imagemagick or something that will leave only highest width characters in image

metaschima · 10-24-2014, 07:45 PM

Let's say I've got an image with letters in it (captcha), is there a function to specifically leave only the highest width characters (the ones in the foreground) ? Maybe something to eat away at the edges of non-white (usually black) characters and thus leave only the highest width characters ?

I've tried other things like convolve blur and sharpen, but these are unreliable. No java programs or proprietary programs please. I'm just testing and trying to learn about captcha.

ondoho · 10-25-2014, 06:14 AM

sorry, i don't understand.
can you give some examples? convert command, before and after images?

metaschima · 10-25-2014, 11:22 AM

Well, I was looking at one example in particular and I actually found something that works well. I was looking at this example because I'm actually looking into image filtering that will help me clear up images before using an OCR like tesseract. I figure if this will work on captchas the same method tweaked a bit should work on documents too. I have attached an example that I made using GIMP and here is the imagemagick filter and tesseract command.

Code:

convert owntest.jpg -convolve 0,0,0,1,0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,0,0,0,0,0,1,0,0,0 -level 0,30% -unsharp 30x900 t2.jpg
tesseract -psm 7 t2.jpg t2 config
tesseract -psm 8 t2.jpg t2 config

The config file has

Code:

load_system_dawg F
load_freq_dawg F
tessedit_char_whitelist 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ

for captchas at least, for documents this can be fixed or not used.

The output is

Code:

 D O J C Y

ondoho · 10-25-2014, 03:22 PM

i think i understand what you mean with "highest width" - you want to graphically seperate and isolate the actual letters.
fwiw, i tried your convert command with owntest.jpg from above, but the result is very different.

Code:

convert --version
Version: ImageMagick 6.8.9-8 Q16 x86_64 2014-10-09 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2014 ImageMagick Studio LLC
Features: DPC HDRI Modules OpenCL OpenMP
Delegates: bzlib cairo fontconfig freetype gslib jng jp2 jpeg lcms lqr ltdl lzma pangocairo png ps rsvg tiff webp wmf x xml zlib

metaschima · 10-25-2014, 05:28 PM

Hmm, that's not good, I was hoping this would be portable at least within imagemagick. I have version

Code:

bash-4.2$ convert --version
Version: ImageMagick 6.8.6-10 2013-09-18 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2013 ImageMagick Studio LLC
Features: DPC
Delegates: bzlib cairo djvu fftw fontconfig freetype jng jp2 jpeg lcms lzma openexr pango pangocairo png png rsvg tiff x xml zlib

The command is correct, I checked it.

ondoho · 10-27-2014, 01:35 PM

my bad, i used the wrong file - i used the thumbnail instead of the actual image.
however, that shows that the command works differently with different resolutions.

attaching the proper result.

metaschima · 10-27-2014, 03:00 PM

Yeah, it's not too reliable, but can be tweaked for similar inputs. It's really no good for captcha, but may be of some use for document OCR.