ImageMagick 5.5.7 --
Semi-Transparent Draw and Overlay Bug -- FIXED

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ImageMagick Examples Preface and Index
Known and Fixed Bugs Index
Demonstrantion of a Semi-transparent white on transparent black bug, found in ImageMagick 5.5.7 and earlier. They were fixed during the ImageMagick version 6 development.

The background pattern on this page was choosen so that any transparency would be visible in a web browser, that understands PNG transparency (like Mozilla basied browsers).

This page is for reference for older IM users who may still have to deal with this bug. The examples on this page have not been re-created when/if the bug was fixed.


Generate an opaque rectangle

    convert -size 200x200 xc:none -fill "#808080" \
            -draw "rectangle 0,0 150,100" rectangle.png

[IM Output]

and a white circle (Semi-Transparent Bug in -draw command happens here)

    convert -size 200x200 xc:none -fill "#FFFFFF" \
            -draw "circle 125,125 73,73" circle_white.png

[IM Output]

If the bug happened the you will see dark, near black, edges to the pure white circle.

Make it semi-transparent

    convert circle_white.png  -fill "#FFFFFF80" -opaque "#FFFFFF" circle.png

[IM Output]

WARNING: This is not a practical example of generating a semi-transparent white circle, and should not be taken as a the best method to do so.

Overlay circle on rectangle (Semi-Transparency Bug in overlays happens here)

    composite circle.png rectangle.png alpha.png

[IM Output]

If the bug happens here, the result should be that while the overlays grey rectangle becomes a lighter color, the rest of the semi-transparent white circle becomes grey in color, instead of remaining unchanged.

Overlay the above result onto a white background, so we can see the result on browsers like IE.

    composite alpha.png  -size 200x200 xc:white  result.png

[IM Output]

The final result above should be a only a slight whitening of the grey rectangle, where the circle overlaps. Anywhere else and the white circle should disappear (semi-transparent white on white gives white!).

If the bug is present you will see a grey circle instead, and due to the presence of the bug in the "-draw" command, the edges of the circle will appear even darker, instead of shading into the white background.


Correct results with IM version 6

This is what the above image should look like (generated with IM version 6)

[IM Output]

NOTE: Due to the inaccurate way the white circle is made semi-transparent (using "-fill" and "-opaque") there is a thin almost white' border between the dark rectangle and the overlayed circle. This is not a bug, just incorrect image handling.

If I had generated the semi-transparent circle using this (more correct) command instead (only valid in IMv6)...

    convert circle_white.png  -channel A -fx A/2  circle.png

Then we will not see the thin white border in the final result.
[IM Output]


Created: 15 December 2003
Updated: 28 April 2005
Author: Anthony Thyssen, <A.Thyssen@griffith.edu.au>
Examples Generated with: [version image]
URL: http://www.imagemagick.org/Usage/bugs/trans_bug/