Compose Mask and Transparency using magick.. -composite

[svengineer99]

Running Windows ImageMagick 7.0.1-2 Q16 (64-bit) (2016-05-08)

I'm having trouble with alpha masking. Unfortunately the documentation at http://imagemagick.org/Usage/compose/#mask is obsolete (using composite command). Trying to use magick.. -composite I'm not getting the expected result:

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  magick -size 60x60 xc:none -fill red  -draw "circle 35,21 35,3"  m_src.png
  magick -size 60x60 xc:none -fill blue -draw "circle 21,39 24,57" m_bgnd.png
  magick -size 60x60 xc:   -draw "polygon 0,59 59,0, 0,0"          m_mask.png

  magick m_bgnd.png m_src.png -composite                 m_over.png
  magick m_bgnd.png m_src.png   m_mask.png -composite     m_over_masked.png

Produces an image with white portion of the mask turned black in what should be transparent background:


I guess I must have some problem with the magick .. -composite masking syntax?

[snibgo]

This looks like a v7 bug.

[fmw42]

It does not work in IM 6 either. I do not think you can use a mask image directly on two transparent images. You will need to multiply the mask times the alpha channel of the two images and then flatten them over a transparent background.

At 6.9.0.0 for example, the output is transparent, but one sees the full blue circle and half the red circle. So even that is not correct.

Both may be a bug in both cases, but I am not sure how IM handles the fact that there are two transparent image and transparency controlled by the mask image.

This will work:

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magick  m_bgnd.png m_src.png \( m_mask.png -write mpr:mask +delete \) \
\( -clone 0 -alpha extract mpr:mask  -compose multiply -composite \
-clone 0 +swap -alpha off -compose over -compose copy_opacity -composite +write show: \) \
\( -clone 1 -alpha extract \( mpr:mask -negate \) -compose multiply -composite \
-clone 1 +swap -alpha off -compose over -compose copy_opacity -composite +write show: \) \
-delete 0,1 -background none -compose over -flatten \
m_over_masked.png

[svengineer99]

Thanks for confirming it. Could you provide an example how to 'multiply the mask times the alpha channel of the two images and then flatten them over a transparent background' for the case above? My experience with getting IM syntax correct on my own is not so good.

Edit -sorry I see you included an example already. IN that case - thank you! I will try it out now..

[snibgo]

At 6.9.0.0 for example, the output is transparent, but one sees the full blue circle and half the red circle. So even that is not correct.

v6.9.2-5 gives that result, which is the correct result.

The mask is black at top-left and white at bottom right. So the effect masked "over" composite should be that we get the "dst" image top-left, and "src over dst" bottom-right.

[fmw42]

snibgo, I am not getting what you imply. I get a full blue circle and a half red circle. It should be two half circles, should it not?

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im6925 convert -size 60x60 xc:none -fill red  -draw "circle 35,21 35,3"  m_src.png
im6925 convert -size 60x60 xc:none -fill blue -draw "circle 21,39 24,57" m_bgnd.png
im6925 convert -size 60x60 xc:   -draw "polygon 0,59 59,0, 0,0"          m_mask.png

im6925 convert m_bgnd.png m_src.png m_mask.png -composite m_over_masked6925.png

Is this what you see?

For reference:

IM6944

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convert -size 60x60 xc:none -fill red  -draw "circle 35,21 35,3"  m_src.png
convert -size 60x60 xc:none -fill blue -draw "circle 21,39 24,57" m_bgnd.png
convert -size 60x60 xc:   -draw "polygon 0,59 59,0, 0,0"          m_mask.png

convert m_bgnd.png m_src.png m_mask.png -compose over -composite m_over_masked_6944.png

IM7016

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im7 magick -size 60x60 xc:none -fill red  -draw "circle 35,21 35,3"  m_src.png
im7 magick -size 60x60 xc:none -fill blue -draw "circle 21,39 24,57" m_bgnd.png
im7 magick -size 60x60 xc:   -draw "polygon 0,59 59,0, 0,0"          m_mask.png

im7 magick m_bgnd.png m_src.png  m_mask.png -composite     m_over_masked_7016.png

So to get two half circle, my code from above was:

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convert m_bgnd.png m_src.png \( m_mask.png -write mpr:mask +delete \) \
\( -clone 0 -alpha extract mpr:mask  -compose multiply -composite \
-clone 0 +swap -alpha off -compose over -compose copy_opacity -composite +write show: \) \
\( -clone 1 -alpha extract \( mpr:mask -negate \) -compose multiply -composite \
-clone 1 +swap -alpha off -compose over -compose copy_opacity -composite +write show: \) \
-delete 0,1 -background none -compose over -flatten \
m_over_masked.png

So clearly there is a bug, at minimum, in the IM 7 processing, since it does not even match that of IM 6.

