Z-buffer Optimizations Patrick Cozzi

Z-Buffer Optimizations

• Patrick Cozzi

• Analytical Graphics, Inc.

Overview

• Z-Buffer Review

• Hardware: Early-Z

• Software: Front-to-Back Sorting

• Hardware: Double-Speed Z-Only

• Software: Early-Z Pass

• Software: Deferred Shading

• Hardware: Buffer Compression

• Hardware: Fast Clear

• Hardware: Z-Cull

• Future: Programmable Culling Unit

Z-Buffer Review

• Also called Depth Buffer

• Fragment vs Pixel

• Alternatives: Painter’s, Ray Casting, etc

Z-Buffer History

• “Brute-force approach”

• “Ridiculously expensive”

• Sutherland, Sproull, and, Schumacker, “A Characterization of Ten Hidden-Surface Algorithms”, 1974

Z-Buffer Quiz

• 10 triangles cover a pixel. Rendering these in random order with a Z-buffer, what is the average number of times the pixel’s z-value is written?

Z-Buffer Quiz

• 1st triangle writes depth

• 2nd triangle has 1/2 chance of writing depth

• 3rd triangle has 1/3 chance of writing depth

• 1 + 1/2 + 1/3 + …+ 1/10 = 2.9289…

Z-Buffer Quiz

• Harmonic Series

Z-Test in the Pipeline

• When is the Z-Test?

Early-Z

Early-Z

Front-to-Back Sorting

• Utilize Early-Z for opaque objects

• Old hardware still has less z-buffer writes

• CPU overhead. Need efficient sorting

• Bucket Sort
• Octtree

• Conflicts with state sorting

Double Speed Z-Only

• GeForce FX and later render at double speed when writing only depth or stencil

• Enabled when

• Color writes are disabled
• Fragment shader discards or write depth
• Alpha-test is disabled

Early-Z Pass

• Software technique to utilize Early-Z and Double Speed Z-Only

• Two passes

• Render depth only. “Lay down depth” – Double Speed Z-Only
• Render with full shaders – Early-Z (and Z-Cull)

Deferred Shading

• Similar to Early-Z Pass

• 1st Pass: Visibility tests
• 2nd Pass: Shading

• Different than Early-Z Pass

• Geometry is only transformed once

Deferred Shading

• 1st Pass

• Render geometry into G-Buffers:

Deferred Shading

• 2nd Pass

• Shading == post processing effects
• Render full screen quads that read from G-Buffers
• Objects are no longer needed

Deferred Shading

• Light Accumulation Result

Deferred Shading

• Eliminates shading fragments that fail Z-Test

• Increases video memory requirement

• How does it affect bandwidth?

Buffer Compression

• Reduce depth buffer bandwidth

• Generally does not reduce memory usage of actual depth buffer

• Same architecture applies to other buffers, e.g. color and stencil

Buffer Compression

• Tile Table: Status for nxn tile of depths, e.g. n=8

• [state, zmin, zmax]
• state is either compressed, uncompressed, or cleared

Buffer Compression

Buffer Compression

• Depth Buffer Write

• Rasterizer modifies copy of uncompressed tile
• Tile is lossless compressed (if possible) and sent to actual depth buffer
• Update Tile Table
• zmin and zmax
• status: compressed or decompressed

Buffer Compression

• Depth Buffer Read

• Tile Status
• Uncompressed: Send tile
• Decompress: Decompress and send tile
• Cleared: See Fast Clear

Fast Clear

• Don’t touch depth buffer

• glClear sets state of each tile to cleared

• When the rasterizer reads a cleared buffer

• A tile filled with GL_DEPTH_CLEAR_VALUE is sent
• Depth buffer is not accessed

Fast Clear

• Use glClear

• Not full screen quads
• No "one frame positive, one frame negative“ trick

• Clear stencil together with depth

Z-Cull

• Cull blocks of fragments before shading

• Coarse-grained as opposed to Early-Z

Z-Cull

• Zmax-Culling

• Rasterizer fetches zmax for each tile it processes
• Compute ztrianglemin for a triangle
• Culled if ztrianglemin > zmax

Z-Cull

• Zmin-Culling

• Support different depth tests
• Avoid depth buffer reads
• If triangle is in front of tile, depth tests for each pixel is unnecessary

Z-Cull

• Automatically enabled on GeForce (6?) cards unless

• glClear isn’t used
• Fragment shader writes depth (or discards?)
• Direction of depth test is changed

• ATI recommends avoiding = and != depth compares and stencil fail and stencil depth fail operations

• Less efficient when depth varies a lot within a few pixels

Programmable Culling Unit

• Cull before fragment shader even if the shader writes depth or discards

• Run part of shader over an entire tile to determine lower bound z value

• Hasselgren and Akenine-Möller, “PCU: The Programmable Culling Unit,” 2007

Summary

• What was once “ridiculously expensive” is now the primary visible surface algorithm for rasterization

Resources

• www.realtimerendering.com

Resources

• developer.nvidia.com/object/gpu_programming_guide.html

Resources

• http://www.graphicshardware.org/previous/www_2000/presentations/ATIHot3D.pdf

Resources

• http://ati.amd.com/developer/dx9/ATI-DX9_Optimization.pdf

Resources

• developer.nvidia.com/object/gpu_gems_home.html

Resources

• developer.nvidia.com/object/gpu-gems-3.html