SDL_GPU Backend

[DeltaW0x]

New PR because I irreparably messed up the commit history on the old one.
Now that SDL3 has reached ABI stability, I've added a SDL_Gpu backend to ImGui. The only notable difference from other backends lies in the fact that SDL_Gpu doesn't allow copy passes to occur during a renderpass.
There are two solutions to this problem (that I thought of):

• Take control of the entire RenderPass for ImGui, which could (and will) limit what the user can do with the rendered texture (like rendering to a different RenderTarget).
• Require the user to call a new function Imgui_ImplSDLGPU_PrepareDrawData before starting a user-created RenderPass, which uploads all the vertex/index buffer data.

I chose the second option because I thought it was the more useful one, but the code can be rewritten easily to support the former if deemed necessary. I've included shaders for SPIRV, DXCB and METALLIB for MacOS, I'll be able to compile the remaining ones for iOS and tvOS as soon as I'm able to install the SDKs for those.

I also wrote an example for the Visual Studio solution and added a Make file for other platforms

[DeltaW0x]

I think I've added everything, now the Makefile works on both Linux and MacOS. Let me know if there's anything else to fix or add or test

[povik]

I might be doing something wrong but when integrating this I had to change the target_info on the pipeline to match my other pipelines in order to have depth and stencil test configuration from the imgui pipeline properly applied. This is on Metal.

Without the patch below the imgui content rendered behind, not in front of the scene:

diff --git a/backends/imgui_impl_sdlgpu3.cpp b/backends/imgui_impl_sdlgpu3.cpp
index cfcf12d4..e2a88b4d 100644
--- a/backends/imgui_impl_sdlgpu3.cpp
+++ b/backends/imgui_impl_sdlgpu3.cpp
@@ -505,7 +505,8 @@ static void ImGui_ImplSDLGPU_CreateGraphicsPipeline()
     SDL_GPUGraphicsPipelineTargetInfo target_info = {};
     target_info.num_color_targets = 1;
     target_info.color_target_descriptions = color_target_desc;
-    target_info.has_depth_stencil_target = false;
+    target_info.depth_stencil_format = SDL_GPU_TEXTUREFORMAT_D16_UNORM;
+    target_info.has_depth_stencil_target = true;
     
     SDL_GPUGraphicsPipelineCreateInfo pipeline_info = {};
     pipeline_info.vertex_shader = bd->VertexShader;

[DeltaW0x]

If you want to apply it on top of other rendered geometry in an existing renderpass then yes, you would need to use a depth buffer, but I believe that usually you would render ImGui as the last renderpass of your application, so it gets drawn on top of everything else without problems or a zbuffer. I can add a custom pipeline input though, so you can pass what you need without problems

[povik]

Sounds like I need to separate out a render pass for imgui, in which case the imgui backend will work as-is. Thanks! (My knowledge of modern rendering APIs is limited.)

[DeltaW0x]

I've added a custom pipeline input in ImGui_ImplSDLGPU_RenderDrawData, you can try that or just do the separate renderpass. Having a separate pass is useful when you want to disable rending ImGui at all too, like when shipping for a game or something similar

[povik]

I've made it work with a separate render pass.

[ocornut]

Good job, thanks!
I'm presently trying to focus on other features but I hope to get back to this soon-ish.

[DeltaW0x]

Please take your time, I'll keep working on it to improve some things anyways