1. Discrete Techniques
Chapter 8

2. Chapter 8 -- Discrete Techniques
Introduction:
Texture mapping, antialiasing, compositing, and alpha blending are only a few of the techniques that become possible when the API allows us to work with discrete buffers.
This chapter introduces these techniques, focusing on those that are supported by OpenGL and by similar APIs.
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Introduction: (cont)
We start by looking at the frame buffer in more detail and how we can read and write to it.
We learn to work with arrays of pixels that form digital images.
We then consider mapping methods.
We will then look at some of the other buffers that are supported by the OpenGL API
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1. Buffers
We have already used two types of buffers: Color buffers and Depth buffers. Later we will introduce others.
What all buffers have in common is that they are inherently discrete.
They have limited resolution, both spatially and in depth.
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Define a buffer by its spatial resolution (n x m) and its depth k, the number of bits/pixel
pixel
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OpenGL defines the frame buffer as consisting of a variety of buffers
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OpenGL buffers
Color buffers can be displayed
Front
Back
Auxiliary
Overlay
Depth
Accumulation
High resolution buffer
Stencil
Holds masks
The depth of these buffers combined can exceed a few hundred bits.
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2. Digital Images
Before we look at how the graphics system can work with digital images through pixel and bit operations, let’s examine what we mean by a digital image.
Within our programs, we generally work with images that are arrays of pixels.
These images can be of a variety of sizes and data types, depending upon the type of image with which we are working.
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Image Formats
We often work with images in a standard format (JPEG, TIFF, GIF)
How do we read/write such images with OpenGL?
No support in OpenGL
OpenGL knows nothing of image formats
Some code available on Web
Can write readers/writers for some simple formats in OpenGL
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Displaying a PPM Image
PPM is a very simple format
Each image file consists of a header followed by all the pixel data
Header P3
# comment 1
# comment 2 .
#comment n
rows columns maxvalue
pixels
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Reading the Header
FILE *fd;
int k, nm;
char c;
int i;
char b[100];
float s;
int red, green, blue;
printf("enter file name\n");
scanf("%s", b);
fd = fopen(b, "r");
fscanf(fd,"%[^\n] ",b);
if(b[0]!='P'|| b[1] != '3'){
printf("%s is not a PPM file!\n", b);
exit(0); }
printf("%s is a PPM file\n",b);
check for “P3”
in first line
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Reading the Header (cont)
fscanf(fd, "%c",&c);
while(c == '#') {
fscanf(fd, "%[^\n] ", b);
printf("%s\n",b); }
ungetc(c,fd);
skip over comments by looking for # in first column
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Reading the Data
fscanf(fd, "%d %d %d", &n, &m, &k);
printf("%d rows %d columns max value= %d\n",n,m,k);
nm = n*m;
image=malloc(3*sizeof(GLuint)*nm);
s=255./k;
for(i=0;i<nm;i++) {
fscanf(fd,"%d %d %d",&red, &green, &blue );
image[3*nm-3*i-3]=red;
image[3*nm-3*i-2]=green;
image[3*nm-3*i-1]=blue; }
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Scaling the Image Data
We can scale the image in the pipeline
glPixelTransferf(GL_RED_SCALE, s);
glPixelTransferf(GL_GREEN_SCALE, s);
glPixelTransferf(GL_BLUE_SCALE, s);
We may have to swap bytes when we go from processor memory to the frame buffer depending on the processor. If so, we can use
glPixelStorei(GL_UNPACK_SWAP_BYTES,GL_TRUE);
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The display callback
void display() {
glClear(GL_COLOR_BUFFER_BIT);
glRasterPos2i(0,0);
glDrawPixels(n,m,GL_RGB,
GL_UNSIGNED_INT, image);
glFlush(); }
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3. Writing into Buffers
Conceptually, we can consider all of memory as a large two-dimensional array of pixels
We read and write rectangular block of pixels
Bit block transfer (bitblt) operations
The frame buffer is part of this memory
memory
source
frame buffer
(destination)
writing into frame buffer
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Note that, from the hardware perspective, the type of processing involved has none of the characteristics of the processing of geometric objects.
