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ImageMagick: Greyscale to black and white conversion

Posted on 2004-04-23
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Last Modified: 2012-08-13
I have the latest ImageMagick installed on my server and I'm trying to figure out how I can take an extracted matte image (grayscale) and convert it to black and white in such a way that all pixels with a value between 0 and 254 are converted to 0 and all pixels with a value of 255 are left white.  In other words, for each pixel that is any level of grey, turn it black, any pixels that are pure white, leave them be.
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Question by:phasevar
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Expert Comment

by:krisnackaerts
ID: 10926380
Start changing the image to GRAY colorspace, to be sure you have an 8bit image:

convert -colorspace GRAY in.png out.png

then threshold the image to the value you want by

convert -threshold 254 out.png bw.png

This should do the trick
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Author Comment

by:phasevar
ID: 10935329
The first convert produces a grayscale image for me, but the second command's output is a 100% white image and it doesn't matter what I set the threshold to, it turns out as just a plain white image every time.

I tried:

convert -black-threshold 254 out.png bw.png

but that didn't work either, its output looks the same as the input.

I'm using ImageMagick 6.0.0.  
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Expert Comment

by:krisnackaerts
ID: 10935690
I used 5.5.7. It worked. I uninstallled it now, installed 6.0.0 Q16, tried it, didn't work. So it seems a bug in this version. I then double checked it by uninstalling v6.0.0 again, downloading v5.5.7 Q8, and it does work with this version.

My tip: uninstall 6.0.0 and install 5.5.7 for the moment. Try to log the problem. I logged something similar a while ago and the bug was solved in a few days time. You could also try the latest beta to see if they solved it already.
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Author Comment

by:phasevar
ID: 10956568
I tried both the latest beta, and the 5.5.7 and both of them produced blank (white) images for me.  

I start with what is clearly a grayscale image when loaded in my browser.  Then after the command

convert -threshold 254 out.png bw.png

bw.png is pure white.
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Accepted Solution

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krisnackaerts earned 500 total points
ID: 10956586
Next question, which operating system are you using? Windows 95/98/2000/NT/XP, Linux, ...?

It's not because an image shows grayscale, it is grayscale (i.e. 8 bit grayscale). That's why I added the comment above. Also, maybe your white is not exactly white (254), have you tried with v 5.5.7 and other values (e.g. 128 to start with)?

I ask this because it works for me.

Does it need to be imagemagick?
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Author Comment

by:phasevar
ID: 10956638
Linux.

Yes, I tried 5.5.7 with a value of 128 and same result.  Doesn't seem to matter what value I use for threshold, the result is always 100% white.

It doesn't have to be ImageMagick I suppose..  I'm not aware of any other command line utilities for this sort of thing.  My app is in Python and unfortunately I can't find all the tools I need there either.

I don't see why it should matter, but the first step in my process is to pull the matte out of a png image like so:

convert light.png -channel matte light_matte.png

Then I convert with -colorspace GRAY, then onto the convert -threshold.
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