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

> Impeller's OpenGL ES backend now stores render-to-texture content top-down, matching Metal and Vulkan, which can affect fragment shaders and Flutter GPU apps that compensated for the previous bottom-up orientation.




:::important
These breaking change docs are accurate, as of the release
under which they are published. Over time, the
workarounds described here might become inaccurate.
We don't, in general, keep these breaking change docs up
to date as of each release.

The [breaking change index file](/release/breaking-changes)
lists the docs created for each release.
:::


## Summary

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.

## Background

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.

## Migration guide

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:

```glsl
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:

```glsl
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.

```glsl
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);
}
```

## Timeline

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

## References

GitHub issue:

* [Issue 186554][]

Relevant PRs:

* [PR 186556][]
* [PR 187316][]

[Issue 186554]: https://github.com/flutter/flutter/issues/186554
[PR 186556]: https://github.com/flutter/flutter/pull/186556
[PR 187316]: https://github.com/flutter/flutter/pull/187316