BufferReadbackRenderer

Remarks

The other half of the seam BufferUploadRenderer opens, and the one five other things were waiting on: a numeric gate that compares a shader against its C# port needs the shader's answer, an auto-exposure chain needs the histogram's, and a particle system that reaps on the device needs to know how many survived. None of them can get it from a device-local buffer — the host cannot map one — so the value comes out through a copy into host-readable memory, which is a pass, which is an edge the graph has to know about.

Declaring the read is what makes the answer mean anything. A copy recorded without it runs wherever it was written and reads whatever the buffer held before the dispatch — which is zeroes on a fresh allocation and last frame's contents on a recycled one. Both look like a plausible result. The declared read is what puts the barrier between the producer and the copy, and it is exactly the thing a hand-written readback gets wrong.

The pass has a side effect, because its whole point is outside the graph. Nothing in the frame reads what this writes, so culling would remove it — see SideEffect.

When the bytes are valid is Latency's question, and it is the only hard one here. There is no implicit wait anywhere in this — see Read for why an RHI that inserted one would be hiding a stall the caller cannot see.

Fields and properties (9)

• public required string Buffer

  The name of the buffer to read.

• public long Offset

  Where in that buffer to start reading.

• public long Size

  How many bytes, or zero for the rest of the buffer from Offset.

• public int Latency

  How many frames sit between the copy and the read.

• public ReadOnlySpan<byte> Data

  What came back, valid once ReadCount is above zero.

• public long Length

  How many bytes each read produces.

• public int ReadCount

  How many times bytes have come back.

• public int CopyCount

  How many frames have been copied.

• public Action<BufferReadbackRenderer>? OnRead

  What to do with the bytes, run by every Fetch that produces some.

Methods (4)

• public ReadOnlySpan<T> As<T>() where T : unmanaged

  What came back, as records.

• public bool Fetch()

  Reads the region copied Latency frames before the most recent one.

• protected internal override void Build(GraphicsCompositor compositor, CompositorFrame frame)

  Declares this node's render-graph passes.

• public void Dispose()

  Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.

Used by (3)

• BufferTransferDeviceTestsVixen.Graphics.Golden.Tests
• BufferTransferTestsVixen.Rendering.Tests
• CompositorBuilderVixen.Rendering