1. CG Summary: OpenGL Shading andTextures Angel, Chapters 5, 7; “Red Book” slides from AW, red book, etc.
CSCI 6360/4360

2. Overview
“Survival kits”
Shading (and illumination)
Texture mapping

3. Shading and Lights Introduce OpenGL shading functions and lights
Discuss polygon shading a bit
Flat
Smooth
Gouraud

4. (Local) Polygon Mesh Shading
Any surface can be illuminated/shaded/lighted by:
1. calculating surface normal at each visible point and
2. applying illumination model
Where efficiency is consideration, e.g., for interactivity (vs. photorealism) approximations are used
Three methods - each treats a single polygon independently of others (non-global)
Constant (flat)
Gouraud (intensity interpolation)
Interpolated Phong (normal-vector interpolation)

5. Flat/Constant Shading
Single illumination value per polygon
Illumination model evaluated just once for each polygon (1 value for all polygon)
Which is as fast as it gets
As “sampling” value of illumination equation (at just 1 point)
Pretty coarse – below is flat vs. smooth (Gouraud) shading
If polygon mesh is an approximation to curved surface,
faceted look is a problem
Also, facets exaggerated by mach band effect
For fast, can (and do) store normal with each surface
Or can, of course, compute from vertices

6. Gouraud Shading
For flat/constant shading, single illumination value per polygon
Gouraud (or smooth or interpolated intensity) shading overcomes problem of discontinuity at edge
“Smooths” where polygons meet
Linearly interpolate intensity along scan lines
eliminates intensity discontinuities at polygon edges

7. Phong Shading The first best “good enough” model Ambient
From everywhere
Diffuse
Reflected from surface
Specular
“shiny spot”

8. Steps in OpenGL shading
Enable shading and select model
Specify normals
Specify material properties
Specify lights

9. Enabling Shading Shading calculations are enabled by:
glEnable(GL_LIGHTING)
Once lighting is enabled, glColor() ignored
Must enable each light source individually:
glEnable(GL_LIGHTi) i=0,1…..
Can choose light model parameters:
glLightModeli(parameter, GL_TRUE)
GL_LIGHT_MODEL_LOCAL_VIEWER
do not use simplifying distant viewer assumption in calculation
GL_LIGHT_MODEL_TWO_SIDED
shades both sides of polygons independently

10. (opt.) Normals In OpenGL normal vector is part of state
Set by glNormal*()
glNormal3f(x, y, z);
glNormal3fv(p);
Usually want to set normal to have unit length so cosine calculations are correct
Length can be affected by transformations
Note that scaling does not preserved length
glEnable(GL_NORMALIZE) allows for autonormalization at a performance penalty

11. (opt.) Normal for Triangle
plane n ·(p - p0 ) = 0
n = (p2 - p0 ) ×(p1 - p0 ) p p1
normalize n  n/ |n| p0
Note that right-hand rule determines outward face

12. Light: Defining a Point Source
For each light source, we can set an RGBA for the diffuse, specular, and ambient components, and for the position
Setting light values for specular, etc. is model
Code is pretty crude
Can have multiple lights with multiple colors that move – go for it
GL float diffuse0[]={1.0, 0.0, 0.0, 1.0};
GL float ambient0[]={1.0, 0.0, 0.0, 1.0};
GL float specular0[]={1.0, 0.0, 0.0, 1.0};
Glfloat light0_pos[]={1.0, 2.0, 3,0, 1.0};
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glLightv(GL_LIGHT0, GL_POSITION, light0_pos);
glLightv(GL_LIGHT0, GL_AMBIENT, ambient0);
glLightv(GL_LIGHT0, GL_DIFFUSE, diffuse0);
glLightv(GL_LIGHT0, GL_SPECULAR, specular0);

