18.4.3 Optimizing Texture Mapping

Follow these guidelines when rendering textured objects:

• Avoid frequent switching between texture maps. If you have many small textures, consider combining them into a single larger, mosaiced texture. Rather than switching to a new texture before drawing a textured polygon choose texture coordinates that select the appropriate small texture tile within the large texture.

• Use texture objects to encapsulate texture data. Place all the glTexImage() calls (including mipmaps) required to completely specify a texture and the associated glTexParameter() calls (which set texture properties) into a texture object and bind this texture object to the rendering context. This allows the implementation to compile the texture into a format that is optimal for rendering and, if the system accelerates texturing, to efficiently manage textures on the graphics adapter.

• Try to keep texture references localized between polygons. Some implementations use caching to optimize texture mapped rendering. Keeping the texture references localized when sending a batch of polygons to OpenGL can reduce the cache misses.

• If possible, use glTexSubImage*D() to replace all or part of an existing texture image rather than the more costly operations of deleting and creating an entire new image.

• Call glAreTexturesResident() to make sure that all your textures are resident during rendering. (On systems where texturing is done on the host, glAreTexturesResident() always returns GL_ TRUE.) If necessary, reduce the size or internal format resolution of your textures until they all fit into memory. If such a reduction creates intolerably fuzzy textured objects, you may use higher resolutions and specify which textures are important to keep in texture memory by using glPrioritizeTextures().

• Use smaller texel sizes. There is often a tradeoff between texel size and the speed of texture filtering, with smaller texel sizes typically performing better. Applications should try to minimize the width of a texel internal format to something like GL_ RGBA4 or GL_ RGB5_A1 for color textures and 8 bit components for luminance or luminance alpha textures unless the application requires the extra color resolution.

• Avoid expensive texture filter modes. On some systems, trilinear filtering is much more expensive than point sampling or bilinear filtering.