Rectangle missing flip use case from deprecated shape Quad

[tripokey]

What problem does this solve or what need does it fill?

The deprecated shape Quad's flip field does not have a clear migration path to the new Rectangle shape.

Current migration guide: https://bevyengine.org/learn/migration-guides/0-12-to-0-13/#deprecate-shapes-in-bevy-render-mesh-shape

What solution would you like?

#10569 specifies that there should be a flipped method to cover this use case, however that seems to be missing.

let flipped_rectangle = Rectangle::new(100.0, 100.0).mesh().flipped();

[alice-i-cecile]

Ah, this is to invert the normals, correct? Should we add this as an option on all of our mesh builders?

[tripokey]

It flipped the texture coordinates horizontally.

From https://docs.rs/bevy/latest/bevy/prelude/shape/struct.Quad.html :

Horizontally-flip the texture coordinates of the resulting mesh

[Jondolf]

This was sort of left out on purpose: #11431 (comment)

It seems a bit strange to me to support flipping a texture in a mesh representation. I feel like it's something that should instead be configurable at a material-level. I would assume that this is possible already? If not, I think it'd be valuable to have.

Only supporting texture flipping for Rectangle/Quad also seems slightly arbitrary to me; why not also support planes? Or circles? Or basically any shape?

Looking at other applications, I'm also not seeing a similar flip property elsewhere. For example, Godot doesn't have it (I also checked editor GUI), Unity doesn't have it, and Blender doesn't have it. To me, it doesn't feel like a quad-specific property, but a general material property.

[tripokey]

I'm not saying that the flipped method needs to be added to Mesh, as a user of Quad::flip I think that a line in the migration guide on how to achieve this in the new bevy release would be enough

[bugsweeper]

I feel like it's something that should instead be configurable at a material-level

Do you mean just add to StandardMaterial additional flip parameter, or mention in migration guide that making custom flipping material is the correct way for replacement of Quad::flip?

[bugsweeper]

@tripokey For any UV transformations you can use StandardMaterial::uv_transform, which is pretty fresh, because it had been added for KHR_texture_transform support to current main and it will be available from next 0.14 release.
For example for flip you can try set uv_transform to
Affine2{matrix2: -Mat2::IDENTITY, translation: Vec2::ONE,} for both axes flip
or

Affine2 {
    matrix2: Mat2::from_cols(-Vec2::X, Vec2::Y),
    translation: Vec2::X,
} // for horizontal flip

instead of default uv_transform or premultiply it with uv_transform that you wanted to use earlier, if so.
@alice-i-cecile How do you think, should we add flipped method to StandardMaterial, or maybe add FLIPPING / HORIZONTAL_FLIPPING const value to Affine2 consts? Or just copy valuable part of this message to migration log?

[alice-i-cecile]

I think that StandardMaterial is the most predictable place to find this. I think we should do that, but also add some nice constants to Affine2 (and possibly Affine3): those are great simple constants that will come up repeatedly and improve readability.