyph_extents->x_bearing;
  float y0 = (float) glyph_extents->y_bearing;
  float x1 = (float) glyph_extents->x_bearing + glyph_extents->width;
  float y1 = (float) glyph_extents->y_bearing + glyph_extents->height;

  float xmin = hb_min (x0, x1);
  float xmax = hb_max (x0, x1);
  float ymin = hb_min (y0, y1);
  float ymax = hb_max (y0, y1);

  float px[4] = {xmin, xmin, xmax, xmax};
  float py[4] = {ymin, ymax, ymin, ymax};

  float tx, ty;
  hb_raster_draw_transform_point (draw, px[0], py[0], tx, ty);
  float tx_min = tx, tx_max = tx;
  float ty_min = ty, ty_max = ty;

  for (unsigned i = 1; i < 4; i++)
  {
    hb_raster_draw_transform_point (draw, px[i], py[i], tx, ty);
    tx_min = hb_min (tx_min, tx);
    tx_max = hb_max (tx_max, tx);
    ty_min = hb_min (ty_min, ty);
    ty_max = hb_max (ty_max, ty);
  }

  int32_t ex0 = hb_clamp_to<int32_t> (floorf (tx_min));
  int32_t ey0 = hb_clamp_to<int32_t> (floorf (ty_min));
  int32_t ex1 = hb_clamp_to<int32_t> (ceilf  (tx_max));
  int32_t ey1 = hb_clamp_to<int32_t> (ceilf  (ty_max));

  if (ex1 <= ex0 || ey1 <= ey0)
  {
    draw->fixed_extents = {};
    draw->has_extents = false;
    hb_raster_draw_update_flatten_clip (draw);
    return false;
  }

  draw->fixed_extents = {
    ex0, ey0,
    (unsigned) hb_min ((int64_t) ex1 - ex0, (int64_t) HB_RASTER_MAX_AUTO_DIMENSION),
    (unsigned) hb_min ((int64_t) ey1 - ey0, (int64_t) HB_RASTER_MAX_AUTO_DIMENSION),
    0
  };
  draw->has_extents = true;
  hb_raster_draw_update_flatten_clip (draw);
  return true;
}

/**
 * hb_raster_draw_clear:
 * @draw: a rasterizer
 *
 * Discards accumulated geometry and extents so @draw can be reused
 * for another render.  User configuration (transform, scale factors)
 * is preserved.  Call hb_raster_draw_reset() to also reset user
 * configuration to defaults.
 *
 * Since: 14.2.0
 **/
void
hb_raster_draw_clear (hb_raster_draw_t *draw)
{
  draw->fixed_extents     = {};
  draw->has_extents = false;
  draw->has_clip_box = false;
  draw->flatten_clip_active = false;
  draw->flatten_work_left = HB_RASTER_MAX_DRAW_WORK;
  draw->external_work = nullptr;
  draw->edges_left = HB_RASTER_MAX_DRAW_EDGES;
  draw->edges.clear ();
  draw->active_edges.clear ();
}

/**
 * hb_raster_draw_reset:
 * @draw: a rasterizer
 *
 * Resets the rasterizer to its initial state, clearing all accumulated
 * geometry, the transform, and fixed extents.  The object can then be
 * reused for a new glyph.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_reset (hb_raster_draw_t *draw)
{
  draw->transform         = {1, 0, 0, 1, 0, 0};
  draw->x_scale_factor    = 1.f;
  draw->y_scale_factor    = 1.f;
  hb_raster_draw_clear (draw);
}

void
hb_raster_draw_set_clip_box (hb_raster_draw_t *draw,
			     float x0, float y0,
			     float x1, float y1)
{
  draw->has_clip_box = true;
  draw->clip_x0 = x0;
  draw->clip_y0 = y0;
  draw->clip_x1 = x1;
  draw->clip_y1 = y1;
  hb_raster_draw_update_flatten_clip (draw);
}

void
hb_raster_draw_set_external_work (hb_raster_draw_t *draw,
				  int64_t *work_left)
{
  draw->external_work = work_left;
}

int64_t
hb_raster_draw_get_edge_work (hb_raster_draw_t *draw, unsigned max_rows)
{
  int64_t work = 0;
  for (const auto &e : draw->edges)
    work += 1 + hb_min (((int64_t) e.yH - (int64_t) e.yL) >> HB_RASTER_PIXEL_BITS,
			(int64_t) max_rows);
  return work;
}

/**
 * hb_raster_draw_recycle_image:
 * @draw: a rasterizer
 * @image: a raster image to recycle
 *
 * Recycles @image for reuse by a subsequent hb_raster_draw_render()
 * call, avoiding per-render memory allocation.  The caller transfers
 * ownership of @image to @draw and must not use it afterwards.
 *
 * If @draw already holds a recycled image, the previously recycled
 * image is destroyed.
 *
 * Since: 13.0.0
 **/
void
hb_raster_draw_recycle_image (hb_raster_draw_t  *draw,
			      hb_raster_image_t *image)
{
  hb_raster_image_destroy (draw->recycled_image);
  draw->recycled_image = image;
}


/*
 * Draw callbacks — flatten on the fly into hb_raster_edge_t
 */

static inline void
transform_point (const hb_raster_draw_t *draw,
		 float  x,  float  y,
		 float &tx, float &ty)
{
  hb_raster_draw_transform_point (draw, x, y, tx, ty);
}

static void
emit_segment (hb_raster_draw_t *draw,
	      float x0, float y0,
	      float x1, float y1)
{
  if (unlikely (draw->edges_left <= 0))
    return;

  int32_t X0 = hb_clamp_to<int32_t> (roundf (x0 * HB_RASTER_ONE_PIXEL));
  int32_t Y0 = hb_clamp_to<int32_t> (roundf (y0 * HB_RASTER_ONE_PIXEL));
  int32_t X1 = hb_clamp_to<int32_t> (roundf (x1 * HB_RASTER_ONE_PIXEL));
  int32_t Y1 = hb_clamp_to<int32_t> (roundf (y1 * HB_RASTER_ONE_PIXEL));

  if (Y0 == Y1) return; /* horizontal — skip */

  hb_raster_edge_t e;
  if (Y0 < Y1) {
    e.xL = X0; e.yL = Y0; e.xH = X1; e.yH = Y1; e.wind = +1;
  } else {
    e.xL = X1; e.yL = Y1; e.xH = X0; e.yH = Y0; e.wind = -1;
  }
  e.slope = (((int64_t) e.xH - (int64_t) e.xL) * (int64_t) 65536) /
	    ((int64_t) e.yH - (int64_t) e.yL);

  if (likely (draw->edges.push_or_fail (e)))
    draw->edges_left--;
  else
    draw->edges_left = 0;
}


