OpenGL ES render-to-texture content is stored top-down

Impeller's OpenGL ES backend now stores render-to-texture content top-down, the same as the Metal and Vulkan backends. A fragment shader or Flutter GPU app that sampled a render-target texture on OpenGL ES and added a vertical flip to compensate for the old bottom-up orientation now renders that content upside down.

The OpenGL ES backend previously stored render-to-texture content bottom-up, unlike the Metal and Vulkan backends. The renderer carried that orientation difference through every texture sample as a per-sampler Y-coordinate scale. Impeller now absorbs the difference once, at the vertex stage, so render-target textures are stored top-down on every backend.

This change is invisible to the framework and to Flutter's 2D rendering. Application code can only observe the old orientation through the FragmentShader API and through Flutter GPU, the experimental flutter_gpu package, where a shader samples a render-target texture directly.

Shaders written for OpenGL ES often flipped the Y coordinate inside an IMPELLER_TARGET_OPENGLES block to make render-target textures match the other backends. Render-target textures are now top-down on every backend, so that flip is no longer needed.

Code before migration:

void main() {
  vec2 uv = FlutterFragCoord().xy / u_size;
#ifdef IMPELLER_TARGET_OPENGLES
  uv.y = 1.0 - uv.y;
#endif
  fragColor = texture(u_texture, uv);
}

If the shader only needs to run on a Flutter release that includes this change, remove the flip.

Code after migration:

void main() {
  vec2 uv = FlutterFragCoord().xy / u_size;
  fragColor = texture(u_texture, uv);
}

To keep one source working across releases, guard the flip with the IMPELLER_OPENGLES_UNFLIPPED_DEPRECATED macro. That macro is only defined on releases that store render-target textures top-down, so the flip still runs on older releases that need it.

void main() {
  vec2 uv = FlutterFragCoord().xy / u_size;
#if defined(IMPELLER_TARGET_OPENGLES) && !defined(IMPELLER_OPENGLES_UNFLIPPED_DEPRECATED)
  uv.y = 1.0 - uv.y;
#endif
  fragColor = texture(u_texture, uv);
}

Landed in version: not yet released
In stable release: the next stable release after 3.44.0

GitHub issue:

• Issue 186554

Relevant PRs:

• PR 186556
• PR 187316