Martin Nordholts writes: > Nicolas Robidoux wrote: > > My working assumption is that GEGL should be as consistent as possible > > with GIMP. > > That seems weird to me, why not go for the "best" resampling approach > and ignore how GIMP does it? Well, it certainly makes my life easier if I don't have to try to be aligned with how GIMP generally does things. However, I am a bit concerned that users may expect a certain GENERAL behavior, and be upset if they don't get it. So, I'd like feedback regarding: "Coke Classic or New Coke"? Alignment is more or less independent of "best resampling methods" (although currently all samplers implemented are "point" samplers and consequently are more friendly to the "center-based physical coordinate" approach). Every resampling method can pretty much be used with every "image alignment convention." Some resampling methods may be a bit trickier to implement with one convention than another or may lose some desirable properties (most notably near the boundary, although this also depends on the abyss policy). But with a bit of tweaking, reasonable results can be obtained irregardless of the "image alignment convention" provided one makes sure that things are consistent. To give an example, (classic) ImageMagick uses the usual "exact area transformation" convention (corner-based physical coordinates, which are friendly to box filtering) even though most of its many resamplers are point resamplers (if image is N pixel wide and its geometrical width is understood to be from 0 to 1, then the first pixel center is understood to be located at position 1/(2*N)), but I routinely compile it with minor source code changes so that when scaling an image it uses the usual "point transformation" convention (center-based physical coordinates, which are friendly to nearest neighbour) (under the above conditions, the horizontal position of the center of the first pixel is 0). In order to avoid undesirable/unexpected shifts, it is important to make sure that the convention used inside the sampler is the same as the one used by, say, a scaling operator. These issues are addressed, for example, in http://www.vtk.org/Bug/view.php?id=6558 >From the sampler code already in GEGL, I assumed that the GEGL convention is "center-based physical coordinates:" image starts at 0, and the first pixel is centered at 0; in addition, assuming unit inter-pixel distance, the last pixel is located at N-1 if there are N pixels in total in the horizontal direction. The most common alternative being: "corner-based physical coordinates," which is commonly expressed in two different ways: version 1: image starts at -.5, first pixel is centered at 0, the last pixel is located at N-1 and the image ends at N-.5. version 2: image starts at 0, first pixel is centered at .5, the last pixel is located at N-.5 and the image ends at N. So, my questions are: A) Is "center-based physical coordinates" indeed the intended convention for GEGL? B) For GIMP? C) If the intended conventions are different, should I align GEGL's convention with GIMP's? Or leave it be? ---------------------------------------------------------------------- Apologies for the longish email, but I'd like to set this issue to rest. Nicolas Robidoux Laurentian University _______________________________________________ Gegl-developer mailing list Gegl-developer@xxxxxxxxxxxxxxxxxxxxxx https://lists.XCF.Berkeley.EDU/mailman/listinfo/gegl-developer