Consequently the hardware that optimizes bitblt operations has a completely different architecture from the pipeline hardware that we used for geometric operations.
Thus, the OpenGL architecture contains both a geometric pipeline and a pixel pipeline, each of which is usually implemented separately
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3.1 Writing Modes
Read destination pixel before writing source
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Writing Modes
Source and destination bits are combined bitwise
16 possible functions (one per column in table)
XOR
replace OR
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3.2 Writing with XOR
Recall from Chapter 3 that we can use XOR by enabling logic operations and selecting the XOR write mode
XOR is especially useful for swapping blocks of memory such as menus that are stored off screen
If S represents screen and M represents a menu
the sequence
S  S  M
M  S  M
swaps the S and M
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21. 4. Bit and Pixel Operations in OpenGL
Not only does OpenGL support a separate pixel pipeline and a variety of buffers, but also data can be moved among these buffers and between buffers and the processor memory.
The plethora of formats and types can make writing efficient code for dealing with bits and pixels a difficult task. We shall not discuss what the details are, but instead shall look at what capabilities are supported.
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4.1 OpenGL Buffers and the Pixel Pipeline
OpenGL has a separate pipeline for pixels
Writing pixels involves
Moving pixels from processor memory to the frame buffer
Format conversions
Mapping, Lookups, Tests
Reading pixels
Format conversion
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Raster Position
OpenGL maintains a raster position as part of the state
Set by glRasterPos*()
glRasterPos3f(x, y, z);
The raster position is a geometric entity
Passes through geometric pipeline
Eventually yields a 2D position in screen coordinates
This position in the frame buffer is where the next raster primitive is drawn
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Buffer Selection
OpenGL can draw into or read from any of the color buffers (front, back, auxiliary)
Default to the back buffer
Change with glDrawBuffer and glReadBuffer
Note that format of the pixels in the frame buffer is different from that of processor memory and these two types of memory reside in different places
Need packing and unpacking
Drawing and reading can be slow
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Bitmaps
OpenGL treats 1-bit pixels (bitmaps) differently from multi-bit pixels (pixelmaps)
Bitmaps are masks that determine if the corresponding pixel in the frame buffer is drawn with the present raster color
0  color unchanged
1  color changed based on writing mode
Bitmaps are useful for raster text
GLUT font: GLUT_BIT_MAP_8_BY_13
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Raster Color
Same as drawing color set by glColor*()
Fixed by last call to glRasterPos*()
glColor3f(1.0, 0.0, 0.0);
glRasterPos3f(x, y, z);
glColor3f(0.0, 0.0, 1.0);
glBitmap(…….
glBegin(GL_LINES);
glVertex3f(…..)
Geometry drawn in blue
Ones in bitmap use a drawing color of red
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4.2 Bitmaps
OpenGL treats arrays of one-bit pixels (bitmaps) differently from multibit pixels (pixelmaps) and provides different programming interfaces for the two cases.
Bitmaps are used for fonts and for displaying the cursor, often making use of the XOR writing mode.
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glBitmap(width, height, x0, y0, xi, yi, bitmap)
offset from raster
position
increments in
raster
position after
bitmap drawn
first raster position
second raster position
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Example: Checker Board
GLubyte wb[2] = {0 x 00, 0 x ff};
GLubyte check[512];
int i, j;
for(i=0; i<64; i++) for (j=0; j<64, j++)
check[i*8+j] = wb[(i/8+j)%2];
glBitmap( 64, 64, 0.0, 0.0, 0.0, 0.0, check);
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4.4 Pixels and Images
OpenGL works with rectangular arrays of pixels called pixel maps or images
Pixels are in one byte ( 8 bit) chunks
Luminance (gray scale) images 1 byte/pixel
RGB 3 bytes/pixel
Three functions
Draw pixels: processor memory to frame buffer
Read pixels: frame buffer to processor memory
Copy pixels: frame buffer to frame buffer
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OpenGL Pixel Functions
glReadPixels(x,y,width,height,format,type,myimage)
size
type of pixels
start pixel in frame buffer
type of image
GLubyte myimage[512][512][3];
glReadPixels(0,0, 512, 512, GL_RGB,
GL_UNSIGNED_BYTE, myimage);
glDrawPixels(width,height,format,type,myimage)
starts at raster position
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4.5 Lookup Tables
In OpenGL, all colors can be modified by lookup tables as they are placed into buffers.
