Hello Sven: > ... To see this, create a new image, apply a standard grid on it > using Filter->Render->Patterns->Grid and scale it down in one direction > by a scale factor smaller than 0.5. The one pixel wide grid lines will > become blurry. I don't think this is acceptable and so far the only > choice we have is to apply either gimp-decimate.diff or > gimp-decimate-2.diff...
A cartoon of what I understand about downsampling, which may be useful when people debate the various merits of downsampling methods: There are two extremes in what most people expect from downsampling. What they expect basically depends on what the downsampling is applied to: --- Old school CG type images (e.g., Super Mario or the old Wilbur, the Gimp mascot). Then, nearest neighbour (and analogous methods) will, in most situation, do better than box filtering (and most LINEAR interpolatory methods). The reason is that the picture is made up of flat colour areas with sharp boundaries, and anything (linear) which deviates a lot from nearest neighbour will not preserve the property of begin made up of flat colour areas separated by sharp lines. For most people, blur, in this context, is more annoying than aliasing. --- Digital photographs, in which the image is usually made up of smooth colour areas with blurry boundaries, and in addition, there is noise and demosaicing artifacts. Then, in general, nearest neighbour is not acceptable, because it amplifies the noise (which is not present in CG images) and aliasing is more visually jarring than blur. In this situation, box filtering (especially its exact area variant) and analogous methods will, in most situations, do better than nearest neighbour. Consequence: Linear methods cannot make both groups of people happy. Making most people happy will require TWO (linear) downsampling methods. Alternatively, it will require having a parameter (called blur?) which, when equal 0, gives a method which is close to nearest neighbour, and when equal to 1, gives a method which is close to box filtering. I can help with this. ------- Another important point which was raised is that if the image is enlarged in one direction and reduced in the other, one single method is unlikely to do well. Within the GEGL approach, it may be that such situations are better handled by upsampling first (using a good upsample method) in the upsampling direction, then feeding the result to the downsampler in the downsampling direction. That is: Don't expect one single method/"plug-in" to do both. In summary: To stretch in one direction and shrink in the other, first do a one direction stretch, followed by a one direction shrink. ------- Also, in previous emails about this, I saw the following valid point being made: Suppose that the following strategy is followed for downsampling. To make things more explicit, I'll use specific numbers. Suppose that we want to downsample an image from dimensions 128x64 to 15x9 (original pixel dimensions are powers of two for the sake of simplicity). First, box filter down (by powers of two, a different number of times in each direction) to 16x16, then use a standard resampling method (bilinear, say) to downsample to 15x9. The point that was made was that doing things this way is not continuous, meaning that scaling factors which are almost the same will not give images which are almost the same. For example, if one followed this strategy to downsample to 17x9 instead of 15x9, one would first box filter down to 32x9, then apply bilinear. It should surprise no one that this may produce a fairly different picture. The point I want to make about this is that it is possible to fix this "discontinuous" behavior, as follows. Produce TWO box filtered down images. In the case of downsampling from 128x64 to 15x9, the two downsamples would be of dimensions 16x16 and 8x8. Then, downsample the 16x16 to 15x9 using, say, bilinear, and upsample the 8x8 to 15x9 using, again, bilinear, making sure that the sampling keeps the alignment of the images (I know how to do this: it is not hard). Then, blend the two images as follows: Let Theta = ((15-8)/(16-8)+(9-8)/(16-8))/2. Final image = Theta * downsample + (1-Theta) * upsample. If you think about what this does, you will realize that this satisfies the criterion that nearby downsampling factors give nearby images. (WARNING: nearest neighbour is discontinuous, so the nearby images can actually be quite different. But they will be less different than with standard nearest neighbour.) If someone wants to implement the above, I can help. ----- I hope someone finds the above useful when thinking about the downsampling issue. With regards, Nicolas Robidoux Laurentian University/Universite Laurentienne _______________________________________________ Gimp-developer mailing list Gimp-developer@lists.XCF.Berkeley.EDU https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