[svengineer99]

@fmw42 - Thanks for the example. It works for me as you described. However, what I was really after was the half red circle over the full blue circle (masking one alpha image placed over another alpha image). Could I trouble you for another example how to do that? Actually just masking the half red circle (just masking an alpha image similar to http://stackoverflow.com/questions/1364 ... magemagick) would be enough for me, if there's a simpler implementation.

[snibgo]

Fred: Half a red circle, with a complete blue circle, yes, that's the image I get, and it's the correct result.

Where the mask is black, the result should be m_bgnd.png. It is.

Where the mask is white, the result should be m_src OVER m_bgnd. It is.

[fmw42]

I was interpreting the mask composite operation so as to take the blue transparent circle image where it was black and red transparent circle image where it was white. That is top left of first image and bottom right of second image. Thus two half circles.

[fmw42]

@fmw42 - Thanks for the example. It works for me as you described. However, what I was really after was the half red circle over the full blue circle (masking one alpha image placed over another alpha image). Could I trouble you for another example how to do that? Actually just masking the half red circle (just masking an alpha image similar to http://stackoverflow.com/questions/1364 ... magemagick) would be enough for me, if there's a simpler implementation.

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convert swCkD.png gHRFF.png \
\( -clone 0 -alpha extract \) \
\( -clone 1 -clone 0 -compose multiply -composite \) \
-delete 1,2 \
-alpha off -compose copy_opacity -composite \
result.png

[snibgo]

I was interpreting the mask composite operation so as to take the blue transparent circle image where it was black and red transparent circle image where it was white. That is top left of first image and bottom right of second image. Thus two half circles.

That's not what a masked composition should do.

Where the mask is white, the result should be the same as the un-masked composite.

[fmw42]

Where the mask is white, the result should be the same as the un-masked composite.

That has not been how I have interpretted masked composite with -compose overlay. When the two images are not transparent, you get the upper left for where the mask is black and the bottom right where the mask is white. So you should see two half circles.

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convert -size 60x60 xc:gray -fill red  -draw "circle 35,21 35,3"  m_src2.png
convert -size 60x60 xc:gray -fill blue -draw "circle 21,39 24,57" m_bgnd2.png
convert -size 60x60 xc:   -draw "polygon 0,59 59,0, 0,0"          m_mask2.png
convert m_bgnd.png m_src.png m_mask.png -compose over -composite m_over_masked2.png

So I assumed that the same would happen even if the images were transparent.

[snibgo]

Your inputs are opaque, so the result of "src OVER dst" is src.

But when src has any transparency, the result of "src OVER dst" is neither src nor dst.

[svengineer99]

Code: Select all

convert swCkD.png gHRFF.png \
\( -clone 0 -alpha extract \) \
\( -clone 1 -clone 0 -compose multiply -composite \) \
-delete 1,2 \
-alpha off -compose copy_opacity -composite \
result.png

[/quote]

Sorry, I'm not sure which should be the mask and which should be the source, but I'm not getting a good result with either:

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convert m_mask.png m_src.png ( -clone 0 -alpha extract ) ( -clone 1 -clone 0 -compose multiply -composite ) -delete 1,2 -alpha off -compose copy_opacity -composite result.png

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convert m_src.png m_mask.png ( -clone 0 -alpha extract ) ( -clone 1 -clone 0 -compose multiply -composite ) -delete 1,2 -alpha off -compose copy_opacity -composite result.png

Sorry, I have no idea why my images aren't embedding correctly..

[fmw42]

OK. You know the code better than I. I had hoped that functionally masking would be the same whether it was transparent or not. But that was why I was concerned at the beginning of my first reply regarding masking of two transparent images, leading me to create my command line that properly produced two half circles.