1. Textures
Lecture 11
Wed, Oct 5, 2005

2. Textures A texture is an image that can be “pasted” onto a surface.
The image may be a simple one that was generated within the program, such as a checkerboard, or it may be a photograph (.bmp file).

3. Texture Format There are many internal formats for textures.
GL_RGB – Each texel stored in 3 bytes (red, green, blue)
GL_RGBA – Each texel stored in 4 bytes (red, green, blue, alpha)
GL_R3_G3_B2 – Each texel stored in 1 byte (3 red bits, 3 green bits, 2 blue bits), etc.
We will use the GL_RGB format.

4. Creating Textures
The GL_RGB texture image is created as a 3-dimensional array.
GLubyte image[rows][cols][3];
We think of it as a 2-dimensional array of vectors.
Each vector contains the red, green, and blue values as unsigned integers from 0 to 255.

5. Creating Textures
There are also 1-dimensional textures (pasted onto lines) and 3-dimensional textures (pasted onto solids), but we will do only 2-dimensionsal textures.

6. Creating Textures
The dimensions of a texture image must be powers of 2, but they need not be the same power of 2.
Typical dimensions
256  256
256  512
32  32

7. Example A brick wall texture may be designed as a 8  8 texture.
(192, 0, 0)
(208, 208, 208)

8. Tiling Textures Often textures are “tiled” across a surface.
When they are tiled, it is desirable that the images match left-to-right and top-to-bottom.
The brick wall texture:

9. Tiling Textures
Tiling textures works well if the texture is supposed to have a regular pattern, such as in the case of the brick wall.
Other textures look better if there is no discernable pattern, such as leaves.

10. Leaf Texture

11. Another Leaf Texture

12. Window Texture
Other textures are not meant to be tiled.

13. Example of Photograph as Texture
Read
Run

14. Another Example
Read
Run

15. Applying Textures
When a texture is applied to a polygon, the texels are mapped to the pixels of the rasterized polygon.
This usually leads to one of two issues.
One texel maps to several pixels.
Several texels map to one pixel.

16. Applying Textures
Mapping one texel to several pixels is called magnification (because the texel is magnified).
Mapping several texels to one pixel is called minification. (The opposite of magnify is minify?)

17. Applying Textures When mapping texels to pixels, we may choose between
Applying the nearest texel.
Interpolating among texels.
Interpolation uses a weighted average of the 4 nearest texels, weighted by distance.
This is called bilinear interpolation.

18. A Checkerboard Texture
Read
Run

19. Magnification of Texels (Nearest)
64 Pixels
4 Texels

20. Magnification of Texels (Nearest)
64 Pixels
4 Texels

21. Magnification of Texels (Nearest)
Image copied and pasted from demo program:

22. Magnification of Texels (Interpolated)
64 Pixels
4 Texels

23. Magnification of Texels (Interpolated)

24. Magnification of Texels (Interpolated)
Image copied and pasted from demo program:

25. Close-up of Texture 1:1 (Nearest)

26. Close-up of Texture 2:3 (Nearest)

27. Close-up of Texture 1:1 (Interpolated)

28. Close-up of Texture 2:3 (Interpolated)

29. Minification of Texels (Nearest)
4 Pixels
64 Texels

30. Minification of Texels (Nearest)
“Nearest” texel
4 Pixels
64 Texels

31. Minification of Texels (Nearest)
100% gray
4 Pixels
64 Texels
100% red

32. Minification of Texels (Interpolated)
4 Pixels
64 Texels

33. Minification of Texels (Interpolated)
Nearest 4 texels
4 Pixels
64 Texels

34. Minification of Texels (Interpolated)
100% blue
4 Pixels
64 Texels
75% green, 25% blue

35. The Mapping is not Exact (Nearest)
64 Pixels
36 Texels

36. The Mapping is not Exact (Nearest)
64 Pixels
36 Texels

37. The Mapping is not Exact (Nearest)
64 Pixels
36 Texels

38. The Mapping is not Exact (Interpolated)
64 Pixels
36 Texels

39. The Mapping is not Exact (Interpolated)
42% green, 58% blue
86% green, 14% blue
64 Pixels
36 Texels

40. The Mapping is not Exact (Interpolated)
64 Pixels
36 Texels

41. Applying Textures
The situation is often more complicated.

42. Applying Textures
The situation is often more complicated.

43. Texture Example
Read
Run

44. Other Problems
When the texel contains a regular pattern, there can be undesirable effects.

45. Other Problems
When the texel contains a regular pattern, there can be undesirable effects.

46. Other Problems
When the texture contains a regular pattern, there can be undesirable effects.