/* Quadratic Bézier flattener — iterative de Casteljau at t=0.5. */
static inline void
flatten_quadratic_recursive (hb_raster_draw_t *draw,
			     float x0, float y0,
			     float x1, float y1,
			     float x2, float y2,
			     int depth = 0)
{
  struct quad_node_t
  {
    float x0, y0, x1, y1, x2, y2;
    int depth;
    bool check_clip;
  };

  quad_node_t stack[16];
  unsigned top = 0;

  bool check_clip = draw->flatten_clip_active;
  int64_t *work = draw->external_work ? draw->external_work : &draw->flatten_work_left;
  int64_t work_left = *work;

  while (true)
  {
    bool emit_chord = depth >= 16;

    /* Entirely outside the clip box: the chord is exact for coverage.
       Subdivided control points are convex combinations of their
       parent's, so once a node is entirely inside the box no
       descendant can leave it and the test is skipped below. */
    if (!emit_chord && check_clip)
    {
      float bx0 = hb_min (x0, hb_min (x1, x2));
      float bx1 = hb_max (x0, hb_max (x1, x2));
      float by0 = hb_min (y0, hb_min (y1, y2));
      float by1 = hb_max (y0, hb_max (y1, y2));
      if (bx1 < draw->flatten_clip_x0 || bx0 > draw->flatten_clip_x1 ||
	  by1 < draw->flatten_clip_y0 || by0 > draw->flatten_clip_y1)
	emit_chord = true;
      else if (bx0 >= draw->flatten_clip_x0 && bx1 <= draw->flatten_clip_x1 &&
	       by0 >= draw->flatten_clip_y0 && by1 <= draw->flatten_clip_y1)
	check_clip = false;
    }

    if (!emit_chord)
    {
      bool is_flat;
      if (false)
      {
	/* Old behavior: midpoint deviation from chord midpoint. */
	float mx = x0 * 0.25f + x1 * 0.5f + x2 * 0.25f;
	float my = y0 * 0.25f + y1 * 0.5f + y2 * 0.25f;
	float chord_mx = (x0 + x2) * 0.5f;
	float chord_my = (y0 + y2) * 0.5f;
	float dx = mx - chord_mx;
	float dy = my - chord_my;
	static const float flat_thresh = HB_RASTER_FLAT_THRESH * HB_RASTER_FLAT_THRESH;
	is_flat = (dx * dx + dy * dy) <= flat_thresh;
      }
      else
      {
	/* FreeType behavior: control-point deviation from chord center. */
	const float flat_thresh = 0.25f;
	float dx = x0 + x2 - 2.f * x1;
	float dy = y0 + y2 - 2.f * y1;
	if (dx < 0) dx = -dx;
	if (dy < 0) dy = -dy;
	is_flat = dx <= flat_thresh && dy <= flat_thresh;
      }
      emit_chord = is_flat;
    }

    /* Charge one unit per subdivision; degrade to the chord when the
       session work budget is spent. */
    if (!emit_chord)
    {
      if (unlikely (work_left <= 0))
	emit_chord = true;
      else
	work_left--;
    }

    if (emit_chord)
    {
      emit_segment (draw, x0, y0, x2, y2);
      if (!top)
      {
	*work = work_left;
	return;
      }
      const quad_node_t &n = stack[--top];
      x0 = n.x0; y0 = n.y0;
      x1 = n.x1; y1 = n.y1;
      x2 = n.x2; y2 = n.y2;
      depth = n.depth;
      check_clip = n.check_clip;
      continue;
    }

    float x01 = (x0 + x1) * 0.5f, y01 = (y0 + y1) * 0.5f;
    float x12 = (x1 + x2) * 0.5f, y12 = (y1 + y2) * 0.5f;
    float xm  = (x01 + x12) * 0.5f, ym  = (y01 + y12) * 0.5f;

    /* Depth is capped at 16, so stack capacity 16 is sufficient. */
    stack[top++] = {xm, ym, x12, y12, x2, y2, depth + 1, check_clip};
    x2 = xm; y2 = ym;
    x1 = x01; y1 = y01;
    depth++;
  }
}

/* Quadratic Bézier flattener using forward differencing.
   The error (midpoint deviation) shrinks exactly 4× per de Casteljau
   subdivision, so we compute the subdivision count upfront and iterate
   with constant-cost additions instead of recursive branching. */
static inline void
flatten_quadratic_fd (hb_raster_draw_t *draw,
		      float x0, float y0,
		      float x1, float y1,
		      float x2, float y2)
{
  /* Deviation of curve midpoint from chord midpoint (squared). */
  float devx = (x0 - 2 * x1 + x2) * 0.25f;
  float devy = (y0 - 2 * y1 + y2) * 0.25f;
  float err2 = devx * devx + devy * devy;

  static const float flat_thresh = HB_RASTER_FLAT_THRESH * HB_RASTER_FLAT_THRESH;

  if (err2 <= flat_thresh)
  {
    emit_segment (draw, x0, y0, x2, y2);
    return;
  }

  /* err² shrinks 16× per subdivision level.  Find n such that
     err2 / 16^n <= flat_thresh, i.e. n = ceil(log₁₆(err2/flat_thresh)). */
  unsigned n = 1;
  {
    float ratio = err2 / flat_thresh;
    while (ratio > 16.f) { ratio *= (1.f / 16.f); n++; }
    if (n > 16) n = 16;
  }
  unsigned N = 1u << n;   /* number of line segments */
  float h = 1.f / N;

