Compute magazine (retry)

Holger Veit wrote: ... ... Holger, others have pointed out rightly that for low contrast pages, you have to spend some time playing with thresholding to get it to digitize OK. What I want to address here it the file size. If you assemble a PDF from a collection of page images (jpgs, TIFF) acrobat appears to simply puts a wrapper around them and that is that. If you read in a PDF and use the File/Reduce File Size... menu, it often doesn't have much effect (I think if you have high DPI images it may resample them, which may not be what you want). I tried it on your file and there was barely any reduction in size. However, if you scan the files from within acrobat (Create PDF/From scanner...), it applies a lot more intelligence to the task. It is also affected by a few of the preferences you can set. Something that has worked for me for reprocessing existing files is this procedure (I've only used it for 1 bpp images). Read in the PDF. File/Save As TIFF. This produces one TIFF page for each source page. Then use the Create PDF/From Multiple Files... and read back in all the TIFF images. It will recompress them. Now if you are using G4 compression, this will help only if the page images were encoded with something worse (like LZW). The important step is to set the preference for reading in TIF files to allow JBIG2 compression -- this saves about 20% for typical pages, and can be dramatically better for images with halftoning. The real savings come when you select JBIG2 (lossy). Yes, it does change the image in imperceptible ways. For some, this is heresy, but i'd point out that you are scanning at 1 bpp, so why be a stickler about what you get? To make this concrete, your original document is 28 pages and is 2266 KB. After my preprocessing step, it is 849 KB and looks every bit as good to my eyes. See for yourself: http://home.pacbell.net/frustum/v2n7-repacked.pdf Looking closely (like 800% magnification) your scan shows a LOT of dithering on all characters. Perhaps this is the result of the low contrast source, but more likely your scanner is doing dithering. I've seen this on the high speed office scanner at my work -- the dithering is nice visually when making copies, but it introduces a lot of edge deltas that G4 compression spend a lot of bits encoding. My cheap home scanner doesn't dither so agressively and produces smaller scans. Then zoom in on the repacked pdf that I made. The vertical edges of characters have a lot less dithering. No doubt this leads to the smaller file size. One problem that I'm wrestling with is this. If you scan via acrobat and use a grayscale or color option, acrobat tries to identify regions of pages, perhaps the whole page, that can be quantized down to 1 bpp B&W for the best compression. Sometimes it works brilliantly, other times it decides sections of a page are best encoded as jpeg images, resulting in barfalicious artifacts.