PDF to tiff ImageMagick problem

Question

I'm trying to convert pdfs to tiff images for following OCR. I use "-density 300x300 -depth 8" as parameters. The first problem is that from 500 KB pdf file i get 72 MB tiff file. The second problem is bad quality of resulting image causing OCR failing. Here you can see it yourself. Adobe acrobat reader generated (printed) tiff image: enter image description here

ImageMaggick tiff image: enter image description here

The difference is huge. How can i get as good as Adobe generated image using ImageMaggick? Not tiff neccesary, other formats also will be good.

UPD: i've found 'antialias' option. Now it's much more better. But still OCR result not so accurate as for Adobe version.

Actually Adobe's one is worse, as it's not anti-aliased and looks much more pixelated. — Bartosz Ciechanowski, May 14 '11 at 14:06
For human - yes. But for Tesseract Adobe version is the best. — clumpter, May 14 '11 at 15:47
Why are you converting bilevel images to 8-bit gray (-depth 8)? If you just need to change the format from PDF to TIFF and not alter the image data, leave the bit depth as original. — BitBank, May 14 '11 at 16:26
It's for Tesseract. Otherwise it will fail saying that image format isn't supported. — clumpter, May 14 '11 at 17:19
Why do you want to convert text to image and then resulting image to text? Maybe extracting text from PDFs will better suit your needs? — Bobrovsky, May 16 '11 at 14:07
@Bobrovsky: because i don't need to OCR the whole document but some specific parts of it. — clumpter, May 16 '11 at 17:24

score 7 · Accepted Answer · answered May 15 '11 at 18:36

My suggestion is: use a Ghostscript commandline. Because ImageMagick uses Ghostscript anyway, in the background (the technical IM term for this is: Ghostscript is a "delegate" for some of the conversions, such as PDF->TIFF).

Here is a commandline that should work well for letter-sized pages of a multi-page PDF file:

gswin32c.exe ^
   -o page_%03d.tif ^
   -sDEVICE=tiffg4 ^
   -r720x720 ^
   -g6120x7920 ^
    input.pdf

The -g... parameter controls the absolute width+height of the output pages using 'device points'... (and with 6120x7920 at 720dpi this happens to be letter-sized).

These TIFF pages...

...will be black+white,
...will have a resolution of 720dpi,
...will be G4-compressed and
...will be much smaller than your un-compressed 300dpi from the IM commandline

Your IM parameter of -depth 8 isn't suited to give good results from the p.o.v. of later OCR, since it will create shades of gray around letters which don't help with this.

Your OCR results should now be much better than before.

If your OCR can't handle TIFF G4 format (which I doubt), then you could generate other TIFF subformats with the help of Ghostscript. For example:

gswin32c.exe ^
   -o page_%03d.tif ^
   -sDEVICE=tiffgray ^
   -r720x720 ^
   -g6120x7920 ^
   -sCompression=lzw ^
    input.pdf

.

gswin32c.exe ^
   -o page_%03d.tif ^
   -sDEVICE=tiff24nc ^
   -r720x720 ^
   -g6120x7920 ^
   -sCompression=lzw ^
    input.pdf

The tiffgray device creates 8-bit gray output. The tiff24nc device creates 8-bit RGB color output. Both types of TIFF will of course be bigger than the tiffg4 output.

Thanks a lot, that looks fantastic! One last question: do you know to make it produce one multipage tiff file from multipage pdf? — clumpter, May 15 '11 at 20:57
@dumpter: For multipage TIFF just use `-o output.tif`. If there is no `%03d` name part for the output, there will be no single-page TIFF.... — Kurt Pfeifle, May 16 '11 at 05:15

score 2 · Answer 2 · edited Aug 01 '17 at 07:54

2

For european paper format A4 and unix/linux use:

gs -o output.tif -sDEVICE=tiffg4 -r720x720 -sPAPERSIZE=a4 input.pdf

edited Aug 01 '17 at 07:54

GhostCat

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answered Aug 01 '17 at 07:29

sneaky

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According to this [post](http://www.bardecode.com/en1/convert-adobe-pdf-tif-linux-using-ghostscript/), for colored images `-sDEVICE=tiffscaled24` has to be used. – fitzberg Aug 08 '21 at 15:16

PDF to tiff ImageMagick problem

2 Answers2