CS 354Compression &Project 2

Mark KilgardUniversity of TexasMarch 27, 2012

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Today’s material

In-class quiz On shadows + scene graph lecture

Lecture topic Project 2 Compression for Computer Graphics

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My Office Hours

Tuesday, before class Painter (PAI) 5.35 8:45 a.m. to 9:15

Thursday, after class ACE 6.302 11:00 a.m. to 12

Randy’s office hours Monday & Wednesday 11 a.m. to 12:00 Painter (PAI) 5.33

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Last time, this time

Last lecture, we discussed Shadow generation algorithms Scene graph

Data structures to organize the rendering of 3D scenes

This lecture Project 2 on Programmable Shaders Compression for Computer Graphics

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Daily Quiz

1. Multiple choice: Which is not a form of culling used in scene graph implementations

a) network video culling

b) view frustum culling

c) occlusion culling

d) portal culling

2. True or False: Zpass shadow volume rendering has catastrophic artifacts when shadow volumes are clipped by the near clip plane.

3. Name the dark and less dark regions of a shadow generated by an area light source.

4. Multiple choice: When a scene graph implementation has profiling to visualize the depth complexity of a scene, that means:

a) Monitoring the maximum stack depth to avoid overflows

b) Counting and displaying how many times each pixel in the framebuffer is updated during a frame

c) Underwater rendering

On a sheet of paper• Write your EID, name, and date• Write #1, #2, #3, #4 followed by its answer

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Mid-term Feedback

Look for answer key on web site

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Mid-term feedbackTotal questions: 43Maximum possible: 83

Percentage Grade Distribution

Top grade: 75

Median: 59.0Average: 58.0Standard deviation: 9.63

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Project 2

Look for assignment on the web site PDF + code to use

Requirements Mostly writing GLSL shaders Small amount of C/C++ code

Convert height map to normal map

Intermediate milestone April 2nd

Submit snapshots and GLSL code for first two shaders

Complete project Due April 6th

All 8 shaders Embellish for additional credit

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What project code renders

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Your job

Write torus shader Wrap a flat patch into a torus Compute lighting vectors

tangent, bi-normal, normal light vector, eye vector, half-angle vector

Write fragment shaders to implement various lighting effects ambient, diffuse, specular, reflection, decal

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Procedurally Generating aTorus from a 2D Grid

2D grid over (s,t)[0,1]

Tessellated torus

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Bumpy Diffuse

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Bumpy reflection combo

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Compression inComputer Graphics

Why? Lots of data Bandwidth intensive

Strategies to deal with data Compact representations Mesh compression Image compression Framebufffer compression

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Numerics

Poor Man’s Compression… Fixed-point

4-bit, 8-bit, 16-bit, etc. Floating-point

Single precision, 32-bit s23e8 Half-precision, 16-bit s10e5

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Image Compression

Lossless compression Entropy encoding Run-length encoding

Good when images are repetitive in X direction

Lossy compression Palette schemes

Vector quantization approaches Discrete Cosine Transform schemes

JPEG Wavelet schemes

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JPEG Compression

Compression and Decompression pipeline

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Color Space

YUV, YIQ Maintain chrominance data at lower precision

and frequency than luminance data Recall this from color space discussion TV and digital video standards rely on this

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Chroma Sampling

Store chrominance data at lower precision and frequency than luminance data

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Geometry Compression

Along with images, geometric models are also very data-intensive

Strategies Avoid vertex redundancies Scalar quantization Compact vertex attribute choices Mesh simplification

Whether automatic or manual Progressive representations Higher-level primitives

Saying a “sphere of radius 5 at (x,y,z)” is much more compact (and exact) than its tessellation

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Mesh Simplification

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Progressive Meshes

Incrementally transition between mesh Levels-of-Detail (LOD) Avoids “popping” artifacts

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Progressive Mesh Geomorph

Interpolate smoothly between modellevel-of-details

Edge collapse &vertex split

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Amplification of StandardTriangle Meshes

Original faceted mesh

Tessellationresults in triangleamplificationwith curvature

Results of Phongshading of curved

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Normal Compression

Floating-point for normalized normals is incredibly wasteful Half-precision values 10-10-10 signed fixed point Palette of normals

[Deering 1995]

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Texture Compression

Conventional image compression For storage and transmission

Texture mapping Requires random access For storage and reduced bandwidth So tends to use “block” schemes

Often 4:1 compression relying on 4x4 tiles

Assumes lossy compression

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DXT1 Compression 4:1 compression

DXT1 Compressed Uncompressed

blocking artifacts

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GeForce2GTS

GeForce3 GeForce4 Ti4600

GeForce FX GeForce6800 Ultra

GeForce7800 GTX

Rawbandwidth

Effective rawbandwidthwithcompression

Expon.(Effective rawbandwidthwithcompression)Expon. (Rawbandwidth)

GeForce PeakMemory Bandwidth Trends

128-bit interface 256-bit interface

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Effective GPUMemory Bandwidth

Compression schemes Lossless depth and color (when multisampling)

compression Lossy texture compression (S3TC / DXTC) Typically assumes 4:1 compression

Avoidance useless work Early killing of fragments (Z cull) Avoiding useless blending and texture fetches

Very clever memory controller designs Combining memory accesses for improved coherency Caches for texture fetches

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Multi-sample Coverage Positions

4x jittered1x

(aliased)

8x jittered

4x orthogonal

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Depth Values Compress Well

Linear values Store plane equations instead of point samples

Smooth transitions, discrete edges

Rendered view Depth buffer view

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Next Class

Project 2 today (!?) On texturing, shading, & lighting Study GLSL Two weeks to complete

Next lecture Interaction How do users interact with 3D scenes?