96 lines
3.7 KiB
Markdown
96 lines
3.7 KiB
Markdown
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---
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uid: urp-render-graph-framebuffer-fetch
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---
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# Get the current framebuffer with framebuffer fetch
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To speed up rendering, use framebuffer fetch to read the frame that Unity has rendered so far.
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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.
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Framebuffer fetch is supported if you use one of the following graphics APIs:
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- Vulkan
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- Metal
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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.
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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](render-graph-viewer-reference.md).
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## Use framebuffer fetch
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To use framebuffer fetch in a render pass, use the following:
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- The `SetInputAttachment` API to set the output of the previous render pass as the input of the new render pass.
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- The `LOAD_FRAMEBUFFER_X_INPUT` macro in your fragment shader code to sample the output of the previous render pass, instead of a macro such as `SAMPLE_TEXTURE2D`.
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The following steps assume you already have one render pass that writes to a `TextureHandle` called `sourceTextureHandle`.
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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](writing-custom-shaders-urp.md).
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2. Inside the HLSLPROGRAM of your custom shader, make sure you import the `Packages/com.unity.render-pipelines.core/Runtime/Utilities/Blit.hlsl` file:
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```hlsl
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HLSLPROGRAM
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...
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#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
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...
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ENDHLSL
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```
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3. Inside the HLSLPROGRAM, use one of the following to declare the texture from the previous render pass. For example:
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- `FRAMEBUFFER_INPUT_X_HALF`
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- `FRAMEBUFFER_INPUT_X_FLOAT`
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- `FRAMEBUFFER_INPUT_X_INT`
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- `FRAMEBUFFER_INPUT_X_UINT`
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For example:
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```hlsl
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FRAMEBUFFER_INPUT_X_HALF(0);
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```
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4. In the fragment function, use `LOAD_FRAMEBUFFER_X_INPUT` to sample the texture using framebuffer fetch. For example:
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```hlsl
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half4 frag() : SV_Target
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{
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half4 colorFromPreviousRenderPass = LOAD_FRAMEBUFFER_X_INPUT(0, input.positionCS.xy);
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return colorFromPreviousRenderPass;
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}
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```
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5. In a new render graph render pass, add the material you created to your `PassData`. For example:
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```csharp
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class PassData
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{
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public Material frameBufferFetchMaterial;
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}
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```
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6. Use `builder.SetInputAttachment` to set the output of the previous render pass as the input of the new render pass. For example:
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```csharp
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builder.SetInputAttachment(sourceTextureHandle, 0, AccessFlags.Read);
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```
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7. In your `SetRenderFunc` method, use a command such as `BlitTexture` to render using the material. For example:
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```csharp
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static void ExecutePass(PassData data, RasterGraphContext context)
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{
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Blitter.BlitTexture(context.cmd, new Vector4(1, 1, 0, 0), frameBufferFetchMaterial, 1);
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}
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```
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## Example
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For a full example, refer to the example called **FrameBufferFetch** in the [render graph system URP package samples](package-samples.md).
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## Additional resources
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- [Writing custom shaders](writing-custom-shaders-urp.md)
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- [Write a render pass using the render graph system](render-graph-write-render-pass.md)
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- [Transfer a texture between render passes](render-graph-pass-textures-between-passes.md)
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