Composite with drawn mask requires TrueColor

[tom_dl]

I have used the -composite option with 3 images to make a Masked Alpha Composition in the past like so:

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convert img1.tif img2.tif mask.tif -composite output.tif

But when I draw the mask rather than feed Imagemagick an image, this doesn't give expected results:

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convert input.tif \( -clone 0 -threshold 50372 \) \( -clone 0 -gamma -1 -fill black -draw "rectangle 128,504 425,826" \) -composite output.tif

Instead I need to use the -type option like so:

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convert input.tif \( -clone 0 -threshold 50372 \) \( -clone 0 -gamma -1 -fill black -draw "rectangle 128,504 425,826" \) -type TrueColor -composite output.tif

Why is the mask determining the image type rather than the 1st or 2nd image? Is it a bug that I should required -type TrueColor?

This is present in 7.0.3-7 but I can't upgrade to the latest version to see if this has been changed.

[fmw42]

Can you post your input TIFF file, so we can try your command? Why are you using -gamma -1? I thought -gamma was limited to values >0. What is the purpose of the -gamma -1?

Does this work for you? It seems to work fine for me using the logo: image. Note I have replace -gamma -1 with -fill white -colorize 100%, which seems to do the same as your -gamma -1 does. The output is type=palette.

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convert logo: \( -clone 0 -threshold 50% \) \( -clone 0 -fill white -colorize 100% -fill black -draw "rectangle 128,128 400,300" \) -composite output.tif

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convert logo: \( -clone 0 -threshold 50% \) \( -clone 0 -gamma -1 -fill black -draw "rectangle 128,128 400,300" \) -composite output.tif

http://www.fmwconcepts.com/misc_tests/output.tif

Tested with IM 6.9.8.3 Q16 Mac OSX and also IM 7.0.5.4 Q16 Mac OSX

[tom_dl]

Thanks for your reply! This TIFF file demonstrates the behaviour: https://www.dropbox.com/s/48ia9nquplfpa ... 9.tif?dl=0

I'm using -gamma -1 because http://www.imagemagick.org/Usage/canvas/#color_misc suggested it (it's really fast).

Neither of your lines works as intended for me, but adding -type TrueColor fixes both. I therefore assume this behaviour (I want to call it a bug, but I'm not sure what the intended behaviour is) is not caused by -gamma -1 or by the input image (I tried logo: and my TIFF and got the same results).

I get this with your line: https://www.dropbox.com/s/kfuebbphs9lk1 ... 2.tif?dl=0

And this when I add -type TrueColor:https://www.dropbox.com/s/0b5cw487yju6x ... T.tif?dl=0

Any idea if this is intended behaviour?

[snibgo]

I'm using -gamma -1 because http://www.imagemagick.org/Usage/canvas/#color_misc suggested it (it's really fast).

When Anthony wrote that page, "-gamma" probably just changed pixel values. But now it also changes metadata, by multiplying the previous gamma value by the value you give. So the image now has negative gamma value. Try to save that as a PNG, and it is changed to a more sensible value, and you get a warning. But save it as a miff, and the negative metadata is saved.

I suggest that, if you use "-gamma -1" or "-gamma 0" or whatever, you should then immediately "-set gamma 0.454545", which changes just the metadata.

[snibgo]

Neither of your lines works as intended for me, but adding -type TrueColor fixes both.

Leaving aside the gamma issue, the problem occurs because some versions of v7, eg v7.0.3-6, generally regards the alpha channel when v6 generally ignored it. So the cure is to "-alpha off" the mask image.