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.

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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Mats

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.

Quote:

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Originally Posted by hk_mp5kpdw

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.

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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.

Quote:

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Originally Posted by MK27

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.

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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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Mats

Quote:

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Originally Posted by matsp

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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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.

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.

Quote:

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Originally Posted by brewbuck

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.

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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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Mats

Great, thanks people, imagemagick worked perfectly, i used

Code:

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chii:~/Desktop/Stimuli$convert hab14d1_007.bmp -fill blue -opaque white hab14d1_007done.gmp

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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

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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Mats

e.g.

Code:

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$ mkdir converted
$ for file in *.bmp; do convert $file -fill blue -opaque white converted/$file; done

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Ah great, that worked really well. Done them all now, thanks again