[Feature request] Generate texture mipmap

[rom1v]

I would like a feature to enable mipmap generation on a given SDL_Texture.

For now, if the renderer is opengl >= 3 or opengl es >= 3, then I use:

SDL_Texture *texture = ...;
SDL_GL_BindTexture(texture, NULL, NULL); 
glGenerateMipmap(GL_TEXTURE_2D);

It works very well. However, I have no solution for other renderers (direct3d, metal, etc.).

Do you think such a feature could be added to SDL directly?

Or at least, is it possible to generate mipmaps by calling the native APIs (with some ifdefs) for the other renderers, similar to glGenerateMipmap() for OpenGL? (but I want to keep using the SDL API like SDL_Texture for the remaining code)

Ref: Genymobile/scrcpy#40 (comment)

[bibendovsky]

IMO The 3D mipmapping has no sense in SDL 2D renderer.

[rom1v]

It makes sense if you downscale (especially by a factor of more than 2). See Genymobile/scrcpy#40 (comment).

[cjxgm]

@bibendovsky
Texture prefiltering is a useful technique to reduce aliasing when textures are scaled down. (Linear filtering has its effect only when scaling up).
Mipmapping and anisotropic filtering are just two hardware provided mechanisms of texture prefiltering for uniform (keeping aspect-ratio) and non-uniform (not keeping aspect-ratio) downscaling, respectively.

[bibendovsky]

@cjxgm Thanks for explanation.
Just never used such technique for 2D.

[bibendovsky]

@rom1v According to this commit you not only need to generate mipmaps but also to modify texture properties - minification filter and LOD bias.

[rom1v]

@bibendovsky Hmm, yes, that's true.

[JianYuhaha]

I have a problem.
When I run the scrcpy, it reminds me that Trilinear filtering disabled (not an OpenGL renderer).
I don't know whether it's a mobile phone problem or a computer problem.

[dingogames]

I saw that SDL now includes SDL_RenderGeometry - this makes SDL a lot more useful for making many types of 2d games. However without mipmapping the use cases are much more limited - you can't really scale down graphics because they turn into an aliased mess. So that's something to consider - mipmapping goes hand in hand with the new SDL_RenderGeometry functionality.

[slouken]

@icculus, does this make sense for SDL3?

[flibitijibibo]

Running through SDL_Render issues tagged for 3.0 ABI, sorry if you get multiple e-mails for this...

FWIW the GPU API at #9312 includes support for generating mipmaps and is supported on all backends, so it could also be exposed to SDL_Render if needed.

[icculus]

I don't think we're going to add this to the 2D API.

[danpla]

@icculus Why not?

[icculus]

Because it's a 2D API.

For special cases that can benefit from it, like scrcpy using it to do downscaling on the hardware, they can force a specific renderer and/or write some custom code to interact with the SDL backend.

[danpla]

As far as I understand, the special case is actually not using mipmapping, e.g. when you have a guarantee that images will not be scaled down. Because if they do, billinear filtering will only help with small downscaling factors, and larger factors will still give you aliasing.

The aliasing problem is not specific to "real" 3D rendering, so it seems odd that SDL has bilinear filtering while mipmapping is considered out of scope, as if it were something unrelated.

[dingogames]

I also have to argue the case for this. I’ve used quite a few different 2D APIs over the past 20 years or so, and they’ve pretty much all supported mipmapping. It's just such a broadly useful feature! The idea that mipmapping is mostly a 3D feature doesn’t square with my experience.

One super common use case is any game with the idea of “zooming”... and that’s a LOT of games. Maybe MOST games? If you size your assets exactly right for the current display resolution and only do a small amount of zooming, you might be fine. But if your assets are sized for, say, 4K and you’re running at 1080p, ANY amount of zooming will look horrible. Think of how many 2D games have a camera that can zoom out:

• Zooming out for a more tactical view/map
• Zooming out to frame the scene/level differently
• Split-screen views
• Stretchy co-op camera zoom to show both players
• Slight camera zoom animation common in many games

Really, there are endless reasons to zoom out.

Even if you just have a simple application with a graphical UI. If you don't size the UI art assets correctly you're going to get aliasing artifacts when you run at a low resolution.

You can fix SOME of these issues by optimizing and managing assets for different screen resolutions. However, that can be a lot of work and it doesn’t even solve all the issues.

This isn’t a niche use case. If you’re making a production-ready 2D game that supports different resolutions, you want mipmapping.

[icculus]

Don't have the strength to win fights this week, so what the heck, here's a first shot at this in #10254.

I don't know all the ramifications and corner cases, we should probably have something in the docs saying "don't use this unless you know what you're doing, as it could have surprising performance results for no difference in quality if you just slap it around your code."

Metal will need the same code that texture updating requires, where it ends any current command encoder and switching to a Blit encoder. The higher level flushes the SDL renderer command queue if the texture is used in it, like it does for render target switching, etc, so the state is correct between previous draws and future ones if the texture is going to change at this point. It's smart enough to not flush the queue if the texture isn't pending use, though, same as other actions.

Vulkan and later D3D revisions offer something for this...D3D9 (I recall maybe incorrectly) only offered this in the D3DX utility library that we don't use, and there it was probably just manually uploading mips from the CPU. We could roll that ourselves, but I'm inclined to leave it unsupported on D3D9.

[icculus]

Anyhow, please do try this out, it's completely untested and I want feedback on whether this is what you want/need. It should work on the OpenGL and OpenGLES2 backends in that PR.

[slouken]

Has anyone tested this? Does anyone have an SDL rendering test case that shows how this improves things?

[icculus]

The PR needs work still.

[icculus]

In the interest of time, I'm bumping this out of the 3.0 ABI milestone.

The idea is the PR needs to change to specify mipmapping as a texture creation property, as opposed to a function that generates mipmaps on demand...which is to say we don't need to change the API to add it later.

(EDIT: if we decide to do this.)

[slouken]

We are scoping work for the SDL 3.2.0 release, so please let us know if this is a showstopper for you.

[dingogames]

Speaking for myself, it is a showstopper before I can migrate to SDL3. I'm currently using SDL2 with mipmapping and texture wrap/repeat hacked into it. All good for now, since this is for my production games so I want to be on the most stable version of SDL. Just hoping at some point I'll be able to migrate all my stuff to SDL3, and keep my games working for many years to come.

As for test cases - the type of graphic used will be important. A low contrast photograph with smooth transitions won't show big differences until it's scaled down a lot. Noisy images, or images with quick hard transitions will show the difference much much faster. Test patterns like black/white vertical or horizontal lines, or checkerboard pattern will look horrible without mipmapping as the sampler picks different sets of pixels and the whole image shimmers. (I was going to try to set up some samples, but I don't think the mipmap generation was working in this PR)