Computer Graphics Texture Mapping CO2409 Computer Graphics Week 13

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

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

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

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

Artist UV Mapping Tool

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…

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

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

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

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

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]

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

Comparison

High Resolution Comparison

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