CS 354 Computer Graphics; March 27, 2012 lecture on Project 2 and Compression; University of Texas at Austin

1. CS 354
Compression &
Project 2
Mark Kilgard
University of Texas
March 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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On a sheet of paper
Daily Quiz • Write your EID, name, and date
• Write #1, #2, #3, #4 followed by its answer
 Multiple choice: Which is not  Name the dark and less dark
a form of culling used in scene regions of a shadow generated
graph implementations by an area light source.
a) network video culling
b) view frustum culling  Multiple choice: When a scene
graph implementation has
c) occlusion culling profiling to visualize the depth
complexity of a scene, that
d) portal culling means:
a) Monitoring the maximum
 True or False: Zpass shadow stack depth to avoid overflows
volume rendering has
catastrophic artifacts when b) Counting and displaying how
shadow volumes are clipped by many times each pixel in the
the near clip plane. framebuffer is updated during a
frame
c) Underwater rendering

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Mid-term Feedback
 Look for answer key on web site
Percentage Grade Distribution
Mid-term feedback 16
Total questions: 43 14
Maximum possible: 83
12
Top grade: 75 10
Median: 59.0 8
Average: 58.0 6
Standard deviation: 9.63
4
2
0
40-49 50-59 60-69 70-79 80-89 90-100

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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 a
Torus 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 in
Computer 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 model
level-of-details
Edge collapse &
vertex split

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Amplification of Standard
Triangle Meshes
Tessellation
results in triangle
amplification
with curvature
Original faceted mesh
Results of Phong
shading 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 blocking artifacts
DXT1 Compressed Uncompressed

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GeForce Peak
Memory Bandwidth Trends
200
128-bit interface 256-bit interface
180
Raw
160 bandwidth
Gigabytes per second
140
Effective raw
bandwidth
120
with
compression
100
Expon.
(Effective raw
bandwidth
80
with
compression)
60
Expon. (Raw
bandwidth)
40
20
0
GeForce2 GeForce3 GeForce4 T i GeForce FX GeForce GeForce
GT S 4600 6800 Ultra 7800 GT X

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Effective GPU
Memory 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
1x 4x jittered 8x jittered
(aliased)
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?