  /* Quadratic: B(t) = a·t² + b·t + c
     Forward differences with step h:
       d²f = 2·a·h²   (constant)
       df₀ = a·h² + b·h
       f₀  = c = P₀                     */
  float ax = x0 - 2 * x1 + x2;
  float ay = y0 - 2 * y1 + y2;
  float bx = 2 * (x1 - x0);
  float by = 2 * (y1 - y0);

  float d2fx = 2 * ax * h * h;
  float d2fy = 2 * ay * h * h;
  float dfx  = ax * h * h + bx * h;
  float dfy  = ay * h * h + by * h;
  float fx   = x0;
  float fy   = y0;

  for (unsigned i = 1; i < N; i++)
  {
    float nx = fx + dfx;
    float ny = fy + dfy;
    emit_segment (draw, fx, fy, nx, ny);
    fx = nx;
    fy = ny;
    dfx += d2fx;
    dfy += d2fy;
  }
  /* Last segment uses exact endpoint to avoid drift. */
  emit_segment (draw, fx, fy, x2, y2);
}

static void
flatten_quadratic (hb_raster_draw_t *draw,
		   float x0, float y0,
		   float x1, float y1,
		   float x2, float y2)
{
  if (false)
    flatten_quadratic_fd (draw, x0, y0, x1, y1, x2, y2);
  else
    flatten_quadratic_recursive (draw, x0, y0, x1, y1, x2, y2);
}

/* For cubic B(t), the max deviation from its chord on [0,1] is bounded by:
     max||B(t)-L(t)|| <= max_t ||B''(t)|| / 8.
   B''(t) is linear, so max norm is attained at t=0 or t=1:
     B''(0)=6*(P0-2P1+P2), B''(1)=6*(P1-2P2+P3). */
static inline float
cubic_chord_error_bound2 (float x0, float y0,
			  float x1, float y1,
			  float x2, float y2,
			  float x3, float y3)
{
  float d20x = x0 - 2 * x1 + x2;
  float d20y = y0 - 2 * y1 + y2;
  float d21x = x1 - 2 * x2 + x3;
  float d21y = y1 - 2 * y2 + y3;
  float m0 = d20x * d20x + d20y * d20y;
  float m1 = d21x * d21x + d21y * d21y;
  float m = m0 > m1 ? m0 : m1;
  /* (max||B''||/8)^2 = (6/8)^2 * max||d2||^2 = (3/4)^2 * m. */
  return m * (9.f / 16.f);
}

/* Cubic Bézier flattener — iterative de Casteljau at t=0.5. */
static inline void
flatten_cubic_recursive (hb_raster_draw_t *draw,
			 float x0, float y0,
			 float x1, float y1,
			 float x2, float y2,
			 float x3, float y3,
			 int depth = 0)
{
  struct cubic_node_t
  {
    float x0, y0, x1, y1, x2, y2, x3, y3;
    int depth;
    bool check_clip;
  };

  cubic_node_t stack[16];
  unsigned top = 0;

  bool check_clip = draw->flatten_clip_active;
  int64_t *work = draw->external_work ? draw->external_work : &draw->flatten_work_left;
  int64_t work_left = *work;

  while (true)
  {
    bool emit_chord = depth >= 16;

    /* Entirely outside the clip box: the chord is exact for coverage.
       Subdivided control points are convex combinations of their
       parent's, so once a node is entirely inside the box no
       descendant can leave it and the test is skipped below. */
    if (!emit_chord && check_clip)
    {
      float bx0 = hb_min (hb_min (x0, x1), hb_min (x2, x3));
      float bx1 = hb_max (hb_max (x0, x1), hb_max (x2, x3));
      float by0 = hb_min (hb_min (y0, y1), hb_min (y2, y3));
      float by1 = hb_max (hb_max (y0, y1), hb_max (y2, y3));
      if (bx1 < draw->flatten_clip_x0 || bx0 > draw->flatten_clip_x1 ||
	  by1 < draw->flatten_clip_y0 || by0 > draw->flatten_clip_y1)
	emit_chord = true;
      else if (bx0 >= draw->flatten_clip_x0 && bx1 <= draw->flatten_clip_x1 &&
	       by0 >= draw->flatten_clip_y0 && by1 <= draw->flatten_clip_y1)
	check_clip = false;
    }

    if (!emit_chord)
    {
      bool is_flat;
      if (false)
      {
	/* Old behavior: curvature/chord-error bound. */
	float err2 = cubic_chord_error_bound2 (x0, y0, x1, y1, x2, y2, x3, y3);
	static const float flat_thresh = HB_RASTER_FLAT_THRESH * HB_RASTER_FLAT_THRESH;
	is_flat = err2 <= flat_thresh;
      }
      else
      {
	/* FreeType behavior: chord-trisection distance test. */
	const float flat_thresh = 0.5f;

	float d10x = 2.f * x0 - 3.f * x1 + x3;
	float d10y = 2.f * y0 - 3.f * y1 + y3;
	float d20x = x0 - 3.f * x2 + 2.f * x3;
	float d20y = y0 - 3.f * y2 + 2.f * y3;

	if (d10x < 0) d10x = -d10x;
	if (d10y < 0) d10y = -d10y;
	if (d20x < 0) d20x = -d20x;
	if (d20y < 0) d20y = -d20y;

	is_flat = d10x <= flat_thresh &&
		  d10y <= flat_thresh &&
		  d20x <= flat_thresh &&
		  d20y <= flat_thresh;
      }
      emit_chord = is_flat;
    }

    /* Charge one unit per subdivision; degrade to the chord when the
       session work budget is spent. */
    if (!emit_chord)
    {
      if (unlikely (work_left <= 0))
	emit_chord = true;
      else
	work_left--;
    }

    if (emit_chord)
    {
      emit_segment (draw, x0, y0, x3, y3);
      if (!top)
      {
	*work = work_left;
	return;
      }
      const cubic_node_t &n = stack[--top];
      x0 = n.x0; y0 = n.y0;
      x1 = n.x1; y1 = n.y1;
      x2 = n.x2; y2 = n.y2;
      x3 = n.x3; y3 = n.y3;
      depth = n.depth;
      check_clip = n.check_clip;
      continue;
    }

    float x01  = (x0 + x1) * 0.5f, y01  = (y0 + y1) * 0.5f;
    float x12  = (x1 + x2) * 0.5f, y12  = (y1 + y2) * 0.5f;
    float x23  = (x2 + x3) * 0.5f, y23  = (y2 + y3) * 0.5f;
    float x012 = (x01 + x12) * 0.5f, y012 = (y01 + y12) * 0.5f;
    float x123 = (x12 + x23) * 0.5f, y123 = (y12 + y23) * 0.5f;
    float xm   = (x012 + x123) * 0.5f, ym   = (y012 + y123) * 0.5f;

    /* Depth is capped at 16, so stack capacity 16 is sufficient. */
    stack[top++] = {xm, ym, x123, y123, x23, y23, x3, y3, depth + 1, check_clip};
    x3 = xm; y3 = ym;
    x2 = x012; y2 = y012;
    x1 = x01; y1 = y01;
    depth++;
  }
}

/* Cubic Bézier flattener using forward differencing.
   Use a curvature-based chord-error bound (max||B''||/8), then choose a
   uniform subdivision count n such that the bound drops below threshold.
   The cubic adds a constant third difference d³f = 6·a·h³. */
static inline void
flatten_cubic_fd (hb_raster_draw_t *draw,
		  float x0, float y0,
		  float x1, float y1,
		  float x2, float y2,
		  float x3, float y3)
{
  float err2 = cubic_chord_error_bound2 (x0, y0, x1, y1, x2, y2, x3, y3);

  static const float flat_thresh = HB_RASTER_FLAT_THRESH * HB_RASTER_FLAT_THRESH;

  if (err2 <= flat_thresh)
  {
    emit_segment (draw, x0, y0, x3, y3);
    return;
  }

  /* The bound scales with h², so err² shrinks 16× per subdivision level. */
  unsigned n = 1;
  {
    float ratio = err2 / flat_thresh;
    while (ratio > 16.f) { ratio *= (1.f / 16.f); n++; }
    if (n > 16) n = 16;
  }
  unsigned N = 1u << n;
  float h = 1.f / N;

  /* Cubic: B(t) = a·t³ + b·t² + c·t + d
     a = -P₀ + 3P₁ - 3P₂ + P₃
     b =  3P₀ - 6P₁ + 3P₂
     c =  3(P₁ - P₀)
     d =  P₀
     Forward differences with step h:
       d³f  = 6·a·h³              (constant)
       d²f₀ = 6·a·h³ + 2·b·h²
       d¹f₀ = a·h³ + b·h² + c·h
       f₀   = d = P₀              */
  float ax = -x0 + 3*x1 - 3*x2 + x3;
  float ay = -y0 + 3*y1 - 3*y2 + y3;
  float bx =  3*x0 - 6*x1 + 3*x2;
  float by =  3*y0 - 6*y1 + 3*y2;
  float cx =  3*(x1 - x0);
  float cy =  3*(y1 - y0);

  float h2 = h * h, h3 = h2 * h;
  float d3fx = 6 * ax * h3;
  float d3fy = 6 * ay * h3;
  float d2fx = d3fx + 2 * bx * h2;
  float d2fy = d3fy + 2 * by * h2;
  float dfx  = ax * h3 + bx * h2 + cx * h;
  float dfy  = ay * h3 + by * h2 + cy * h;
  float fx   = x0;
  float fy   = y0;

  for (unsigned i = 1; i < N; i++)
  {
    float nx = fx + dfx;
    float ny = fy + dfy;
    emit_segment (draw, fx, fy, nx, ny);
    fx = nx;  fy = ny;
    dfx += d2fx;  dfy += d2fy;
    d2fx += d3fx; d2fy += d3fy;
  }
  /* Last segment uses exact endpoint to avoid drift. */
  emit_segment (draw, fx, fy, x3, y3);
}

static void
flatten_cubic (hb_raster_draw_t *draw,
	       float x0, float y0,
	       float x1, float y1,
	       float x2, float y2,
	       float x3, float y3)
{
  if (false)
    flatten_cubic_fd (draw, x0, y0, x1, y1, x2, y2, x3, y3);
  else
    flatten_cubic_recursive (draw, x0, y0, x1, y1, x2, y2, x3, y3);
}


/* Draw callback implementations */

static void
hb_raster_move_to (hb_draw_funcs_t *dfuncs HB_UNUSED,
		   void *draw_data,
		   hb_draw_state_t *st HB_UNUSED,
		   float to_x HB_UNUSED, float to_y HB_UNUSED,
		   void *user_data HB_UNUSED)
{
  /* no-op: state tracked by hb_draw_state_t */
}

static void
hb_raster_line_to (hb_draw_funcs_t *dfuncs HB_UNUSED,
		   void *draw_data,
		   hb_draw_state_t *st,
		   float to_x, float to_y,
		   void *user_data HB_UNUSED)
{
  hb_raster_draw_t *draw = (hb_raster_draw_t *) draw_data;
  if (unlikely (draw->edges_left <= 0)) return;

  float tx0, ty0, tx1, ty1;
  transform_point (draw, st->current_x, st->current_y, tx0, ty0);
  transform_point (draw, to_x,          to_y,           tx1, ty1);
  emit_segment (draw, tx0, ty0, tx1, ty1);
}

static void
hb_raster_quadratic_to (hb_draw_funcs_t *dfuncs HB_UNUSED,
			void *draw_data,
			hb_draw_state_t *st,
			float control_x, float control_y,
			float to_x, float to_y,
			void *user_data HB_UNUSED)
{
  hb_raster_draw_t *draw = (hb_raster_draw_t *) draw_data;
  if (unlikely (draw->edges_left <= 0)) return;

  float tx0, ty0, tx1, ty1, tx2, ty2;
  transform_point (draw, st->current_x, st->current_y, tx0, ty0);
  transform_point (draw, control_x,     control_y,      tx1, ty1);
  transform_point (draw, to_x,          to_y,           tx2, ty2);
  flatten_quadratic (draw, tx0, ty0, tx1, ty1, tx2, ty2);
}

