Gray outlines around edges of transparent areas in embedded raster images
This issue is best illustrated with an example.
I generated a 5x5 png test image (test1.png) with transparent background and a 1 pixel wide green strip through the middle, embedded it into an svg (test1.svg), converted it to a pdf using inkscape (test1.pdf) and then viewed it in Acrobat Reader DC on windows 10, downloaded and installed today (2020-01-27). The result is shown in the attached image test1_acrobat_windows.png.
Notice how the green band fades first to gray, then to white, instead of fading directly to white. The same pdf file rendered with PDF.js looks like one would expect, as seen in test1_out_pdfjs.png, which looks just like the svg file does.
The gray border goes away if the original png has a white background instead of a transparent background. The corresponding files for this case are test2.png (raw image), test2.svg, test2.pdf and test2_acrobat_windows.png.
This test case is of course a bit contrived, but this behavior has real-world consequences. For example, a pdf figure that includes a raster contour plot with lots of thin colored curves on transparent background, will appear desaturated, especially as one zooms out.
My guess is that the transparent pixels have a black (but invisible) color, and that the interpolation formula uncritically interpolates R, G, B and A between #00ff00ff and #00000000, resulting in intermediates like #00800080, which when alpha composited with white gives #80c080, a slightly green shade of gray. To avoid this happening, I think one would need to alpha-composite before interpolation instead of after.
Since I could only attach a single image to the post (even though it looked like I could upload multiple in the submission form), and the form crashed when I tried to upload a zip file containing them all, I have uploaded the zip file to the following address instead: