TechMetal 2 on A11 — Imageblock Sample Coverage Control

Tech Talks

Multisampling Antialiasing

MSAA on A-Series GPUs

• Edge tracking built into hardware for fast blending
• Efficient memory usage with Memoryless Render Targets
• Optimize memory bandwidth with resolve store action
• Programmable Sample Positions introduced in Metal 2

Enhanced MSAA on A11

• Faster MSAA hardware
  - A11 GPU tracks unique sample data for even faster blending
• Imageblock Sample Coverage Control
  - Access sample coverage tracking data
  - Resolve at any time in your render pass
  - Implement custom resolve with tile shading

Edge Tracking

• A-series GPUs track primitive edges
  - Pixels without edges blend per pixel
  - Pixels that contain edges blend each sample
• A11 GPU also tracks unique samples
  - Pixels that contain edges blend unique samples

Enhanced MSAA on A11

Optimized multisampled blending

• GPU tracks unique colors per pixel
• Primitives can add or merge colors
• Only blend unique values
  - Includes programmable blending

Color Tracking Example

• Initial Value
  - All samples are initialized to a clear color
  - 1 color represents all 4 samples
• New unique color introduced
  - Primitive edge can introduce a new color
  - Covered samples are reassigned to the new color
• Blend each unique color
  - Blend for each covered color
  - New unique colors created as needed
• Merging colors
  - Color count can grow and shrink
  - Opaque geometry can merge colors

Imageblock Sample Coverage Control

• Implement new algorithms and optimizations

• Resolve in the middle of a render pass
  - Resolve sampels before blending
  - Ensure only 1 unique color to blend per pixel

Sample Code — Surface Aggregation

• Merge surfaces heuristically

• Do more in a single render pass
  - Render G-buffer with MSAA
  - Reduce unique G-buffer samples with tile pipeline
  - Light and shade aggregate G-buffer samples