Clever Uses of OpenGL
Kurt Akeley
CS248 Lecture 16
15 November 2007
http://graphics.stanford.edu/courses/cs248-07/
Emphasis
Is on OpenGL mechanisms and their application

OpenGL is a power tool

It can be applied in clever and non-obvious ways
Is not full coverage of useful graphics algorithms

Many will not be covered

But what we do cover will be useful
CS248 Lecture 16
Kurt Akeley, Fall 2007
Reference
Advanced Graphics Programming Using OpenGL

Tom McReynolds (NVIDIA)

David Blythe (Microsoft, Direct3D 10 architect)
CS248 Lecture 16
Kurt Akeley, Fall 2007
Informal taxonomy of clever uses
Accumulation
 Z-buffer
 Transparent surfaces
 Multisample antialiased surfaces with pre-filtered lines
 Image composition
Texture
 Contour mapping
 Image warping
 Billboards
 Implementing pre-filter antialiasing with texture lookup
 Volume rendering
Polygon offset
 Coplanar primitives
 Hidden-line rendering
Stencil
 Capping
 Shadow volumes
GPGPU
CS248 Lecture 16
Kurt Akeley, Fall 2007
Invariance
On a single machine

Appendix A


Invariant enable/disable
Consistent input sequence

E.g., use glFrontFace to reverse facing direction, rather
than reordering the vertexes or reflecting by scaling
Cross-platform


Be careful!

OpenGL’s design emphasized cross-platform compatibility

But there are still many differences between platforms
Endian issues and support
CS248 Lecture 16
Kurt Akeley, Fall 2007
Accumulation
CS248 Lecture 16
Kurt Akeley, Fall 2007
Accumulation
Basic idea:

Build up a final image in the framebuffer by depth
buffering and/or blending multiple images
Examples

Z-buffer

Transparent surfaces

Multisample solids with pre-filtered antialiased lines

Image composition
CS248 Lecture 16
Kurt Akeley, Fall 2007
Z-buffer
glEnable(GL_DEPTH_TEST);
glDisable(GL_DEPTH_TEST);
glDepthFunc(GL_NEVER
| GL_LESS
| GL_EQUAL | GL_LEQUAL |
GL_GREATER | GL_NOTEQUAL | GL+GEQUAL | GL_ALWAYS);
glDepthFunc(GL_ALWAYS);
// invariant disable
glDepthMask(GL_TRUE);
glDepthMask(GL_FALSE);
if (Zfrag depthfunc
if (Rcolormask)
if (Gcolormask)
if (Bcolormask)
if (Acolormask)
if (depthmask)
}
CS248 Lecture 16
// enable writing
// disable writing
Zpixel) {
Rpixel 
Gpixel 
Bpixel 
Apixel 
Zpixel 
Rfrag;
Gfrag;
Bfrag;
Afrag;
Zfrag;
Kurt Akeley, Fall 2007
Transparent surfaces
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
draw opaque objects
glDepthMask(GL_FALSE);
// key OpenGL mode
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_CULL_FACE); // optional
glCullFace(GL_BACK);
draw transparent surfaces in any order
glDisable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glDepthMask(GL_TRUE);
glDisable(GL_BLEND);
glDisable(GL_CULL_FACE);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Multisample and pre-filter antialiasing
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
glEnable(GL_MULTISAMPLE);
draw solid objects (triangles)
glDepthMask(GL_FALSE);
glDisable(GL_MULTISAMPLE);
glEnable(GL_LINE_SMOOTH);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
glDisable(GL_LIGHTING);
// optional
draw pre-filter antialiased lines in any order
glDisable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glDepthMask(GL_TRUE);
glDisable(GL_LINE_SMOOTH);
glDisable(GL_BLEND);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Image composition (fade)
glEnable(GL_BLEND);
glBlendFunc(GL_CONSTANT_ALPHA, GL_ONE);
glBlendColor(0, 0, 0, first weight);
glDrawPixels(first image);
glBlendColor(0, 0, 0, second weight);
glDrawPixels(second image);
glDisable(GL_BLEND);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Image composition (over)
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ZERO);
glDrawPixels(first image);
gllendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDrawPixels(second image);
glDisable(GL_BLEND);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Texture
CS248 Lecture 16
Kurt Akeley, Fall 2007
Texture
Basic idea:

