Depth Test & Z-Buffer

Multiple triangles cover the same pixel. Which one to draw? The one CLOSEST to the camera.

Z-buffer (depth buffer): per-pixel depth value.

Algorithm:

1. Initialize Z-buffer to ∞ (or 1 in NDC).
2. For each triangle, for each covered pixel:
   • Compute interpolated depth.
   • Compare to Z-buffer.
   • If closer: update both Z-buffer and pixel.

Memory:

• Z-buffer: same dimensions as frame buffer.
• Per pixel: 16-32 bit depth value.
• 1920x1080x24 bits = ~6 MiB.

Convention: usually z=0 near plane, z=1 far plane. Smaller z = closer.

Hidden surface removal:

• Z-buffer is "image-space" approach: per-pixel.
• "Object-space" alternatives exist (BSP trees) but are more complex.
• Z-buffer is simple + parallelizable.

Z-fighting:

• Two surfaces at exactly same depth → flickering.
• Numerical precision issue.
• Solutions:
  • Higher precision Z-buffer.
  • Polygon offset (slight push).
  • Sort opaque vs transparent properly.

Reverse Z:

• Modern: use 1/z for better precision distribution.
• Allows huge view distances without z-fighting.

Early Z:

• GPU optimization: reject pixels by depth before running fragment shader.
• 2-3x speedup for occluded geometry.

Depth pre-pass:

• Render scene with only depth (no color).
• Then render again with color, using early Z.
• Skip fragment shader for occluded pixels.
• AAA optimization.

Occlusion culling:

• Higher level: don't even submit hidden objects to GPU.
• Frustum culling: reject objects outside view.
• Hierarchical Z-buffer (HiZ).
• Used in modern engines.

Test: