“Why should the background be treated differently than the foreground, if this is just a simple scan?”
I don’t know, but NBC claims: “That is the whole point of Mixed Raster Compression, the colored background is compressed differently than the foreground ‘text’.”
NBC is claiming that the Xerox did NOT do a “simple scan” but a compressed scan using industry standard algorithms (MCR) which NBC claims can result in replications in foreground shapes (mostly but not always text or numbers, IIRC...could be box shapes for example).
As I understand it, MRC was developed for things like magazine pages. Things with images, text and line art. The BC is not the type of document that was intended to create.
Here is what one of the developers of MRC told John Woodman:
“The question is whether all these artifacts we see after rendering the PDF of POBC are signs of forgery. I do not see that. I see them more likely as a result of inadequate processing.”
“The document has poor quality and it has been aggressively processed, no questions about it. The question is whether the corruptive processing was individual with the intent of forging it, or if it was automated within regular MRC segmentation.”
[skip]
“MRC is about separating the single-image document into multiple layers, hopefully each one with a given characteristic. This has to be done automatically, in what we call segmentation. What I see in the document are signs of MRC segmentation consistent with strategies in line with the techniques pioneered by DjVu. I (and my students) do not advocate doing the segmentation that way, but that is not the point either. In fact, I would not be surprised if the software which segmented the WH document was derived from some DjVu tool.”
[skip]
“I took a birth certificate which has a similar background pattern, scanned and compressed using an older DjVu tool. It has shown the same problems as POBC, like text letters that were missed and sent to background, and multiple text styles. It didn’t have halo, though, because its algorithm decided to obliterate the whole background pattern. Perhaps if I had time to toy around with packages and parameters I might find something very close to what was used to generate the document shown by the WH, but I unfortunately do not have the time right now.”
“In summary I can only say I see much stronger signs of common MRC algorithmic processing of the image rather than some intentional manipulation.”
Sincerely
Ricardo L. de Queiroz