These maps are essentially the same as the color lookup tables that we introduced in Chapter 2
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5. Examples
This section considers three examples of writing pixels.
Maxwell Triangle – Chromacity of color coordinates
Pseudo Color Buffer
Picking
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6. Mapping Methods
Although graphics cards can render over 10 million polygons per second, that number is insufficient for many phenomena
Clouds
Grass
Terrain
Skin
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Modeling an Orange
Consider the problem of modeling an orange (the fruit)
Start with an orange-colored sphere
Too simple
Replace sphere with a more complex shape
Does not capture surface characteristics (small dimples)
Takes too many polygons to model all the dimples
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Modeling an Orange (2)
Take a picture of a real orange, scan it, and “paste” onto simple geometric model
This process is known as texture mapping
Still might not be sufficient because resulting surface will be smooth
Need to change local shape
Bump mapping
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Three Types of Mapping
Texture Mapping
Uses images to fill inside of polygons
Environmental (reflection mapping)
Uses a picture of the environment for texture maps
Allows simulation of highly specular surfaces
Bump mapping
Emulates altering normal vectors during the rendering process
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Texture Mapping
geometric model
texture mapped
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Environment Mapping
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Bump Mapping
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7. Texture Mapping
Textures are patterns.
They can range from regular patterns, such as stripes and checkerboards, to the complex patterns that characterize natural materials.
Textures can be 1, 2, 3, and 4 dimensional
1D – used to create a pattern for a curve.
3D – a block of material used to sculpt an object.
2D is by far the most common.
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Where does mapping take place?
Mapping techniques are implemented at the end of the rendering pipeline
Very efficient because few polygons make it past the clipper
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Is it simple?
Although the idea is simple---map an image to a surface---there are 3 or 4 coordinate systems involved
2D image
3D surface
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Coordinate Systems
Parametric coordinates
May be used to model curved surfaces
Texture coordinates
Used to identify points in the image to be mapped
World Coordinates
Conceptually, where the mapping takes place
Screen Coordinates
Where the final image is really produced
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6.1 Two-Dimensional Texture Mapping
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Mapping Functions
Basic problem is how to find the maps
Consider mapping from texture coordinates to a point a surface
Appear to need three functions
x = x(s,t)
y = y(s,t)
z = z(s,t)
But we really want
to go the other way
(x,y,z) t s
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Backward Mapping
We really want to go backwards
Given a pixel, we want to know to which point on an object it corresponds
Given a point on an object, we want to know to which point in the texture it corresponds
Need a map of the form
s = s(x,y,z)
t = t(x,y,z)
Such functions are difficult to find in general
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Two-part mapping
One solution to the mapping problem is to first map the texture to a simple intermediate surface
Example: map to cylinder
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Cylindrical Mapping
parametric cylinder
x = r cos 2p u
y = r sin 2pu
z = v/h
maps rectangle in u,v space to cylinder
of radius r and height h in world coordinates
s = u
t = v
maps from texture space
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Spherical Map
We can use a parametric sphere
x = r cos 2pu
y = r sin 2pu cos 2pv
z = r sin 2pu sin 2pv
in a similar manner to the cylinder
but have to decide where to put
the distortion
Spheres are used in environmental maps
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Box Mapping
Easy to use with simple orthographic projection
Also used in environmental maps
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Second Mapping
Map from intermediate object to actual object
Normals from intermediate to actual
Normals from actual to intermediate
Vectors from center of intermediate
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Aliasing
Point sampling of the texture can lead to aliasing errors
point samples in u,v
(or x,y,z) space
miss blue stripes
point samples in texture space
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Area Averaging
