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Get the current framebuffer from GPU memory in URP

To speed up rendering, use the SetInputAttachment API to read the frame that Unity has rendered so far.
Read time 3 minutesLast updated 21 days ago
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 shader code to read the pixel from the previous render pass, instead of a sampling 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