GIMP Style Tranparency

[armstrong091]

In Gimp when you use the Color-to-alpha tool - to remove a background color, for example - any selected area will also become semi transparent.

I would like to automate this process using Image magick, but have not been able to find how to do it. So far, any color-to-alpha exapmles that i have found strictly remove the color specified, not producing a gradient of transperency.

Below is an example - made in Gimp - of what i'm looking to be able to do with ImageMagick. I Color-to-alpha'd the white in the image then pasted it onto its original self to illustrate the semi transperent effect.

Any and all help is appreciated!

[snibgo]

I haven't compared this to what Gimp does, but ...

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convert in.png -alpha off ( +clone -negate ) -compose CopyOpacity -composite out.png

... makes a copy of the image, negates that, and copies the tones into the alpha channel. What was white becomes transparent; black becomes opaque.

[armstrong091]

I'm getting an unexpected token error on the '(' . I should have mentioned before that i'm using ImageMagick 6.7.7-10 in Linux

[snibgo]

In Unix commands you need to escape parentheses: \( and \).

[armstrong091]

I had tried that but It gave me a more complicated error...

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 $ convert smiley.png -alpha off \( +clone -negate \) compose CopyOpacity -composite out.png
convert.im6: unable to open image `compose': No such file or directory @ error/blob.c/OpenBlob/2638.
convert.im6: no decode delegate for this image format `compose' @ error/constitute.c/ReadImage/544.
convert.im6: unable to open image `CopyOpacity': No such file or directory @ error/blob.c/OpenBlob/2638.
convert.im6: no decode delegate for this image format `CopyOpacity' @ error/constitute.c/ReadImage/544.

I apologize for being a newbie on this, so far the extend of my experience with IM has been cropping/resizing/converting.

I appreciate your help.

[fmw42]

convert smiley.png -alpha off \( +clone -negate \) compose CopyOpacity -composite out.png

You need -compose CopyOpacity (you left of the minus sign)

[armstrong091]

A HA! It worked. This is exactly what i was looking for . Is there any way to vary the intensity of the transparency? The Result is very transparent...

I guess i could superimpose the image on itself a couple times.. though that seems sloppy.

Thanks again for your help.

[fmw42]

try

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convert smiley.png -alpha off \( +clone -negate -evaluate multiply 2 \) -compose CopyOpacity -composite out.png

or

convert smiley.png -alpha off \( +clone -negate -evaluate add 50% \) -compose CopyOpacity -composite out.png

Adjust the factors 2 or 50% as desired