static void
hb_raster_cubic_to (hb_draw_funcs_t *dfuncs HB_UNUSED,
		    void *draw_data,
		    hb_draw_state_t *st,
		    float control1_x, float control1_y,
		    float control2_x, float control2_y,
		    float to_x, float to_y,
		    void *user_data HB_UNUSED)
{
  hb_raster_draw_t *draw = (hb_raster_draw_t *) draw_data;
  if (unlikely (draw->edges_left <= 0)) return;

  float tx0, ty0, tx1, ty1, tx2, ty2, tx3, ty3;
  transform_point (draw, st->current_x, st->current_y, tx0, ty0);
  transform_point (draw, control1_x,    control1_y,     tx1, ty1);
  transform_point (draw, control2_x,    control2_y,     tx2, ty2);
  transform_point (draw, to_x,          to_y,           tx3, ty3);
  flatten_cubic (draw, tx0, ty0, tx1, ty1, tx2, ty2, tx3, ty3);
}

static void
hb_raster_close_path (hb_draw_funcs_t *dfuncs HB_UNUSED,
		      void *draw_data HB_UNUSED,
		      hb_draw_state_t *st HB_UNUSED,
		      void *user_data HB_UNUSED)
{
  /* no-op: hb_draw_funcs_t already emits closing line_to before us */
}


/* Lazy-loader singleton for draw funcs */

static inline void free_static_raster_draw_funcs ();

static struct hb_raster_draw_funcs_lazy_loader_t : hb_draw_funcs_lazy_loader_t<hb_raster_draw_funcs_lazy_loader_t>
{
  static hb_draw_funcs_t *create ()
  {
    hb_draw_funcs_t *funcs = hb_draw_funcs_create ();

    hb_draw_funcs_set_move_to_func      (funcs, hb_raster_move_to,      nullptr, nullptr);
    hb_draw_funcs_set_line_to_func      (funcs, hb_raster_line_to,      nullptr, nullptr);
    hb_draw_funcs_set_quadratic_to_func (funcs, hb_raster_quadratic_to, nullptr, nullptr);
    hb_draw_funcs_set_cubic_to_func     (funcs, hb_raster_cubic_to,     nullptr, nullptr);
    hb_draw_funcs_set_close_path_func   (funcs, hb_raster_close_path,   nullptr, nullptr);

    hb_draw_funcs_make_immutable (funcs);

    hb_atexit (free_static_raster_draw_funcs);

    return funcs;
  }
} static_raster_draw_funcs;

static inline void
free_static_raster_draw_funcs ()
{
  static_raster_draw_funcs.free_instance ();
}

/**
 * hb_raster_draw_get_funcs:
 * @draw: a rasterizer draw context.
 *
 * Fetches the #hb_draw_funcs_t that feeds outline data into
 * @draw.  Pass @draw as the @draw_data argument when calling
 * the draw functions.
 *
 * Return value: (transfer none):
 * The rasterizer draw functions
 *
 * Since: 14.2.0
 **/
hb_draw_funcs_t *
hb_raster_draw_get_funcs (const hb_raster_draw_t *draw HB_UNUSED)
{
  return static_raster_draw_funcs.get_unconst ();
}

/**
 * hb_raster_draw_glyph_or_fail:
 * @draw: a rasterizer
 * @font: font to draw from
 * @glyph: glyph ID to draw
 *
 * Convenience to draw one glyph.  Equivalent to:
 *
 * |[<!-- language="plain" -->
 * hb_font_draw_glyph_or_fail (font, glyph,
 *   hb_raster_draw_get_funcs (draw), draw);
 * ]|
 *
 * Return value: `true` if the glyph was drawn, `false` if the font has
 * no outlines for @glyph.
 *
 * Since: 14.2.0
 **/
hb_bool_t
hb_raster_draw_glyph_or_fail (hb_raster_draw_t *draw,
			      hb_font_t       *font,
			      hb_codepoint_t   glyph)
{
  return hb_font_draw_glyph_or_fail (font, glyph,
				     hb_raster_draw_get_funcs (draw), draw);
}

/**
 * hb_raster_draw_glyph:
 * @draw: a rasterizer
 * @font: font to draw from
 * @glyph: glyph ID to draw
 *
 * Draws one glyph into @draw using the rasterizer's current
 * transform.  Equivalent to hb_raster_draw_glyph_or_fail() with the
 * return value ignored.
 *
 * Since: 14.2.0
 **/
void
hb_raster_draw_glyph (hb_raster_draw_t *draw,
		      hb_font_t       *font,
		      hb_codepoint_t   glyph)
{
  hb_raster_draw_glyph_or_fail (draw, font, glyph);
}


/*
 * Analytic coverage rasterizer
 *
 * For each line-segment edge and each pixel row it crosses, we compute
 * exact area/cover contributions per pixel cell.  A left-to-right sweep
 * then converts accumulated (area, cover) into alpha values.
 *
 * Coordinates are fixed-point.
 *
 *   cover[x] = Σ dy · wind        — signed vertical extent per cell
 *   area[x]  = Σ (fx₀+fx₁)·dy·wind — twice the signed trapezoidal area
 *
 * Sweep:
 *   cover_accum += cover[x]
 *   α = min(|cover_accum·128 − area[x]|, 8192) · 255 / 8192
 */

/* Add one edge piece's area/cover into a single cell. */
static HB_ALWAYS_INLINE void
cell_add (int32_t *area, int16_t *cover, unsigned width, int col,
	  int32_t fx0, int32_t fy0, int32_t fx1, int32_t fy1, int32_t wind,
	  unsigned &x_min, unsigned &x_max)
{
  if (unlikely ((unsigned) col >= width))
  {
    if (unlikely (col < 0))
    {
      /* Edge is to the left of the surface.  The winding contribution
       * still carries into the visible region, so add the cover delta
       * to column 0.  Area is not added since the edge doesn't cross
       * column 0's cell. */
      int32_t dy = fy1 - fy0;
      cover[0] += (int16_t) (dy * wind);
      x_min = hb_min (x_min, 0u);
      x_max = hb_max (x_max, 0u);
    }
    return;
  }
  int32_t dy = fy1 - fy0;
  area[col]  += (fx0 + fx1) * dy * wind;
  cover[col] += (int16_t) (dy * wind);
  x_min = hb_min (x_min, (unsigned) col);
  x_max = hb_max (x_max, (unsigned) col);
}

