Hello! Before I put in a bug in GitHub, I was hoping someone could confirm if what I am seeing is expected behavior.
I've made a small test case (https://github.com/youngpm/alphabug) where I have two adjacent rasters that abut but do not overlap. They are the same solid color, but contain a alpha band that goes from transparent to opaque along the border where they touch. When I use bilinear resampling and zoom in, I see a pixel of overlap (see https://github.com/youngpm/alphabug/blob/main/example/bilinear.png), which manifests as a darker line when the layers are composited in semi-transparent areas. This doesn't occur with nearest neighbor resampling (see https://github.com/youngpm/alphabug/blob/main/example/nearest.png). I've placed these sample rasters and a mapfile with shp2map command examples that illustrate what I am seeing here via individual raster layers and when using a tile index: https://github.com/youngpm/alphabug For another experiment, in QGIS, I loaded one of the geotiffs directly and again via WMS. With bilinear resampling, one sees that the WMS layer has an additional 1/2 pixel arpon (the pixel size being the native pixel size) all the way around compared with the directly loaded version, but they agree exactly when nearest neighbor is used; bilinear appears to be interpolating past the image boundary by half a pixel. I'm pretty new to using mapserver so it may very well be user error. My other thought is that during compositing, there is some mixup between pixel centers and pixel edges. Thanks for any suggestions! Happy to file an issue if anyone believes this is a bug and not expected behavior. Patrick
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