Get the current framebuffer from GPU memory in URP

To more efficiently read a frame that Unity rendered in a previous pass, use the SetInputAttachment API to set the output of a render pass as the input of the next render pass.

Using SetInputAttachment 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. This process is sometimes called framebuffer fetch.

On mobile devices, rendering speed improves 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 the SetInputAttachment API, URP merges the render passes that write to the framebuffer and read from it. You can check this in the Render Graph Viewer window.

You can also use SetInputAttachment to read other render targets, for example a G-buffer.

Requirements

The SetInputAttachment API is supported if you use one of the following graphics APIs:

• Vulkan
• Metal

If you use SetInputAttachment with an unsupported API, Unity copies the texture to and from video memory, so there’s no effect on rendering speed.

Use the SetInputAttachment API

To use the SetInputAttachment API 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 from on-chip GPU memory. 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 exampleMaterial;
   }
   

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), exampleMaterial, 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