/* Walk one edge through the pixel cells of a single pixel row,
   accumulating area/cover.  py is the integer pixel-row index. */
static HB_ALWAYS_INLINE void
edge_sweep_row (int32_t                *area,
		int16_t                *cover,
		unsigned                width,
		int                     x_org,
		int32_t                 y_top,
		const hb_raster_edge_t &edge,
		unsigned               &x_min,
		unsigned               &x_max)
{
  /* Saturate: y_top can be within ONE_PIXEL of INT32_MAX when the surface
   * sits at the extreme of the fixed-point coordinate space.  Edges are
   * int32-clamped too, so no coverage exists beyond INT32_MAX anyway. */
  int32_t y_bot = (int32_t) hb_min ((int64_t) y_top + HB_RASTER_ONE_PIXEL, (int64_t) INT32_MAX);

  int32_t ey0 = hb_max (edge.yL, y_top);
  int32_t ey1 = hb_min (edge.yH, y_bot);
  if (ey0 >= ey1) return;

  /* X at clipped endpoints (fixed-point). Keep the interpolation in 64-bit
   * so extreme y values do not overflow before the slope multiply. */
  int64_t x0_64 = (int64_t) edge.xL +
		  ((((int64_t) ey0 - (int64_t) edge.yL) * edge.slope) >> 16);
  int64_t x1_64 = (int64_t) edge.xL +
		  ((((int64_t) ey1 - (int64_t) edge.yL) * edge.slope) >> 16);
  int32_t x0 = (int32_t) hb_clamp (x0_64, (int64_t) INT32_MIN, (int64_t) INT32_MAX);
  int32_t x1 = (int32_t) hb_clamp (x1_64, (int64_t) INT32_MIN, (int64_t) INT32_MAX);

  /* Fractional y within this pixel row [0, ONE_PIXEL] */
  int32_t fy0 = ey0 - y_top;
  int32_t fy1 = ey1 - y_top;

  int32_t cx0 = x0 >> HB_RASTER_PIXEL_BITS;
  int32_t fx0 = x0 & HB_RASTER_PIXEL_MASK;
  int32_t cx1 = x1 >> HB_RASTER_PIXEL_BITS;
  int32_t fx1 = x1 & HB_RASTER_PIXEL_MASK;
  int32_t wind = edge.wind;

  /* Fast path: both endpoints in the same pixel column. */
  if (cx0 == cx1)
  {
    cell_add (area, cover, width, cx0 - x_org, fx0, fy0, fx1, fy1, wind, x_min, x_max);
    return;
  }

  int64_t total_dx = (int64_t) x1 - (int64_t) x0;
  int64_t total_dy = (int64_t) fy1 - (int64_t) fy0;

  /* fy increment per pixel column (constant since x_b advances by ONE_PIXEL). */
  int32_t delta_fy = (int32_t) ((int64_t) HB_RASTER_ONE_PIXEL * total_dy / total_dx);

  if (total_dx > 0)
  {
    /* Left-to-right edge. */
    int32_t x_b = (int32_t) hb_clamp (((int64_t) cx0 + 1) * HB_RASTER_ONE_PIXEL,
				      (int64_t) INT32_MIN, (int64_t) INT32_MAX);
    int32_t fy_b = fy0 + (int32_t) ((((int64_t) x_b - (int64_t) x0) * total_dy) / total_dx);
    cell_add (area, cover, width, cx0 - x_org, fx0, fy0, HB_RASTER_ONE_PIXEL, fy_b, wind, x_min, x_max);

    int32_t fy_prev = fy_b;
    for (int32_t cx = cx0 + 1; cx < cx1; cx++)
    {
      fy_b = fy_prev + delta_fy;
      cell_add (area, cover, width, cx - x_org, 0, fy_prev, HB_RASTER_ONE_PIXEL, fy_b, wind, x_min, x_max);
      fy_prev = fy_b;
    }

    cell_add (area, cover, width, cx1 - x_org, 0, fy_prev, fx1, fy1, wind, x_min, x_max);
  }
  else
  {
    /* Right-to-left edge. */
    int32_t x_b = (int32_t) hb_clamp ((int64_t) cx0 * HB_RASTER_ONE_PIXEL,
				      (int64_t) INT32_MIN, (int64_t) INT32_MAX);
    int32_t fy_b = fy0 + (int32_t) ((((int64_t) x_b - (int64_t) x0) * total_dy) / total_dx);
    cell_add (area, cover, width, cx0 - x_org, fx0, fy0, 0, fy_b, wind, x_min, x_max);

    int32_t fy_prev = fy_b;
    for (int32_t cx = cx0 - 1; cx > cx1; cx--)
    {
      fy_b = fy_prev - delta_fy;
      cell_add (area, cover, width, cx - x_org, HB_RASTER_ONE_PIXEL, fy_prev, 0, fy_b, wind, x_min, x_max);
      fy_prev = fy_b;
    }

    cell_add (area, cover, width, cx1 - x_org, HB_RASTER_ONE_PIXEL, fy_prev, fx1, fy1, wind, x_min, x_max);
  }
}

/* Convert cover-delta + area to alpha bytes, then clear.
   Returns final cover accumulator over [x_min, x_max]. */
static int32_t
sweep_row_to_alpha (uint8_t *__restrict row_buf,
		    int32_t *__restrict area,
		    int16_t *__restrict cover,
		    unsigned x_min,
		    unsigned x_max)
{
  const int32_t cover_scale = 2 * HB_RASTER_ONE_PIXEL;
  int32_t cover_accum = 0;
  unsigned x = x_min;

