Get the current framebuffer via framebuffer fetch in URP

To speed up rendering, use framebuffer fetch to read the frame that Unity has rendered so far.

Using framebuffer fetch means the render pass can access the framebuffer from the on-chip memory of the GPU, instead of video memory. This speeds up rendering, because the GPU doesn’t need to copy the texture to and from its video memory.

Framebuffer fetch is supported if you use one of the following graphics APIs:

• Vulkan
• Metal

Framebuffer fetch speeds up rendering a lot on mobile devices that use tile-based deferred rendering (TBDR). The GPU keeps the frame in its on-chip tile memory between render passes, using less memory bandwidth and processing time.

If you use framebuffer fetch, URP merges the render passes that write to the framebuffer and read from it. You can check this in the Render Graph Viewer window.

Use framebuffer fetch

To use framebuffer fetch in a render pass, use the following:

• The SetInputAttachment API to set the output of the previous render pass as the input of the new render pass.
• The LOAD_FRAMEBUFFER_X_INPUT macro in your fragment shaderA program that runs on the GPU. More info
  See in Glossary code to sample the output of the previous render pass, instead of a macro such as SAMPLE_TEXTURE2D.

The following steps assume you already have one render pass that writes to a TextureHandle called sourceTextureHandle.

1. Create a custom shader, then create a material from the shader. For more information about creating a custom shader, refer to Writing custom shaders.

2. Inside the HLSLPROGRAM of your custom shader, make sure you import the Packages/com.unity.render-pipelines.core/Runtime/Utilities/Blit.hlsl file:

   HLSLPROGRAM
   ...
   #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
   ...
   ENDHLSL
   

3. Inside the HLSLPROGRAM, use one of the following to declare the texture from the previous render pass. For example:

   • FRAMEBUFFER_INPUT_X_HALF
   • FRAMEBUFFER_INPUT_X_FLOAT
   • FRAMEBUFFER_INPUT_X_INT
   • FRAMEBUFFER_INPUT_X_UINT

   For example:

   FRAMEBUFFER_INPUT_X_HALF(0);
   

4. In the fragment function, use LOAD_FRAMEBUFFER_X_INPUT to sample the texture using framebuffer fetch. For example:

   half4 frag() : SV_Target
   {
       half4 colorFromPreviousRenderPass = LOAD_FRAMEBUFFER_X_INPUT(0, input.positionCS.xy);
       return colorFromPreviousRenderPass;
   }
   

5. In a new render graph render pass, add the material you created to your PassData. For example:

   class PassData
   {
       public Material frameBufferFetchMaterial;
   }
   

6. Use builder.SetInputAttachment to set the output of the previous render pass as the input of the new render pass. For example:

   builder.SetInputAttachment(sourceTextureHandle, 0, AccessFlags.Read);
   

7. In your SetRenderFunc method, use a command such as BlitTexture to render using the material. For example:

   static void ExecutePass(PassData data, RasterGraphContext context)
   {
       Blitter.BlitTexture(context.cmd, new Vector4(1, 1, 0, 0), frameBufferFetchMaterial, 1);
   }
   

Example

For a full example, refer to the example called FrameBufferFetch in the render graph system URP package samples.

Additional resources

• Writing custom shaders
• Write a render pass using the render graph system
• Transfer a texture between render passes