batch-render-group

Reference

• BatchRendererGroup sample: Achieve high frame rate even on budget devices

• Unity-Technologies/brg-shooter: Unity blog-post sample showing BatchRendererGroup and Burst/JobSystem. Focus is high performance even on budget mobile devices. Unity 2022.3.5 or above required

Concept

BatchRendererGroup (or BRG) is an API that efficiently generates draw commands from C# and produces GPU-instancing draw calls.

相对于 DrawMeshInstanced 好处

• DrawMeshInstanced 需要托管内存, 传入 TRS 数组

• DrawMeshInstanced 需要自定义着色器

• DrawMeshInstanced 每次绘制需要上传 GPU

BRG shader data model

• the standard constant buffer (UnityPerMaterial)

• custom, large GPU buffer (BRG raw buffer)

  • Shader Storage Buffer Object (SSBO, or byte address buffer)

  • The default BRG data layout is the structure of arrays (SoA) type

Properties per instance – or not

BRG metadata

• Bits 0–30 define the offset of the property within the BRG raw buffer,

• and bit 31 tells whether the property value is the same for all instances or the offset is the beginning of an array, with one value per instance.

两种 Buffer

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BRG culling and visibility indices

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BRG matrix format and buffer

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SoA layout

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Animating floor cells

• allocate a similar buffer in system memory (CPU can process data at full speed in system memory).

• Let’s call this second buffer a “shadow copy” of the GPU memory

• When animation is done, we upload the shadow copy buffer to the GPU using the GraphicsBuffer.SetData API.

defined as follows

1. “unity_ObjectToWorld” property is an array starting at offset 0 of the BRG raw buffer

2. “unity_WorldToObject” property is an array starting at offset 153,600

3. “_BaseColor” property is an array, starting at offset 307,200

The devil’s in the details: GLES exception

most GLES 3.0 devices can’t access SSBO during the vertex stage (i.e., the GL_MAX_VERTEX_SHADER_STORAGE_BLOCKS value is 0).

在 OpenGL 上用 UBO 替代 SSBO

A constant buffer can be any size, but only a small part of it (a window) is visible at any given time when the shader is running. The window size depends on the hardware and driver, but a widely accepted value is 16 KiB.

BatchRendererGroup.GetConstantBufferMaxWindowSize() API to get the correct BRG window size.

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Uploading data

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Main BRG user callback

1. To generate all draw commands into the output BatchCullingOut struct

2. To use (or not) information provided in the BatchCullingContext read-only struct within your own culling code

Draw commands generation

A BRG draw command is almost a GPU DrawInstanced call.