#ifdef HB_RASTER_NEON
  int32x4_t clamp_v = vdupq_n_s32 (HB_RASTER_FULL_COVERAGE);
  int32x4_t bias_v  = vdupq_n_s32 (HB_RASTER_FULL_COVERAGE / 2);
  int32x4_t zero32  = vdupq_n_s32 (0);
  int16x8_t zero16  = vdupq_n_s16 (0);
  for (; x + 7 <= x_max; x += 8)
  {
    int32_t ctmp[8];
    for (unsigned i = 0; i < 8; i++)
    {
      cover_accum += cover[x + i];
      ctmp[i] = cover_accum * cover_scale;
    }

    int32x4_t c0  = vld1q_s32 (ctmp + 0);
    int32x4_t c1  = vld1q_s32 (ctmp + 4);
    int32x4_t a0  = vld1q_s32 (area + x);
    int32x4_t a1  = vld1q_s32 (area + x + 4);

    int32x4_t v0 = vabsq_s32 (vsubq_s32 (c0, a0));
    int32x4_t v1 = vabsq_s32 (vsubq_s32 (c1, a1));

    v0 = vminq_s32 (v0, clamp_v);
    v1 = vminq_s32 (v1, clamp_v);

    int32x4_t r0 = vshrq_n_s32 (vmlaq_n_s32 (bias_v, v0, 255), 2 * HB_RASTER_PIXEL_BITS + 1);
    int32x4_t r1 = vshrq_n_s32 (vmlaq_n_s32 (bias_v, v1, 255), 2 * HB_RASTER_PIXEL_BITS + 1);

    int16x4_t h0 = vmovn_s32 (r0);
    int16x4_t h1 = vmovn_s32 (r1);
    int16x8_t h  = vcombine_s16 (h0, h1);
    uint8x8_t b  = vqmovun_s16 (h);
    vst1_u8 (row_buf + x, b);

    vst1q_s32 (area + x,     zero32);
    vst1q_s32 (area + x + 4, zero32);
    vst1q_s16 (cover + x,    zero16);
  }
#elif defined(HB_RASTER_SSE2)
  __m128i clamp_v = _mm_set1_epi32 (HB_RASTER_FULL_COVERAGE);
  __m128i bias_v  = _mm_set1_epi32 (HB_RASTER_FULL_COVERAGE / 2);
  __m128i zero_v  = _mm_setzero_si128 ();
  for (; x + 7 <= x_max; x += 8)
  {
    int32_t ctmp[8];
    for (unsigned i = 0; i < 8; i++)
    {
      cover_accum += cover[x + i];
      ctmp[i] = cover_accum * cover_scale;
    }

    __m128i c0  = _mm_loadu_si128 ((__m128i *) (void *) (ctmp + 0));
    __m128i c1  = _mm_loadu_si128 ((__m128i *) (void *) (ctmp + 4));
    __m128i a0  = _mm_loadu_si128 ((__m128i *) (void *) (area + x));
    __m128i a1  = _mm_loadu_si128 ((__m128i *) (void *) (area + x + 4));

    __m128i v0 = _mm_sub_epi32 (c0, a0);
    __m128i v1 = _mm_sub_epi32 (c1, a1);

    __m128i s0 = _mm_srai_epi32 (v0, 31);
    __m128i s1 = _mm_srai_epi32 (v1, 31);
    v0 = _mm_sub_epi32 (_mm_xor_si128 (v0, s0), s0);
    v1 = _mm_sub_epi32 (_mm_xor_si128 (v1, s1), s1);

    __m128i lt0 = _mm_cmplt_epi32 (v0, clamp_v);
    __m128i lt1 = _mm_cmplt_epi32 (v1, clamp_v);
    v0 = _mm_or_si128 (_mm_and_si128 (lt0, v0), _mm_andnot_si128 (lt0, clamp_v));
    v1 = _mm_or_si128 (_mm_and_si128 (lt1, v1), _mm_andnot_si128 (lt1, clamp_v));

    __m128i r0 = _mm_srai_epi32 (_mm_add_epi32 (_mm_sub_epi32 (_mm_slli_epi32 (v0, 8), v0), bias_v), 2 * HB_RASTER_PIXEL_BITS + 1);
    __m128i r1 = _mm_srai_epi32 (_mm_add_epi32 (_mm_sub_epi32 (_mm_slli_epi32 (v1, 8), v1), bias_v), 2 * HB_RASTER_PIXEL_BITS + 1);

    __m128i h = _mm_packs_epi32 (r0, r1);
    __m128i b = _mm_packus_epi16 (h, h);
    _mm_storel_epi64 ((__m128i *) (void *) (row_buf + x), b);

    _mm_storeu_si128 ((__m128i *) (void *) (area + x),     zero_v);
    _mm_storeu_si128 ((__m128i *) (void *) (area + x + 4), zero_v);
    _mm_storeu_si128 ((__m128i *) (void *) (cover + x),    zero_v);
  }
#endif

  for (; x <= x_max; x++)
  {
    cover_accum += cover[x];
    int32_t val   = cover_accum * cover_scale - area[x];
    int32_t alpha = val < 0 ? -val : val;
    if (alpha > HB_RASTER_FULL_COVERAGE) alpha = HB_RASTER_FULL_COVERAGE;
    row_buf[x] = (uint8_t) (((unsigned) alpha * 255 + HB_RASTER_FULL_COVERAGE / 2) >> (2 * HB_RASTER_PIXEL_BITS + 1));
    area[x]  = 0;
    cover[x] = 0;
  }

  return cover_accum;
}


/**
 * hb_raster_draw_render:
 * @draw: a rasterizer
 *
 * Rasterizes the accumulated outline geometry into a new
 * #hb_raster_image_t.  After rendering, the accumulated edges are
 * cleared so the rasterizer can be reused. Output format is always
 * @HB_RASTER_FORMAT_A8.
 *
 * Return value: (transfer full):
 * A rendered #hb_raster_image_t. Returns `NULL` on allocation/configuration
 * failure. If no geometry was accumulated, returns an empty image.
 *
 * Since: 13.0.0
 **/
hb_raster_image_t *
hb_raster_draw_render (hb_raster_draw_t *draw)
{
  /* ── 1. Compute result extents ─────────────────────────────────── */
  hb_raster_extents_t ext;

  if (draw->has_extents)
  {
    ext = draw->fixed_extents;
  }
  else
  {
    /* Auto-size from edge bounding box */
    if (draw->edges.length == 0)
    {
      /* No edges: produce 0×0 image */
      ext = { 0, 0, 0, 0, 0 };
    }
    else
    {
      int32_t xmin = draw->edges.arrayZ[0].xL, xmax = draw->edges.arrayZ[0].xL;
      int32_t ymin = draw->edges.arrayZ[0].yL, ymax = draw->edges.arrayZ[0].yH;

      for (const auto &e : draw->edges)
      {
	xmin = hb_min (xmin, hb_min (e.xL, e.xH));
	xmax = hb_max (xmax, hb_max (e.xL, e.xH));
	ymin = hb_min (ymin, e.yL);
	ymax = hb_max (ymax, e.yH);
      }

