How to fix poor interpolation of very small rasters in viewed pdfs (evince and chrome)

Question

I want to create a visualization of a matrix for some academic work. I decided to go about this by having the pixels in the image correspond to the values in the matrix. I created the nice small png that follows:

When properly scaled up, you get a very reasonable image:

This is a screenshot from within inkscape. However, when export this as a pdf, both evince and chrome do a terrible job at upscaling what should be very trivial, and instead I get something that looks like:

The pdf itself seems to scale appropriately well for printing, but unfortunately I do a lot of my editing without printing, and this looks unacceptable. I did find this incredibly old thread about people seeming to have a similar issue with chrome's pdf viewer, and the "solution" was to just upscale the raster graphics. This is a solution, but is terribly inefficient.

Is anyone aware of a way to change the pdf so that it gets upscaled appropriately? Maybe a config change in evince or chrome that will render these properly? Even a nice way to go from a raster image to a vector image might be suitable?

Answer 1

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An image dictionary in a PDF has an (optional) boolean entry Interpolate. It is specified as a flag indicating whether image interpolation shall be performed by a conforming reader.

The program used by the OP to create the PDF, Inkscape, seems to have explicitly set this flag to true. Editing the PDF to unset this flag creates a file which looks as desired by the OP.

(This also is a solution proposed in this Inkscape forum thread eventually found by the OP, which is to save the PDF with high-resolution bitmaps embedded. File -> Inkscape Preferences -> Bitmaps -> Resolution for Create Bitmap Copy, and set it to 6000 dpi)

The fact that interpolation looks different in different viewers and different output media, is by design. The PDF specification states on interpolation:

A conforming Reader may choose to not implement this feature of PDF, or may use any specific implementation of interpolation that it wishes.

A different way to get around this problem (especially as some PDF viewers have the tendency to not really live up to the specification and e.g. interpolate ignoring that flag) would be to use vector graphics here, drawing the bitmap pixels as rectangles. The result should be optimal.