13. Light: Distance, Direction, Ambient
Again, source colors are specified in RGBA
Position is given in homogeneous coordinates
If w =1.0, we are specifying a finite location
If w =0.0, we are specifying a parallel source with the given direction vector
Glfloat light0_pos[]={1.0, 2.0, 3,0, 1.0};
Coefficients in distance terms (for attenuation) are by default
a=1.0 (constant terms), b=c=0.0 (linear and quadratic terms).
Change by:
a= 0.80;
glLightf(GL_LIGHT0, GLCONSTANT_ATTENUATION, a);
Ambient light depends on color of light sources
A red light in a white room will cause a red ambient term that disappears when the light is turned off
OpenGL also allows a global ambient term that is often helpful for testing
glLightModelfv(GL_LIGHT_MODEL_AMBIENT, global_ambient)

14. Light: Spotlights f Use glLightv to set Direction GL_SPOT_DIRECTION
Cutoff GL_SPOT_CUTOFF
Attenuation GL_SPOT_EXPONENT
Proportional to cosaf f -q q

15. Moving Light Sources
Light sources are geometric objects whose positions or directions are affected by the model-view matrix
Depending on where we place the position (direction) setting function, we can
Move the light source(s) with the object(s)
Fix the object(s) and move the light source(s)
Fix the light source(s) and move the object(s)
Move the light source(s) and object(s) independently

16. Material Properties
Material properties are also part of OpenGL state and match terms in modified Phong model
Set by glMaterialv()
GLfloat ambient[] = {0.2, 0.2, 0.2, 1.0};
GLfloat diffuse[] = {1.0, 0.8, 0.0, 1.0};
GLfloat specular[] = {1.0, 1.0, 1.0, 1.0};
GLfloat shine = 100.0
glMaterialf(GL_FRONT, GL_AMBIENT, ambient);
glMaterialf(GL_FRONT, GL_DIFFUSE, diffuse);
glMaterialf(GL_FRONT, GL_SPECULAR, specular);
glMaterialf(GL_FRONT, GL_SHININESS, shine);

17. Texture Mapping Responsible for much of today’s photorealistic cg
Puts an image on a facet (polygon)
Using some geometry
Will see example
Lots of variations
OpenGL texture functions and options x y z
image
geometry
display

18. Textures in OpenGL
Texture is 256 x 256 image that has been mapped to a rectangular polygon which is viewed in perspective
Three steps to applying a texture:
Specify the texture
A. read or generate image
B. assign to texture
C. enable texturing
Assign texture coordinates to vertices
Mapping function is set in application
Specify texture parameters
wrapping, filtering

19. Texture Mapping and OpenGL Pipeline
Big idea …
Images and geometry flow through separate pipelines that join during fragment processing
“complex” textures do not affect geometric complexity
geometry pipeline
vertices
pixel pipeline
image
Fragment
processor

20. 1. Specifying a Texture Image
Define texture image from array of texels (texture elements) in memory
Glubyte my_texels[512][512];
Define as any other pixel map
Scanned image, generate by application code, etc.
Enable texture mapping
glEnable(GL_TEXTURE_2D)
OpenGL supports 1-4 dimensional texture maps
Define image as 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);

21. May Need to Convert 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 If

22. 2. Mapping Texture -> Polygon
Based on texture coordinates
glTexCoord*() specified at each vertex
Extraordinary flexibility
glBegin(GL_POLYGON);
glColor3f(r0, g0, b0); //no shading used
glNormal3f(u0, v0, w0); // shading used
glTexCoord2f(s0, t0);
glVertex3f(x0, y0, z0);
glColor3f(r1, g1, b1);
glNormal3f(u1, v1, w1);
glTexCoord2f(s1, t1);
glVertex3f(x1, y1, z1); .
glEnd(); t
Texture Space
Object Space
1, 1
(s, t) = (0.2, 0.8)
0, 1 A a c
(0.4, 0.2) b B C
0, 0
(0.8, 0.4)
1, 0 s

23. Interpolation
OpenGL uses interpolation to find proper texels from specified texture coordinates
Can be distortions
texture stretched
over trapezoid
showing effects of
bilinear interpolation
good selection
of tex coordinates
poor selection
of tex coordinates

24. 3. Texture Parameters
OpenGL has a variety of parameters that determine how texture is applied:
Wrapping parameters determine what happens if 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

25. 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

26. glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param )
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

27. End .