On 27/02/15 14:03, Alan Barnett wrote:
I'm using gimp to increase the contrast of scanned documents. The
scanned images were combined into a multi-page pdf. The images are
music scores; the originals are RGB, but only grayscale information is
I opened the input pdf as layers with a resolution of 300 dpi, changed
the brightness and contrast of each layer, and exported the images as
a .mng file. I then used ImageMagick to convert the nmg back to .pdf
I've done this with two different sets of images. For the first set
of images, the input .pdf files are about 2 Mb, and the output .mng
and .pdf are both about 4 Mb; the process increases the size of the
files by a factor of 2.
For the second set of images, the input pdf files are about 5 MB, and
the output .mng and .pdf files are both about 45 MB, an increase of a
factor of 9!
Any suggestions how to decrease the size of the output files? Any
ideas about why the large difference in the size increase between the
two sets of images. (The first set were scanned by me on an EPSON
1640SU scanner, the second set were scanned by someone else on a
scanner unknown to me.)
The file size is related to the size of the image in pixels, not to its
physical size (which depends on print defintion)
Also, in the PNG format (and AFAIK MNG is a PNG derivative) there is no
compression of random data, so color uniformity is important to keep
size to a minimum.
An album of three pictures(*) on IMGUR:
- The one where the background is pure white is 4K.
- Rather visible solid noise added to the clean image increases it to 20K
- Barely visible HSV noise added to the clean image increases it to 30K
So some pos-tprocessing to cleanup your background may have the required
effect. Thresholding would be perfect for the final size but would be
ugly. High contrast could do it, or a more fine-tuned Curves setting. A
de-speckling filter could also help.
(*) Anyone recognizes the song?
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