6.21.7 Example: Image Warping

A common use of a 2D noise texture is to distort the texture coordinates while drawing a 2D image, thus warping the image. A noise function is created in the framebuffer as described above, read back to the host, and used as texture coordinates (or offsets to texture coordinates) to render the image. Since color values in OpenGL are normalized to the range 0.0 to 1.0, if one is careful the image returned to the host may be used without much conversion; assuming that the modelview and texture matrixes are set up to accept values in this range, the returned data may be used directly for rendering.

Another similar use of a 2D noise texture is to distort the reflection of an image. In OpenGL, reflections on a flat surface can be done by reflecting a scene across the surface. The results can be copied from the framebuffer to texture memory, and in turn drawn with distorted texture coordinates. The shape and form of the distortion can be controlled by modulating the contents of the framebuffer after the noise texture is drawn but before it is copied to texture memory. This can produce interesting effects such as water ripples.