Use texture mapping mechanisms for creative purposes
Examples

Contour mapping

Image warping

Billboards

Implementing pre-filter antialiasing texture lookup

Volume rendering
CS248 Lecture 16
Kurt Akeley, Fall 2007
Contour mapping
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
glEnable(GL_TEXTURE_1D);
glEnable(GL_TEXTURE_GEN_S);
glTexGeni(GL_S, GL_TEXTURE_GEN_MODE, GL_EYE_LINEAR);
glTexGenfv(GL_S, GL_EYE_PLANE, vec4f(f, 0, 10, 0, 0));
draw objects without specifying texture coordinates
glDisable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glDisable(GL_TEXTURE_1D);
glDisable(GL_TEXTURE_GEN_S);
Today a vertex shader is a
more general TexGen mechanism.
But the notion of generated texture
coordinates remains important.
CS248 Lecture 16
Kurt Akeley, Fall 2007
Image warping
glEnable(GL_TEXTURE_2D);
for (y=0; y<(height-1); ++y) {
glBegin(GL_QUAD_STRIP);
for (x=0; x<width; ++x) {
glTexCoord2fv(tex[index(x,y)]);
glVertex2fv (vtx[index(x,y)]);
glTexCoord2fv(tex[index(x,y+1)]);
glVertex2fv (vtx[index(x,y+1)]);
}
glEnd();
}
CS248 Lecture 16
Kurt Akeley, Fall 2007
Billboards
Poster-child application of geometry shaders!
Application
Vertex assembly
Vertex operations
Primitive assembly
Primitive operations
Rasterization
Fragment operations
Advanced Graphics Programming Using OpenGL
Figure 13.4
Framebuffer
Display
CS248 Lecture 16
Kurt Akeley, Fall 2007
The magic of machine shops
Sewing machines make clothes
But machine tools make machine tools

And computers are this century’s machine tools
Hagley Machine Shop, Wilmington, DE
Photo by Incaz, Flickr
CS248 Lecture 16
Kurt Akeley, Fall 2007
Pre-filter antialiasing via texture lookup
Another ideal geometry-shader application
// draw pre-filtered point at (x,y)
const float h = 1.5;
// 3x3 filter
glEnable(GL_TEXTURE_2D);
glBegin(GL_QUADS);
glTexCoord2f(0, 0); glVertex2f(x-h, y-h);
glTexCoord2f(0, 1); glVertex2f(x-h, y+h);
glTexCoord2f(1, 1); glVertex2f(x+h, y+h);
glTexCoord2f(1, 0); glVertex2f(x+h, y-h);
glEnd();
Application
Vertex assembly
Vertex operations
Primitive assembly
Primitive operations
Rasterization
(0 1)
(1 1)
(x-h y+h)
(x+h y+h)
Fragment operations
Framebuffer
(0 0)
CS248 Lecture 16
(1 0)
(x-h y-h)
(x+h y-h)
Display
Kurt Akeley, Fall 2007
Volume rendering
Advanced Graphics Programming Using OpenGL
Figure 20.12
Advanced Graphics Programming Using OpenGL
Figure 20.13
CS248 Lecture 16
Kurt Akeley, Fall 2007
Polygon Offset
CS248 Lecture 16
Kurt Akeley, Fall 2007
Polygon offset
Basic idea:

Avoid depth fighting by biasing Z values
Examples

Coplanar primitives

Hidden lines

Silhouette edges
CS248 Lecture 16
Kurt Akeley, Fall 2007
Polygon mode
glPolygonMode(GLenum face, GLenum mode);
GL_FILL, GL_LINE, GL_POINT
Face culling happens
before conversion
to lines or points!
GL_FRONT, GL_BACK, GL_FRONT_AND_BACK
GL_FILL
CS248 Lecture 16
GL_LINE
GL_POINT
Kurt Akeley, Fall 2007
Polygon offset
Correspond to
polygon modes
glEnable/glDisable(GL_POLYGON_OFFSET_FILL |
GL_POLYGON_OFFSET_LINE |
GL_POLYGON_OFFSET_POINT);
glPolygonOffset(GLfloat factor, GLfloat units);
2
2
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Triangle
(on edge)
Line
(on edge)
Minimum resolvable zbuffer difference
-z
View position
CS248 Lecture 16
Kurt Akeley, Fall 2007
Coplanar primitives
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(maxwidth/2, 1);
draw planar surface
glDepthMask(GL_FALSE);
glDisable(GL_POLYGON_OFFSET_FILL);
draw points, lines, and polygons on the planar surface
glDisable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glDepthMask(GL_TRUE);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Hidden lines
glEnable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glColorMask(false, false, false, false);
glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(maxwidth/2, 1);
draw solid objects
glDepthMask(GL_FALSE);
glColorMask(true, true, true, true);
glColor3f(linecolor);
glDisable(GL_POLYGON_OFFSET_FILL);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
draw solid objects again
glDisable(GL_DEPTH_TEST);
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glDepthMask(GL_TRUE);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Silhouette lines (true hidden-line drawing)
glEnable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glColorMask(false, false, false, false);
glEnable(GL_POLYGON_OFFSET_FILL);
glPolygonOffset(maxwidth/2, 1);
draw solid objects
Additions to the hiddenline algorithm (previous
slide) highlighted in red
glDepthMask(GL_FALSE);
glColorMask(true, true, true, true);
glColor3f(1, 1, 1);
glDisable(GL_POLYGON_OFFSET_FILL);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
draw solid objects again
draw true edges
// for a complete hidden-line drawing
glDisable(GL_DEPTH_TEST);
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
glDepthMask(GL_TRUE);
glDisable(GL_CULL_FACE);
CS248 Lecture 16
Kurt Akeley, Fall 2007
Stencil
CS248 Lecture 16
Kurt Akeley, Fall 2007
Stencil
Basic idea:

Implement a simple state machine in every pixel
Examples

Capping

Shadow volumes
CS248 Lecture 16
Kurt Akeley, Fall 2007
Stencil
glEnable(GL_STENCIL_TEST);
glDisable(GL_STENCIL_TEST);
glStencilFunc(GLenum func, GLint ref, GLuint mask);
GL_NEVER, GL_LESS, GL_LEQUAL,
GL_GREATER, GL_GEQUAL,
GL_EQUAL, GL_NOTEQUAL,
GL_ALWAYS
glStencilOp(GLenum fail, GLenum zfail, GLenum zpass);
Bitmask, not
Boolean flag
glStencilMask(GLuint mask);
CS248 Lecture 16
GL_KEEP, GL_ZERO, GL_REPLACE
(with ref), GL_INCR, GL_DECR,
GL_INVERT
Kurt Akeley, Fall 2007
Z-buffer operation (again)
if (Zfrag depthfunc
if (Rcolormask)
if (Gcolormask)
if (Bcolormask)
if (Acolormask)
if (depthmask)
}
CS248 Lecture 16
Zpixel) {
Rpixel 
Gpixel 
Bpixel 
Apixel 
Zpixel 
Rfrag;
Gfrag;
Bfrag;
Afrag;
Zfrag;
Kurt Akeley, Fall 2007
Stencil operation
if ((ref & mask) stencilfunc (Spixel
if (Zfrag depthfunc Zpixel) {
if (Rcolormask) Rpixel 
if (Gcolormask) Gpixel 
if (Bcolormask) Bpixel 
if (Acolormask) Apixel 
if (depthmask) Zpixel 
StencilOp(zpass);
}
else {
StencilOp(zfail);
}
}
else {
StencilOp(fail);
}
CS248 Lecture 16
& mask)) {
Rfrag;
Gfrag;
Bfrag;
Afrag;
Zfrag;
Z-buffer
operation
Stencil implements a state
machine in each pixel .
(A programmable action occurs
in every cases)
Kurt Akeley, Fall 2007
Capping
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHTING);
for (int i=0; i<max; ++i) {
drawWithCap(model, i);
// remains enabled
…
drawWithCap(int model, int i) {
setMaterial(model, i);
glEnable(GL_CLIP_PLANE0);
glEnable(GL_STENCIL_TEST);
glEnable(GL_CULL_FACE);
glStencilFunc(GL_GEQUAL, 1, 3);
glCullFace(GL_BACK);
glStencilOp(GL_KEEP, GL_KEEP, GL_ZERO);
drawModel(model, i);
// don’t change capped pixels
// render frontfacing only
// clear stencil to 0
glCullFace(GL_FRONT);
// render backfacing only
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE); // set stencil to 1
drawModel(model, i);
glDisable(GL_CULL_FACE);
glDisable(GL_CLIP_PLANE0);
glStencilFunc(GL_EQUAL, 1, 3);
glStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
drawCap();
glDisable(GL_STENCIL_TEST);
CS248
Lecture 16
}
// draw only where stencil is 1
// set stencil to 2
Kurt Akeley, Fall 2007
Shadow volumes
Similar to capping:

