1. Projective Texture Mapping

2. Projective Textures Use to simulate effects: Slide projectors
Spotlight illuminations
Shadows
Reprojection of images

3. Texture Projection
Very similar to projecting the objects to the screen
But now we will project the scene to the light source (or the slide projector, for example)
The key is to calculate the texture coordinates of the projection receiver
Projection geometry
image plane
Slide projector
camera

4. Calculate the Texture Coordinates
Main idea – Project the objects to the “image plane” of the projector and use the projector’s NDC to calculate the texture coordinates
projector
objects
Projection
projector
objects
Projection

5. Calculate the Texture Coordinates
Use OpenGL’s texgen function and set it to GL_OBJECT_LINEAR
Construct the texture matrix as a concatenation of (M_t = T * P * M)
M: Modelview (set the projector’s view using gluLookAt)
P: Projection (set the projector’s “camera” parameters)
T: Bias and Scale: transform from projector’s NDC [-1,1] to [0,1] to look up texture

6. Things to pay attention:
Need to make sure that only the objects meant to receive the projected texture will render with the texture (not every object in the scene)
Need to use 4D texture coordinates to ensure correct perspective projection

7. Example Code
Note that in this example we are using GL_EYE_LINEAR, but it is okay
As long as you remember to transform the eye space back to object space first
(next page to continue)

8. Example Code
… //draw the scene …

9. Spotlight illumination
The OpenGL’s spot light feature requires us to tessellate the polygons
The main idea – project the texture to the scene from the light as a illumination mask
Regular point light source
is used

10. Algorithm
Drawing the scene with depth buffering enabled and color buffer write disabled
Load and enable the spotlight texture and use GL_MODULATE to map the texture
Enable texture coordinate geneartion and load the texture matrix
Enable blending and use GL_ONE, GL_ONE for the blending function
Disable depth buffer write, set the depth function to GL_EQUAL
Draw the scene with the vertex color set to the spot light color
Disable the spot light texture, texgen, and texture transformation
Set the blend function to GL_DST_COLOR
Draw the scene with normal diffuse and specular light

11. Using textures for contouring
Remember the texture coodinates are generated as:
s = Ax + By + Cz+ Dw (same for t, r, q if needed), where A, B, C, D are user specified parameters, and x,y,z,w can be either object space or eye space position
This can be used to measure the distance to the plane Ax+By+Cz+D when w = 1
If we simply apply a 1D texture from the generated texture coordinate S (below), then you are making the object based on its distance to a particular plane

12. Examples