impeller/entity/shaders/geometry/uv.comp

// 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 <impeller/texture.glsl>
#include <impeller/types.glsl>

// Unused, see See PointFieldGeometry::GetPositionBuffer
layout(local_size_x = 16) in;

layout(std430) readonly buffer GeometryData {
  // Size of this input data is frame_info.count;
  vec2 points[];
}
geometry_data;

layout(std430) writeonly buffer GeometryUVData {
  // Size of this output data is frame_info.count;
  // x,y is the original geometry.
  // u,v is the texture UV.
  vec4 points_uv[];
}
geometry_uv_data;

uniform FrameInfo {
  uint count;
  mat4 effect_transform;
  vec2 texture_origin;
  vec2 texture_size;
}
frame_info;

vec2 project_point(mat4 m, vec2 v) {
  float w = v.x * m[0][3] + v.y * m[1][3] + m[3][3];
  vec2 result = vec2(v.x * m[0][0] + v.y * m[1][0] + m[3][0],
                     v.x * m[0][1] + v.y * m[1][1] + m[3][1]);

  // This is Skia's behavior, but it may be reasonable to allow UB for the w=0
  // case.
  if (w != 0) {
    w = 1 / w;
  }
  return result * w;
}

void main() {
  uint ident = gl_GlobalInvocationID.x;
  if (ident >= frame_info.count) {
    return;
  }

  vec2 point = geometry_data.points[ident];

  vec2 coverage_coords =
      (point - frame_info.texture_origin) / frame_info.texture_size;
  vec2 texture_coords =
      project_point(frame_info.effect_transform, coverage_coords);

  geometry_uv_data.points_uv[ident] = vec4(point, texture_coords);
}