Render the scene

Render shadow volumes


Don’t change color or depth

Use stencil to determine in/out
Reduce intensities of pixels in
shadow
Common game technique

E.g., Quake, Doom
Simple frustum culling fails!

Must keep light sources and
occluders that cast shadows on
geometry within the frustum
CS248 Lecture 16
Kurt Akeley, Fall 2007
GPGPU
CS248 Lecture 16
Kurt Akeley, Fall 2007
GPGPU
Basic idea:

General-purposes computing on GPUs

Take advantage of the huge compute power of modern
GPUs
CS248 Lecture 16
Kurt Akeley, Fall 2007
Multi-pass vector processing (2000)

Treat OpenGL as a very long
instruction word

Compute vector style

Apply inst. to all pixels

Build up final image in many
passes

Peercy, Olano, Airey, and
Ungar, Interactive Multi-Pass
Programmable Shading,
SIGGRAPH 2000

(Figure adapted from the
SIGGRAPH paper)
CS248 Lecture 16
#include “marble.h”
surface marble() {
varying color a;
uniform string fx;
uniform float x; x = ½;
fx = “noisebw.tx”;
FB = texture(tx, scale(x,x,x));
repeat(3) {
x = x * 0.5;
FB *= 0.5;
FB += texture(tx, scale(x,x,x));
}
FB = lookup(FB,tab);
a = FB;
FB = diffuse;
FB *= a;
FB += environment(“env”);
}
Kurt Akeley, Fall 2007
GPGPU
Still operates on images

Conceptually 2-D arrays of data elements
Deemphasizes VLIW thinking

Most pipeline stages are not used

What is used:

Rasterization (to generate and schedule data elements)

Fragment operations (specifically the programmable shader)

Texture lookup and filter (gather, not a stream processor)

Fragment/framebuffer operations (usually limited to write)
Emphasizes data-parallel programmability
Clever solutions have been developed for

Scatter

Reduction

Sorting …
CS248 Lecture 16
Kurt Akeley, Fall 2007
Modern GPGPU
Graphics APIs (OpenGL, Direct 3D) being replaced:

CUDA (NVIDIA)

CTM (AMD)
Great results being achieved:

Technical: 10x performance improvement in some cases

Business: multi-billion dollars anticipated soon
Coming soon:

IEEE double precision arithmetic

Greater exposure of hardware details (AMD)

Intel Larrabee

…
CS248 Lecture 16
Kurt Akeley, Fall 2007
Summary
Powerful OpenGL mechanisms (some introduced by IRIS/OpenGL):

8-way comparison and masks (depth, stencil, alpha, …)

Texture features:

3-D

TexGen and Texture coordinate matrix

Homogeneous coordinates

Application to all primitives (not just triangles)

glPolygonOffset

Stencil (state machine in a pixel)
Shaders have devalued some of these (e.g., TexGen) but most
remain valuable
It’s fun and productive to devise clever uses of OpenGL
CS248 Lecture 16
Kurt Akeley, Fall 2007
Assignments
No class next week
Next lecture: Color theory (Tuesday 27 November)
Reading assignment: FvD 13.2 through 13.6
CS248 Lecture 16
Kurt Akeley, Fall 2007
End
CS248 Lecture 16
Kurt Akeley, Fall 2007