A better but slower option is to use area averaging
pixel
preimage
Note that preimage of pixel is curved
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6.2 Texture Mapping in OpenGL
Three steps to applying a texture
specify the texture
read or generate image
assign to texture
enable texturing
assign texture coordinates to vertices
Proper mapping function is left to application
specify texture parameters
wrapping, filtering
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Texture Mapping x y z
geometry
screen s t
image
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Texture Example
The texture (below) is a 256 x 256 image that has been mapped to a rectangular polygon which is viewed in perspective
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Texture Mapping and the OpenGL Pipeline
Images and geometry flow through separate pipelines that join at the rasterizer
“complex” textures do not affect geometric complexity
geometry pipeline
vertices
pixel pipeline
image
rasterizer
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Specify Texture Image
Define a texture image from an array of texels (texture elements) in CPU memory
Glubyte my_texels[512][512];
Define as any other pixel map
Scanned image
Generate by application code
Enable texture mapping
glEnable(GL_TEXTURE_2D)
OpenGL supports 1-4 dimensional texture maps
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Define Image as a Texture
glTexImage2D( target, level, components, w, h, border, format, type, texels );
target: type of texture, e.g. GL_TEXTURE_2D
level: used for mipmapping (discussed later)
components: elements per texel
w, h: width and height of texels in pixels
border: used for smoothing (discussed later)
format and type: describe texels
texels: pointer to texel array
glTexImage2D(GL_TEXTURE_2D, 0, 3, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE, my_texels);
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Converting A Texture Image
OpenGL requires texture dimensions to be powers of 2
If dimensions of image are not powers of 2
gluScaleImage( format, w_in, h_in, type_in, *data_in, w_out, h_out, type_out, *data_out );
data_in is source image
data_out is for destination image
Image interpolated and filtered during scaling
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Mapping a Texture
Based on parametric texture coordinates glTexCoord*() specified at each vertex
Texture Space
Object Space t
1, 1
(s, t) = (0.2, 0.8)
0, 1 A a c
(0.4, 0.2) b B C s
(0.8, 0.4)
0, 0
1, 0
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Typical Code
glBegin(GL_POLYGON);
glColor3f(r0, g0, b0);
glNormal3f(u0, v0, w0);
glTexCoord2f(s0, t0);
glVertex3f(x0, y0, z0);
glColor3f(r1, g1, b1);
glNormal3f(u1, v1, w1);
glTexCoord2f(s1, t1);
glVertex3f(x1, y1, z1); .
glEnd();
Note that we can use vertex arrays to increase efficiency
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Interpolation
OpenGL uses bilinear interpolation to find proper texels from specified texture coordinates. There can be distortions
texture stretched
over trapezoid
showing effects of
bilinear interpolation
good selection
of tex coordinates
poor selection
of tex coordinates
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Texture Parameters
OpenGL has a variety of parameters that determine how texture is applied
Wrapping parameters determine what happens of s and t are outside the (0,1) range
Filter modes allow us to use area averaging instead of point samples
Mipmapping allows us to use textures at multiple resolutions
Environment parameters determine how texture mapping interacts with shading
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Wrapping Mode
Clamping: if s,t > 1 use 1, if s,t <0 use 0
Wrapping: use s,t modulo 1
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP )
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT )
texture s t
GL_CLAMP
wrapping
GL_REPEAT
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Magnification and Minification
More than one texel can cover a pixel (minification) or more than one pixel can cover a texel (magnification)
Can use point sampling (nearest texel) or linear filtering ( 2 x 2 filter) to obtain texture values
Texture
Polygon
Magnification
Minification
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Filter Modes
Modes determined by
glTexParameteri( target, type, mode )
glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER,
GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER,
GL_LINEAR);
Note that linear filtering requires a border of an
extra texel for filtering at edges (border = 1)
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Mipmapped Textures
Mipmapping allows for prefiltered texture maps of decreasing resolutions
Lessens interpolation errors for smaller textured objects
Declare mipmap level during texture definition
glTexImage2D( GL_TEXTURE_*D, level, … )
GLU mipmap builder routines will build all the textures from a given image
gluBuild*DMipmaps( … )
