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:
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.
To use framebuffer fetch in a render pass, use the following:
SetInputAttachment
API to set the output of the previous render pass as the input of the new render pass.LOAD_FRAMEBUFFER_X_INPUT
macro in your fragment shaderA program that runs on the GPU. More infoSAMPLE_TEXTURE2D
.The following steps assume you already have one render pass that writes to a TextureHandle
called sourceTextureHandle
.
Create a custom shader, then create a material from the shader. For more information about creating a custom shader, refer to Writing custom shaders.
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
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);
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;
}
In a new render graph render pass, add the material you created to your PassData
. For example:
class PassData
{
public Material frameBufferFetchMaterial;
}
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);
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);
}
For a full example, refer to the example called FrameBufferFetch in the render graph system URP package samples.