Use an average of the RGB components or the alpha channel as the height source in the generation of the normals.
Useful for creating normalmaps from a tiling bumpmap. This allows the filters to wrap around the image. These include the filters found in the NVIDIA plugin (4 sample, 3x3, 5x5, 7x7 and 9x9) with the addition of 3x3 and 5x5 Sobel and Prewitt edge detection filters (yields the best results IMO). The goal is to completely clone NVIDIA's photoshop plugin, with a few new useful features. It allows you to convert images into RGB normal maps for use in per-pixel lighting applications. A plugin for GIMP that aids in the authoring of tangent-space normal maps for use in per-pixel lighting applications.
