Table of Contents Glide Programming Guide
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- 5 6Texture Filtering
- 7Texture Clamping
56Texture FilteringTexture maps are square or rectangular, but after being mapped to a polygon or surface and transformed into screen coordinates, the individual texels of a texture map rarely correspond to screen pixels on a one-to-one basis. Depending on the transformations used and the texture mapping applied, a single pixel on the screen can correspond to anything from a tiny portion of a texel, resulting in magnification, to a large collection of texels, resulting in minification. In either case it is unclear exactly which texel values should be used and how they should be averaged or interpolated. Consequently, Glide allows an application to choose between two types of filtering: point sampling and bilinear interpolation. Figure Texture Mapping.3 Point sampling and bilinear filtering. Glide supports two methods of choosing a texel within a texture map. If the pixel maps to less than one texel, as shown in diagram (a), texture magnification is called. If the pixel maps to more than one texel, as shown in diagram (b), then minification is required. The user can select between point-sampling and bilinear filtering during the minification or magnification. When using point sampling, the texel whose (s, t) coordinates are nearest the center of the pixel is chosen. Bilinear filtering computes a weighted average of the 2 by 2 array of texels that lie nearest the center of the pixel. The magnification and minification filters are independent: one can specify point sampling and the other bilinear filtering, or both can be the same. Magnification of a texture map occurs when a texture map is “blown up” on screen (see Figure Texture Mapping.3(a)). For example, if a 64´64 texture map is rendered onto a polygon that covers 128´128 pixels on the screen, an average of four pixels will cover each texel in the texture map, causing noticeable blockiness. The Voodoo Graphics hardware supports bilinear interpolation of texels that greatly reduces the blockiness and pixelization of texture magnification. Minification of a texture map occurs when a texture map is compressed on screen (see Figure Texture Mapping.3(b)). For example, if a 64´64 texture map is rendered onto a polygon that only covers 16´16 pixels on the screen, an average of 16 texels will cover each pixel on the screen. This leads to disturbing artifacts known as “texture aliasing”. The Voodoo Graphics hardware remedies this problem by supporting both mipmapping and filtering. If a Voodoo Graphics subsystem is performing point sampled filtering, the texel with coordinates nearest the center of the pixel being rendered is used to generate the color output on the screen (see Figure Texture Mapping.3(c)). Point sampling, also known as nearest neighbor sampling, may result in pixelization and blockiness during magnification and “texture jerking” during minification. One way of reducing the blockiness of point sampling is by linearly interpolating between the colors of the texels that are adjacent to the source pixel, which results in a much smoother image than point sampling (see Figure Texture Mapping.3(d)). Bilinear interpolation is performed by the Voodoo Graphics hardware with no incurred additional performance overhead. Minification and magnification filtering are controlled by the Glide function grTexFilterMode() and are independently selectable. void grTexFilterMode( GrChipID_t tmu, GrTextureFilterMode_t minFilterMode, GrTextureFilterMode_t magFilterMode ) The first argument, tmu, selects the texture mapping unit that the filter selections apply to. Valid values are GR_TMU0, GR_TMU1, and GR_TMU2. The minification filter, minFilterMode , can be either GR_TEXTUREFILTER_POINT_SAMPLED or GR_TEXTUREFILTER_BILINEAR, as can the magnification filter, magFilterMode. The magnification filter is used when the LOD calculated for a pixel indicates that the pixel covers less than one texel. Otherwise, the minification filter is used. 7Texture ClampingWhen texture s and t coordinates have overflowed during a texture mapped rendering operation, the hardware can either clamp the coordinates to a maximum value or, alternatively, wrap them around. This choice is up to the developer depending on whether tiled or non-tiled texture mapping is desired. Texture clamping also allows for interesting effects, for example, out of range s and t coordinates can be passed with a very small texture in a large polygon. Such an approach will effectively place the texture somewhere in the interior of the polygon with the rest of the polygon rendered with the border color of the texture. This can potentially save texture memory if small composite textures are used on a predominantly monotone surface, e.g., a window on the side of a space ship. Figure Texture Mapping.4 Texture clamping. The texture clamp mode specifies what to do when texture coordinates are outside the range of the texture map. If wrapping is enabled, then texture maps will tile, i.e., values greater than 255 will wrap around to 0. If clamping is enabled, then texture map indices will be clamped to 0 and 255. Both modes should always be set to GR_TEXTURECLAMP_CLAMP when using projected textures. Note that s and t coordinates may be individually wrapped or clamped, as shown in Figure Texture Mapping.4. void grTexClampMode( GrChipID_t tmu, GrTextureClampMode_t sClampMode, GrTextureClampMode_t tClampMode ) The first argument, tmu, selects the TMU in which the mipmap resides and may be GR_TMU0, GR_TMU1, or GR_TMU2. The other two arguments set the clamping mode for s and t individually; they may be set to either GR_TEXTURECLAMP_CLAMP or GR_TEXTURECLAMP_WRAP. If wrapping is enabled, texture maps will tile: values greater than 255 will wrap around to 0. If clamping is enabled, texture map indices will be clamped to 0 and 255. Both modes should always be set to GR_TEXTURECLAMP_CLAMP when using projected textures. Download 6.22 Mb. Share with your friends:
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