iluFlipImage inverts an image over the x axis. The image will be upside-down after calling this function. If this function is called twice in succession, the image is restored to its original state. A version of this function in OpenIL is used throughout internally when loading images to correct images that would otherwise be upside-down. Using ilOriginFunc will essentially tell the library which way is up.
Ilu Class | Tao.DevIl Namespace