Pro OpenGL ES for iOS

156 CHAPTER 5: Textures^

than a pixel, and that is called minification. Filtering is the process used to determine

how to correlate a pixel’s color with the underlying texel, or texels. Tables 5-3 and 5-4,

respectively, show the possible variants of this.

Table 5-3. Texture Filter Types in OpenGL ES for Minification

Name Purpose

GL_LINEAR Smooths texturing using the four nearest texels closest to the

center of the pixel being textured

GL_LINEAR_MIPMAP_LINEAR Similar to GL_LINEAR but uses the two nearest mipmaps closest

to the rendered pixel

GL_LINEAR_MIPMAP_NEAREST Similar to GL_LINEAR but uses the one nearest mipmap closest to

the rendered pixel

GL_NEAREST Returns the nearest texel value to the pixel being rendered

GL_NEAREST_MIPMAP_NEAREST Similar to GL_NEAREST but uses the texel from the nearest

mipmap

Table 5-4. Texture Filter Types in OpenGL ES for Magnification

Name Purpose

GL_LINEAR Smooths texturing using the four nearest texels closest to the center of the pixel

being textured

GL_NEAREST Returns the nearest texel value to the pixel being rendered

There are three main approaches to filtering:

„ Point sampling (called nearest in OpenGL lingo): A pixel’s color is

based on the texel that is nearest to the pixel’s center. This is the

simplest, is the fastest, and naturally yields the least satisfactory

image.

„ Bilinear sampling, otherwise called just linear: A pixel’s coloring is

based on a weighted average of a 2 ̄2 array of texels nearest to the

pixel’s center. This can smooth out an image considerably.

„ Trilinear sampling: Requires mipmaps and takes the two closest

mipmap levels to the final rendering on the screen, performs a bilinear

selection on each, and then takes a weighted average of the two

individual values.

It’s the trilinear sampling that you saw in action in the previous exercise, and it results in

a pretty dramatic increase in perceived image quality.

Figure 5-20a shows a close-up of Hedly with the filtering off, while Figure 5-20b has it

switched on. If you go back and look at the mipmap code, you’ll see that both images