I am analysing a large number of PDF files from scientific articles, all of which contain images which (I think) were originally monochrome ( black/white).
The images have been extracted with Apache PDFBox and I am doing image processing on
The images have severe colour distortion, and blurring which I do not think is due to the extraction process or to subpixel antialising (see https://stackoverflow.com/questions/16242731/reconstructing-line-diagrams-from-subpixel-rendering). Note that the effect is present on both lines and text.
The effect is present in all PNG files though in different amounts. I suspect it comes from the tool/s used to create the PDFs from the original images which apply some softening process (antialising). I have analysed this visually and it consists of a narrow plane in HSV with only two colours mixed with a lot of white/grey/black.
I'd like to know what the algorithm is that creates this blurring and colour so that I can reverse it. Traditional sharpening is only partially successful. I'd be delighted if anyone has code that does this (any language will do).
NOTE: I do not have originals.
UPDATE. Here are some analyses of the colors with ImageJ ColorInspector3D plugin in YCbCr space
= YCbCr frequencies = Note the very sharp plane in which the colors lie
I intend to ask another question on how to sharpen this. (This Q is *where" does the effect arise; the next will be how to correct it even if we don't know the origin)