Texture, or the arrangement of surface markings, is an important cue that can be used to identify objects in an image. More often than not, object recognition requires estimating the surface orientation of the constituent surfaces. If texture is used to recover the surface orientation, then separating the surfaces to form objects will require discriminating the textured surfaces when the markings have undergone an oblique projection. How-ever, many of the most widely used methods for discriminating textures are not applicable for discriminating textures distorted by oblique projection since they are all based on measurement of distances and angles. Prior work has focused on using the cross ratio of distances between four collinear points chosen appropriately. The results of the experiments with real textures indicate that although the cross ratio performed well, using other projective invariants should be investigated. Two ratios of distances between three points that are invariant under orthographic projection are considered. The two invariants are described first, followed by the results of using these invariants to discriminate natural textures.