rasterize pdf that embeds non-regular dpi image using `cupsfilter` introduces nearest-neighbor like aliasing artifacts.

I was investigating a problem that the Canon Selphy CP1500 prints images 107% larger.
The printer uses the 'Canon SELPHY CP1500, driverless, cups-filter, 2.0.1' driver.

So I started to print an image of the 10x10mm and 1x1mm grid pattern in various software and various options.

Here's the grid image:

254 dpi, 100mmx148mm(Hagaki size) png image. thick lines are 4px thick, 10mm spacing, thin lines are 2px thick, 1mm spacing.

and the printed results have nearest-neighbor-like aliasing artifacts.

also found that it only happens with GIMP and non-300 dpi images (shotwell and darktable have no problem with non-300 dpi images).

GIMP generates a PDF file /tmp/gtkprint* and streams the file to CUPS driver. I could intercept the generated pdf file: grid.pdf
The file has perfectly fine image, but the printed result had aliasing artifacts. This is where I started to suspect the rasterization filter in the CUPS driver.

I noticed this code here
https://github.com/OpenPrinting/libcupsfilters/blob/a02c49054ff828772b9edea083a3fe58e90e9b15/cupsfilters/imagetoraster.c#L1544-L1547

so I've modified the image to include colors to make the embedded image to have 8-bit color. here's the generated pdf file: color.pdf
But the result wasn't any better.

I found that I can manually invoke cupsfilter with a ppd file. Here's the ppd that I somehow salvaged from /tmp/ cp1500.txt (changed the extension to upload to GitHub)

cupsfilter -p cp1500.ppd -m image/urf -i application/pdf ./grid.pdf > grid.urf
urftobmp grid.urf

You can get urftobmp here: https://github.com/mbevand/urf2image
I got this error when I tried to convert it to jpeg.

cupsfilter: No filter to convert from application/pdf to image/jpeg.

Here's the urf result that has been converted to bmp.
urf_to_bmp_results.zip

You can see that both color.bmp and grid.bmp look like they were resized using the nearest-neighbor algorithm.

Here's the GIMP issue for reference: https://gitlab.gnome.org/GNOME/gimp/-/issues/15558

Start with the imagetoraster.c code linked in the report and reproduce the conversion using cupsfilter with cp1500.ppd, grid.pdf, and color.pdf. Inspect the generated URF with urftobmp and compare it with the attached BMP results; done means identifying and correcting the rasterization path that produces nearest-neighbor-like aliasing for non-300 DPI embedded images.