engine/src/flutter/impeller/entity/contents/clip_contents.cc

// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <cmath>
#include <optional>

#include "fml/logging.h"
#include "impeller/core/formats.h"
#include "impeller/core/vertex_buffer.h"
#include "impeller/entity/contents/clip_contents.h"
#include "impeller/entity/contents/content_context.h"
#include "impeller/entity/contents/pipelines.h"
#include "impeller/entity/entity.h"
#include "impeller/renderer/render_pass.h"
#include "impeller/renderer/vertex_buffer_builder.h"

namespace impeller {

static Scalar GetShaderClipDepth(uint32_t clip_depth) {
  // Draw the clip at the max of the clip entity's depth slice, so that other
  // draw calls with this same depth value will be culled even if they have a
  // perspective transform.
  return std::nextafterf(Entity::GetShaderClipDepth(clip_depth + 1), 0.0f);
}

/*******************************************************************************
 ******* ClipContents
 ******************************************************************************/

ClipContents::ClipContents(Rect coverage_rect, bool is_axis_aligned_rect)
    : coverage_rect_(coverage_rect),
      is_axis_aligned_rect_(is_axis_aligned_rect) {}

ClipContents::~ClipContents() = default;

void ClipContents::SetGeometry(GeometryResult clip_geometry) {
  clip_geometry_ = std::move(clip_geometry);
}

void ClipContents::SetClipOperation(Entity::ClipOperation clip_op) {
  clip_op_ = clip_op;
}

ClipCoverage ClipContents::GetClipCoverage(
    const std::optional<Rect>& current_clip_coverage) const {
  if (!current_clip_coverage.has_value()) {
    return ClipCoverage{.coverage = std::nullopt};
  }
  switch (clip_op_) {
    case Entity::ClipOperation::kDifference:
      // This can be optimized further by considering cases when the bounds of
      // the current stencil will shrink.
      return {
          .is_difference_or_non_square = true,  //
          .coverage = current_clip_coverage     //
      };
    case Entity::ClipOperation::kIntersect:
      if (coverage_rect_.IsEmpty() || !current_clip_coverage.has_value()) {
        return ClipCoverage{.coverage = std::nullopt};
      }
      return {
          .is_difference_or_non_square = !is_axis_aligned_rect_,            //
          .coverage = current_clip_coverage->Intersection(coverage_rect_),  //
      };
  }
  FML_UNREACHABLE();
}

bool ClipContents::Render(const ContentContext& renderer,
                          RenderPass& pass,
                          uint32_t clip_depth) const {
  if (!clip_geometry_.vertex_buffer) {
    return true;
  }

  using VS = ClipPipeline::VertexShader;

  VS::FrameInfo info;
  info.depth = GetShaderClipDepth(clip_depth);
  info.mvp = clip_geometry_.transform;

  auto options = OptionsFromPass(pass);
  options.blend_mode = BlendMode::kDst;
  options.primitive_type = clip_geometry_.type;

  pass.SetVertexBuffer(clip_geometry_.vertex_buffer);

  // kNormal and kPreventOverdraw geometries can do a difference clip by writing
  // depth directly without stencil-and-cover.
  if ((clip_geometry_.mode == GeometryResult::Mode::kNormal ||
       clip_geometry_.mode == GeometryResult::Mode::kPreventOverdraw) &&
      clip_op_ == Entity::ClipOperation::kDifference) {
    options.depth_write_enabled = true;
    pass.SetPipeline(renderer.GetClipPipeline(options));

    VS::BindFrameInfo(pass,
                      renderer.GetTransientsDataBuffer().EmplaceUniform(info));

    return pass.Draw().ok();
  }

  /// Stencil preparation draw.

  pass.SetStencilReference(0);

  options.depth_write_enabled = false;
  switch (clip_geometry_.mode) {
    case GeometryResult::Mode::kNonZero:
      pass.SetCommandLabel("Clip stencil preparation (NonZero)");
      options.stencil_mode =
          ContentContextOptions::StencilMode::kStencilNonZeroFill;
      break;
    case GeometryResult::Mode::kEvenOdd:
      pass.SetCommandLabel("Clip stencil preparation (EvenOdd)");
      options.stencil_mode =
          ContentContextOptions::StencilMode::kStencilEvenOddFill;
      break;
    case GeometryResult::Mode::kNormal:
    case GeometryResult::Mode::kPreventOverdraw:
      pass.SetCommandLabel("Clip stencil preparation (Increment)");
      options.stencil_mode =
          ContentContextOptions::StencilMode::kStencilIncrementAll;
      break;
  }
  pass.SetPipeline(renderer.GetClipPipeline(options));

  VS::BindFrameInfo(pass,
                    renderer.GetTransientsDataBuffer().EmplaceUniform(info));

  if (!pass.Draw().ok()) {
    return false;
  }

  /// Write depth.

  options.depth_write_enabled = true;
  options.primitive_type = PrimitiveType::kTriangleStrip;
  Rect cover_area;
  switch (clip_op_) {
    case Entity::ClipOperation::kIntersect:
      pass.SetCommandLabel("Intersect Clip");
      options.stencil_mode =
          ContentContextOptions::StencilMode::kCoverCompareInverted;
      cover_area = Rect::MakeSize(pass.GetRenderTargetSize());
      break;
    case Entity::ClipOperation::kDifference:
      pass.SetCommandLabel("Difference Clip");
      options.stencil_mode = ContentContextOptions::StencilMode::kCoverCompare;
      cover_area = coverage_rect_;
      break;
  }
  auto points = cover_area.GetPoints();
  pass.SetVertexBuffer(
      CreateVertexBuffer(points, renderer.GetTransientsDataBuffer()));

  pass.SetPipeline(renderer.GetClipPipeline(options));

  info.mvp = pass.GetOrthographicTransform();
  VS::BindFrameInfo(pass,
                    renderer.GetTransientsDataBuffer().EmplaceUniform(info));

  return pass.Draw().ok();
}

}  // namespace impeller