Changing pixel colour

This is a discussion on Changing pixel colour within the C Programming forums, part of the General Programming Boards category; Hi, i have 2300 bitmap images very similar to this one http://mage.me.uk/files/hab12d1_001.bmp and what i basically need to do is ...

  1. #1
    Registered User redruby147's Avatar
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    Changing pixel colour

    Hi, i have 2300 bitmap images very similar to this one

    http://mage.me.uk/files/hab12d1_001.bmp


    and what i basically need to do is change every white pixel in the image to blue. I haven't manipulated any images before in c and I'd like some pointers on how to go about it. In addition, after one image is done, ideally the program would switch to the next bitmap file.

    So an English summary of the program would be.

    open bitmap image.
    look at the first pixel in the image.
    if the pixel is anything other than white skip it.
    If the pixel is white, change it to blue and go to next pixel.
    if all pixels have been examined, go to next image in the directory and repeat process.

    If anyone knows of existing software or code which does this, that would be great too. Thanks.

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    That sounds like the correct solution. I'm not sure there are any software out there to do that.

    If you have photoshop, you could of course do it with "Magic Wand", but with thousands of images, it would certainly be faster to do it by (slow) software.

    You should look for a library package to load and save whatever image format you are using.

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    Registered User hk_mp5kpdw's Avatar
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    Start with writing a program to display the filenames of all your bitmap images that you will want to later edit, iterating through a directory/folder location on your computer. Just display them one-to-a-line to the console or output them to a file.

    Once you've got that working you'll need to figure out the bitmap format and work on opening and editing the individual files as required.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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    spurious conceit MK27's Avatar
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    Quote Originally Posted by hk_mp5kpdw View Post
    Once you've got that working you'll need to figure out the bitmap format and work on opening and editing the individual files as required.
    Almost certainly, you read the file in as binary with fread("rb") to an unsigned char array, and go byte by byte thru the array. What each byte means depends on the nature of the format.

    Unfortunately, as I have noticed before, the significant links at "wotsit" are sometimes dead. So you will have to do some digging around and reading of technical specs.
    Last edited by MK27; 06-08-2009 at 12:18 PM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
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    Quote Originally Posted by MK27 View Post
    Almost certainly, you read the file in as binary with fread("rb") to an unsigned char array, and go byte by byte thru the array. What each byte means depends on the nature of the format.
    Sure. But without knowing what for example the headers mean, you won't know a lot of other things you need to know to process the file. The header may not even be the same size for all files [some headers have "optional" content that may or may not be there].

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    spurious conceit MK27's Avatar
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    Quote Originally Posted by matsp View Post
    Sure. But without knowing what for example the headers mean, you won't know a lot of other things you need to know to process the file. The header may not even be the same size for all files [some headers have "optional" content that may or may not be there].
    Even worse, some file formats allow a limited/customized palette, so you might have to do some work just to add "blue" as a possibility! But I don't thing bmp is like that.

    Definitely, dissecting/dissembling/interpreting/understanding the header is essential. Those "technical docs" should elaborate.
    Last edited by MK27; 06-08-2009 at 12:44 PM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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    Captain Crash brewbuck's Avatar
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    I would use Image Magick. You could use "compose" to treat the white color as transparent, and underlay the image with a solid color (whatever color you want to convert white into). The result will be all white pixels changing to your chosen color.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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    Quote Originally Posted by brewbuck View Post
    I would use Image Magick. You could use "compose" to treat the white color as transparent, and underlay the image with a solid color (whatever color you want to convert white into). The result will be all white pixels changing to your chosen color.
    That does sound like a solution. I was thinking that on hardware platforms, you'd use "color-keying", which is similar to this solution.

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    Registered User redruby147's Avatar
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    Great, thanks people, imagemagick worked perfectly, i used
    Code:
    chii:~/Desktop/Stimuli$convert hab14d1_007.bmp -fill blue -opaque white hab14d1_007done.gmp
    which did the job. I'm tempted to just put all the file names in a shell script and be done with it as I'm having difficulty finding examples on using the c api. There seems to be ample information on using imagemagick in the command line. does anyone know of a good place where examples of the api in use could be found. Excuse me if there are some obvious ones, i have not spotted them :P. Thanks again

  10. #10
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    How about using ls | xargs to perform the processing. Just make sure you output to a differnet directory, or you will end up with an infinite loop.

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  11. #11
    Frequently Quite Prolix dwks's Avatar
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    e.g.
    Code:
    $ mkdir converted
    $ for file in *.bmp; do convert $file -fill blue -opaque white converted/$file; done
    dwk

    Seek and ye shall find. quaere et invenies.

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  12. #12
    Registered User redruby147's Avatar
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    Ah great, that worked really well. Done them all now, thanks again

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