      /* Convert fixed-point → pixels (floor for min, ceil for max).  Edge
	 coordinates are saturated to int32 range in emit_segment, so the
	 +MASK ceil step must be widened to avoid signed overflow. */
      int x0 = xmin >> HB_RASTER_PIXEL_BITS;
      int y0 = ymin >> HB_RASTER_PIXEL_BITS;
      int x1 = (int) (((int64_t) xmax + HB_RASTER_PIXEL_MASK) >> HB_RASTER_PIXEL_BITS);
      int y1 = (int) (((int64_t) ymax + HB_RASTER_PIXEL_MASK) >> HB_RASTER_PIXEL_BITS);

      ext.x_origin = x0;
      ext.y_origin = y0;
      ext.width    = (unsigned) hb_min (hb_max (0, x1 - x0), HB_RASTER_MAX_AUTO_DIMENSION);
      ext.height   = (unsigned) hb_min (hb_max (0, y1 - y0), HB_RASTER_MAX_AUTO_DIMENSION);
      ext.stride   = 0; /* filled below */
    }
  }

  /* ── 2. Compute stride ─────────────────────────────────────────── */
  if (ext.stride == 0)
    ext.stride = (ext.width + 3u) & ~3u;

  /* ── 3. Allocate or reuse image ─────────────────────────────────── */
  /* Reset one-shot state on every exit path. */
  HB_SCOPE_GUARD (hb_raster_draw_clear (draw));

  hb_unique_ptr_t<hb_raster_image_t> image;
  if (draw->recycled_image)
  {
    image = hb_unique_ptr_t<hb_raster_image_t> (draw->recycled_image);
    draw->recycled_image = nullptr;
  }
  else
  {
    image = hb_unique_ptr_t<hb_raster_image_t> (hb_raster_image_create_or_fail ());
    if (unlikely (!image)) return nullptr;
  }

  if (unlikely (!image->configure (HB_RASTER_FORMAT_A8, ext)))
    return nullptr;
  image->clear ();

  /* ── 4. Bucket edges by starting row and rasterize scanlines ──── */
  if (draw->edges.length && ext.width && ext.height)
  {
    if (unlikely (!draw->row_area.resize_dirty (ext.width) ||
		  !draw->row_cover.resize_dirty (ext.width)))
      return nullptr;
    hb_memset (draw->row_area.arrayZ,  0, ext.width * sizeof (int32_t));
    hb_memset (draw->row_cover.arrayZ, 0, ext.width * sizeof (int16_t));

    /* Bucket edges by their starting pixel row.
       Only grow the outer vector; clear inner vectors without freeing. */
    unsigned old_buckets = draw->edge_buckets.length;
    if (ext.height > old_buckets)
    {
      if (unlikely (!draw->edge_buckets.resize (ext.height)))
	return nullptr;
    }
    for (unsigned i = 0; i < hb_min (ext.height, old_buckets); i++)
      draw->edge_buckets.arrayZ[i].clear ();
    /* New buckets (if any) are already empty from resize's zero-init. */

    for (unsigned i = 0; i < draw->edges.length; i++)
    {
      int row = (draw->edges.arrayZ[i].yL >> HB_RASTER_PIXEL_BITS) - ext.y_origin;
      if (row < 0) row = 0;
      if ((unsigned) row >= ext.height) continue;
      draw->edge_buckets.arrayZ[row].push (i);
    }

    /* Scanline loop with active edge list. */
    draw->active_edges.clear ();

    for (unsigned row = 0; row < ext.height; row++)
    {
      int64_t y_top_64 = ((int64_t) ext.y_origin + (int64_t) row) * HB_RASTER_ONE_PIXEL;
      int32_t y_top = (int32_t) hb_clamp (y_top_64, (int64_t) INT32_MIN, (int64_t) INT32_MAX);

      /* Add new edges from this row's bucket. */
      draw->active_edges.extend (draw->edge_buckets.arrayZ[row]);

      /* Process active edges and compact live ones in one linear pass. */
      unsigned x_min = ext.width, x_max = 0;
      unsigned write = 0;
      unsigned active_len = draw->active_edges.length;
      for (unsigned j = 0; j < active_len; j++)
      {
	unsigned edge_idx = draw->active_edges.arrayZ[j];
	const auto &e = draw->edges.arrayZ[edge_idx];
	if (e.yH <= y_top)
	  continue;

	edge_sweep_row (draw->row_area.arrayZ, draw->row_cover.arrayZ,
			ext.width, ext.x_origin, y_top, e, x_min, x_max);
	draw->active_edges.arrayZ[write++] = edge_idx;
      }
      draw->active_edges.resize (write);

      if (x_min <= x_max)
      {
	int32_t cover_accum = sweep_row_to_alpha (image->buffer.arrayZ + row * ext.stride,
						   draw->row_area.arrayZ, draw->row_cover.arrayZ,
						   x_min, x_max);

	/* If cover doesn't cancel, memset the constant-alpha tail. */
	if (cover_accum != 0)
	{
	  int32_t alpha = cover_accum * (2 * HB_RASTER_ONE_PIXEL);
	  alpha = alpha < 0 ? -alpha : alpha;
	  if (alpha > HB_RASTER_FULL_COVERAGE) alpha = HB_RASTER_FULL_COVERAGE;
	  uint8_t byte = (uint8_t) (((unsigned) alpha * 255 + HB_RASTER_FULL_COVERAGE / 2) >> (2 * HB_RASTER_PIXEL_BITS + 1));

	  uint8_t *row_buf = image->buffer.arrayZ + row * ext.stride;
	  hb_memset (row_buf + x_max + 1, byte, ext.width - 1 - x_max);
	}
      }
    }
  }

  return image.release ();
}