Add this function in a subclass to get a notification when a texture has completed importing just before.

The texture is optionally compressed and saved to disk.

At this point it is too late to choose compression format, it is still possible to compress the texture using texture.Compress but this is not adviced and the compression format will not be displayed in the editor. Use OnPreprocessTexture if you wish to change compression format based on filename or other attributes of the texture.

If the texture is modified as in the example below it is required to be readable. The flag isReadable must to set True in importer settings either from the editor(Read/Write Enabled) or in the OnPreprocessTexture function. If the texture does not have to be readable at runtime use texture.Apply(true, true) to update the mipmaps and make the texture unreadable at runtime.

	// Postprocesses all textures that are placed in a folder
	// "invert color" to have their colors inverted.
	class InvertColor extends AssetPostprocessor {
		function OnPostprocessTexture (texture : Texture2D) {			
			// Only post process textures if they are in a folder
			// "invert color" or a sub folder of it.
			var lowerCaseAssetPath = assetPath.ToLower();
			if (lowerCaseAssetPath.IndexOf ("/invert color/") == -1)
				return;
			
			for (var m : int  = 0; m < texture.mipmapCount; m++) {
				var c : Color[] = texture.GetPixels(m);
				for (var i : int = 0 ;i < c.Length; i++) {
					c[i].r = 1 - c[i].r;
					c[i].g = 1 - c[i].g;
					c[i].b = 1 - c[i].b;
				}
				texture.SetPixels(c, m);
			}
			
			// Instead of setting pixels for each mip map levels, you can also
			// modify only the pixels in the highest mip level. And then simply use
			// texture.Apply(true); to generate lower mip levels.
		}
	}