SDL_Render (Clamp to) Border Texture Address Mode

[DctrNoob]

I am longing for a texture address mode akin to OpenGL's CLAMP_TO_BORDER in SDL_Render. The big graphics APIs (OpenGL, D3D11 and 12, Vulkan, Metal as of macOS 12 and iOS 14) all support this and I was wondering why it is not offered in SDL_Render (and not in SDL_GPU afaik). All the workarounds I tried so far suck in some regard while just setting CLAMP_TO_BORDER solves things for me very nicely.

After some digging, I realized that setting the texture address mode in SDL_Render has in fact only recently been added to main and is part of the 3.4.0 milestone. Still, the available address modes do not include CLAMP_TO_BORDER or the like.

Obviously, adding it would entail to add API functions to define the border color as well.

Maybe you didn't add it so far because not every backend supports it (OpenGL ES does not, I think) and maybe you are strict about feature parity between backends, so feel free to reject this suggestion. However, it would make me a little sad (though I could imagine contributing this feature if it's a matter of workload).

Keep it up!

Best regards,

Thomas

[slouken]

As mentioned on discourse, not all the SDL render backends support clamp to border, and I don't think you can emulate it if the texture sampler doesn’t support it.

Out of curiosity, is this possible to implement using a texture with a 1 pixel border and adjusting texture coordinates to match?

[DctrNoob]

So my current workaround is to overwrite the 1px boundary of my input image data before making a texture from it, then using CLAMP address mode. I don't adjust texture coordinates so that is a slight error but it largely does the job.

An emulating workaround for unsupported backends could be to not touch pixel or texture data but to have a shader that does the job of the sampler, i. e. returns the border color when UVs are out-of-bounds.

[DctrNoob]

So I thought that maybe I can contribute this feature to SDL and went to work on this using my own fork. I got pretty far but then encountered the problem that SDL_RenderGeometry does not seem to use a blend mode that incorporates transparency (for cases where the border color is transparent). If I set it explicitly beforehand via SDL_SetTextureBlendMode it works on macOS using the Metal backend but not on Windows (neither using one of the D3D backends nor using the OpenGL backend).

(The documentation says that SDL_RenderGeometry ignores SDL_SetTextureColorMod and SDL_SetTextureAlphaMod so I guess that is also true for SDL_SetTextureBlendMode despite it making a difference for the Metal backend.)

Maybe there is an easy fix for this so any comment is appreciated.

[DctrNoob]

For completeness, here's another problem when trying to implement this feature: the Vulkan backend distinguishes border colors into INTEGER and FLOAT types. These must then match the texture image format on which they are "applied". Also see discussion in #13401.