1. Computer Graphics Texture Mapping
CO2409 Computer Graphics
Week 13

2. Lecture Contents Textures Texture Coordinates Texture Addressing Modes
Texture Filtering
Mip-Mapping

3. Textures
We can vastly improve on simple coloured and lit models by using textures
Sometimes called texture maps or more simply maps
Textures are rather like a wallpaper, shrink-wrapped around the model geometry
A texture is simply a bitmap held off-screen
Typically loaded from standard bitmap file formats
Each pixel in the bitmap appears as a square on the geometry called a texel
Texture resolution usually high enough so that we don’t notice the texels

4. Texture Coordinates (UVs)
Need to define exactly how a texture is wrapped around the geometry
Which part of the texture is attached to each vertex
So each vertex in the geometry is assigned a texture coordinate:
A texture coordinate is a pair of (float) values usually from 0.0 to 1.0
Measured on U & V axes
Written (U,V) and hence very often referred to as UVs
The vertex UVs define exactly how the texture maps onto the geometry
This is Texture Mapping

5. Defining Texture Mapping
The UV coordinate (0, 0) specifies the top-left of the texture bitmap, (1,1) specifies the bottom-right
So (1, 0) defines the top-right
(0, 1) the bottom-left
(0.5, 0.5) the centre
See diagram on last slide
The UV for each vertex specifies which part of the texture is on that vertex
Choosing a texture pixel for a given UV is sampling the texture
UVs are interpolated across the polygon surface to define how the texture is applied
This polygon is mapped with the yellow square on the last slide

6. Artist UV Mapping Tool

7. Texture Addressing Modes
UVs do not have to be in the range 0.0 to 1.0
The texture position represented by UVs outside this range depends on the texture addressing mode:
(Note this is DirectX terminology)
Wrap: Use only the fractional part of the UVs (e.g. U=2.7 means U=0.7). This is the usual mode.
Mirror: Similar to wrap mode, but the texture is mirrored for odd U and V
Clamp: UVs are clamped to the nearest valid value (U=2.7 means U=1, V=–2.3 means V=0)
Border: UVs out of range return a fixed colour regardless of the texture colours
Diagrams on next slide…

8. Addressing Modes Examples
Wrap addressing mode repeats the texture over for large UV ranges
Useful for mapping a large polygon with a repeated texture (e.g. a tiled wall)
Mirror addressing looks similar but the texture alternately mirrors
Clamp addressing ensures we see only one texture
Can be used to reduce bleeding problems at the edges of geometry
Border addressing is similar
Wrap
Mirror
Clamp
Border

9. Texture Filtering
If we zoom in on a textured polygon each texel will cover a small, but noticeable area
If all pixels in a texel the same colour we see small small quads:
If we zoom out, will be a choice of several texels within a single pixel
Could cause aliasing shown in a later slide
So when sampling, we blend texels to smooth out these effects
Called Texture Filtering
Magnification: zoom in
Minification: zoom out
Texels Visible as Squares

10. Texture Filtering Modes
Choose a filtering mode for both texture minification & magnification:
Can be different
Point Sampling
No filtering, sample nearest texel
Bilinear Filtering
Sampled colour is a linear blend of the nearest four texels
Called bilinear because it blends neighbour texels in X & Y direction
Anisotropic Filtering
An advanced blending mode
Considers polygon angle to the camera
Improves the clarity of polygons going into the distance especially
Point Sampled
Bilinear
Bilinear Sampling

11. Mip-Mapping
When a texture is drawn far away, it will use the minification filter
However, the available runtime filters are poor when considerable scaling down is required
Certain textures will likely exhibit unpleasant aliasing effects
Aliasing is when the resolution is not sufficient to display fine detail
E.g. “strobing” on regular patterns
or noise effects on naturalistic detail
Reduce problem by pre-creating smaller versions of the texture to use when it is far away
This is called mip-mapping No
Mip-Maps
With
Mip-Maps

12. Mip-Map Creation
A mip-map is a smaller version of a texture created using a high-quality resizing algorithm
Done in advance, not during scene rendering
A sequence of mip-maps is created, each 50% smaller than the one before
Making mip-map chain from original texture potentially down to a 1x1 pixel version
When rendering a texture, the mip-map closest to the polygon size is used
This minimises aliasing effects
[It is also faster: more likely to sequentially sample neighbouring texels with correct sized texture, which is more cache-efficient]

13. Tri-linear Filtering
When a polygon moves toward or away from the viewer, the change in mip-map choice can be seen
A clear visual dividing line between mip-maps (hard to show on static diagram – see lab)
To remove this we can linearly blend the nearest two mip-maps
Instead of just choosing the nearest one
Combined with bilinear filtering within each texture, this gives trilinear filtering
Blends the mip-map dividing line
There is an anisotropic equivalent too
Bilinear
Trilinear

14. Comparison

15. High Resolution Comparison

16. Texture Blending A polygon can have more than one texture applied
Can blend several textures on top of each other to create a composite texture
Possible uses:
Reflections on a texture
Shadows on a texture
Merging textures (above)
Normal mapping - upcoming lab
Use blending modes
Like the earlier sprite lab
We may need to provide multiple sets of UVs, one for each texture