OpenGL supports texture images which are 1D or 2D and have dimensions that are a power of two. Some implementations have been extended to support 3D and 4D textures. Texture coordinates are assigned to the vertices of all primitives (including pixel images). The texture coordinates are part of a three dimensional homogeneous coordinate system (s,t,r,q). When a primitive is rasterized a texture coordinate is computed for each pixel fragment. The texture coordinate is used to look up a texel value from the currently enabled texture map. The coordinates of the texture map range from [0..1]. OpenGL can treat coordinate values outside the range [0,1] in one of two ways: clamp or repeat. In the case of clamp, the coordinates are simply clamped to [0,1] causing the edge values of the texture to be stretched across the remaining parts of the polygon. In the case of repeat the integer part of the coordinate is discarded resulting in a texture tile that repeats across the surface. The texel value that results from the lookup can be used to modify the original surface color value in one of several ways, the simplest being to replace the surface color with texel color, either by modulating a white polygon or simply replacing the color value. Simple replacement was added as an extension by some vendors to OpenGL 1.0 and is now part of OpenGL 1.1.