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Example
point
sampling
linear
filtering
mipmapped
point
sampling
mipmapped
linear
filtering
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Texture Functions
Controls how texture is applied
glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param )
GL_TEXTURE_ENV_MODE modes
GL_MODULATE: modulates with computed shade
GL_BLEND: blends with an environmental color
GL_REPLACE: use only texture color
GL(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
Set blend color with GL_TEXTURE_ENV_COLOR
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Perspective Correction Hint
Texture coordinate and color interpolation
either linearly in screen space
or using depth/perspective values (slower)
Noticeable for polygons “on edge”
glHint( GL_PERSPECTIVE_CORRECTION_HINT, hint )
where hint is one of
GL_DONT_CARE
GL_NICEST
GL_FASTEST
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Generating Texture Coordinates
OpenGL can generate texture coordinates automatically
glTexGen{ifd}[v]()
specify a plane
generate texture coordinates based upon distance from the plane
generation modes
GL_OBJECT_LINEAR
GL_EYE_LINEAR
GL_SPHERE_MAP (used for environmental maps)
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6.3 Texture Objects
Texture is part of the OpenGL state
If we have different textures for different objects, OpenGL will be moving large amounts data from processor memory to texture memory
Recent versions of OpenGL have texture objects
one image per texture object
Texture memory can hold multiple texture obejcts
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Applying Textures II
specify textures in texture objects
set texture filter
set texture function
set texture wrap mode
set optional perspective correction hint
bind texture object
enable texturing
supply texture coordinates for vertex
coordinates can also be generated
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Other Texture Features
Environmental Maps
Start with image of environment through a wide angle lens
Can be either a real scanned image or an image created in OpenGL
Use this texture to generate a spherical map
Use automatic texture coordinate generation
Multitexturing
Apply a sequence of textures through cascaded texture units
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77. 9. Compositing Techniques
OpenGL provides a mechanism, alpha blending, than can (among other effects) create images with transparent objects
The objective of this section is to learn to use the A component in RGBA color for
Blending for translucent surfaces
Compositing images
Antialiasing
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9.1 Opacity and Blending
Opaque surfaces permit no light to pass through
Transparent surfaces permit all light to pass
Translucent surfaces pass some light translucency = 1 – opacity (a)
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Physical Models
Dealing with translucency in a physically correct manner is difficult due to
the complexity of the internal interactions of light and matter
Using a pipeline renderer
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Writing Model
Use A component of RGBA (or RGBa) color to store opacity
During rendering we can expand our writing model to use RGBA values
blend
source blending factor
destination
component
source
component
destination blending
factor
Color Buffer
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Blending Equation
We can define source and destination blending factors for each RGBA component
s = [sr, sg, sb, sa]
d = [dr, dg, db, da]
Suppose that the source and destination colors are
b = [br, bg, bb, ba]
c = [cr, cg, cb, ca]
Blend as
c’ = [br sr+ cr dr, bg sg+ cg dg , bb sb+ cb db , ba sa+ ca da ]
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9.2 Image Compositing
The most straightforward use of a blending is to combine and display several images that exist as pixel maps, or equivalently, as sets of data that have been rendered independently
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9.3 Blending and Compositing in OpenGL
Must enable blending and pick source and destination factors
glEnable(GL_BLEND)
glBlendFunc(source_factor,
destination_factor)
Only certain factors supported
GL_ZERO, GL_ONE
GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA
GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA
See Redbook for complete list
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Example
Suppose that we start with the opaque background color (R0,G0,B0,1)
This color becomes the initial destination color
We now want to blend in a translucent polygon with color (R1,G1,B1,a1)
Select GL_SRC_ALPHA and GL_ONE_MINUS_SRC_ALPHA as the source and destination blending factors
R’1 = a1 R1 +(1- a1) R0, ……
Note this formula is correct if polygon is either opaque or transparent
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Clamping and Accuracy
All the components (RGBA) are clamped and stay in the range (0,1)
However, in a typical system, RGBA values are only stored to 8 bits
Can easily loose accuracy if we add many components together
Example: add together n images
Divide all color components by n to avoid clamping
Blend with source factor = 1, destination factor = 1
But division by n loses bits
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86. Chapter 8 -- Discrete Techniques
9.4 Antialiasing
Line Aliasing
Ideal raster line is one pixel wide
All line segments, other than vertical and horizontal segments, partially cover pixels
Simple algorithms color
only whole pixels
Lead to the “jaggies”
or aliasing
Similar issue for polygons
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87. Chapter 8 -- Discrete Techniques
Antialiasing
Can try to color a pixel by adding a fraction of its color to the frame buffer
Fraction depends on percentage of pixel covered by fragment
Fraction depends on whether there is overlap
no overlap
overlap
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88. Chapter 8 -- Discrete Techniques
Area Averaging
Use average area a1+a2-a1a2 as blending factor
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89. Chapter 8 -- Discrete Techniques
OpenGL Antialiasing
Can enable separately for points, lines, or polygons
glEnable(GL_POINT_SMOOTH);
glEnable(GL_LINE_SMOOTH);
glEnable(GL_POLYGON_SMOOTH);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
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90. Chapter 8 -- Discrete Techniques
9.5 Back-to-Front and Front-to-Back Rendering
Is this image correct?
Probably not
Polygons are rendered
in the order they pass
down the pipeline
Blending functions
are order dependent
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91. Chapter 8 -- Discrete Techniques
Opaque and Translucent Polygons
Suppose that we have a group of polygons some of which are opaque and some translucent
How do we use hidden-surface removal?
Opaque polygons block all polygons behind them and affect the depth buffer
Translucent polygons should not affect depth buffer
Render with glDepthMask(GL_FALSE) which makes depth buffer read-only
Sort polygons first to remove order dependency
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92. Chapter 8 -- Discrete Techniques
9.6 Depth Cueing and Fog
We can composite with a fixed color and have the blending factors depend on depth
Simulates a fog effect
Blend source color Cs and fog color Cf by
Cs’=f Cs + (1-f) Cf
f is the fog factor
Exponential
Gaussian
Linear (depth cueing)
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93. Chapter 8 -- Discrete Techniques
Fog Functions
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94. Chapter 8 -- Discrete Techniques
OpenGL Fog Functions
GLfloat fcolor[4] = {……}:
glEnable(GL_FOG);
glFogf(GL_FOG_MODE, GL_EXP);
glFogf(GL_FOG_DENSITY, 0.5);
glFOgv(GL_FOG, fcolor);
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95. 10. Multirendering and the Accumulation Buffer
Compositing and blending are limited by resolution of the frame buffer
Typically 8 bits per color component
The accumulation buffer is a high resolution buffer (16 or more bits per component) that avoids this problem
Write into it or read from it with a scale factor
Slower than direct compositing into the frame buffer
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96. Chapter 8 -- Discrete Techniques
12. Summary and Notes
In the early days of computer graphics, people worked with only three-dimensional geometric objects, whereas those people who were involved with only two-dimensional images were considered to be working in image processing.
Advances in hardware have made graphics and image processing systems practically indistinguishable.
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97. Chapter 8 -- Discrete Techniques
The idea that a two-dimensional image or texture can be mapped to a three dimensional surface in no more time than it takes to render the surface with constant shading would have been unthinkable a few years ago.
Now these techniques are routine.
In this chapter we have concentrated on techniques that are supported by recently available hardware and API’s
Many of the techniques introduced here are new, many more are just appearing in the literature; even more remain to be discovered.
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98. Chapter 8 -- Discrete Techniques
13. Suggested Readings
Bump mapping was first suggested by Blinn (77). Environmental mapping was developed by Blinn and Newell (76)
Hardware support for texture mapping came with the SGI Reality Engine
Many of the compositing techniques, including the use of the a channel, were suggested by Porter and Duff (84). The OpenGL Programmer’s Guide contains many examples of how buffers can be used.
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99. Exercises -- Due next class
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Chapter 8 -- Discrete Techniques