[magick-users] Where is the background coming from

[Anthony Thyssen]

- Tong - on  wrote...
| Hi,=20
| 
| The background of pic1.png has been cleared (using gimp). After the
| following command (to produce the sketch effect), the cleared background
| shows up again in pic1.gif. I'm wondering why the cleared background can
| still managed to show up, and how can I fix the problem.=20
| 
|   convert pic1.png \( +clone -median 2 -blur 0x7 -negate \) \
|     -matte -channel A -evaluate set 50% -compose plus -composite pic1.gif
| 
Of course the background shows up again.  You replaced all transparnency
values with 50% in BOTH images, added together that is 100% opaque!

Try using
     -evaluate multiply .5
or   -evaluate divide 2
Instead.  Do not use a percentage, as that is a percentage of color
and not 1/2.

Also note that GIF does not allow semi-transparency, so your source
image better not be using any semi-transparency.

| Here are the picture and the result.=20
| 
| http://xpt.sourceforge.net/download/pic1.png
| http://xpt.sourceforge.net/download/pic1.gif
| 
| I've also tried to specify -channel RGBA, but the result is all grayed.
| 
|   convert pic1.png \( +clone -median 2 -blur 0x7 -negate \) \
|     -matte -channel RGBA -evaluate set 50% -compose plus -composite pic1.=
| gif
| 
Naturally you just SET every color to 50% grey and 50% transparent!

See IM Examples
  Evaluate, Faster Simple Math Operations
     http://www.imagemagick.org/Usage/transform/#evaluate


  Anthony Thyssen ( System Programmer )    <A.Thyssen at griffith.edu.au>
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   Never play leap frog with a Unicorn    -- Grand Wizard Murphy
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     Anthony's Home is his Castle     http://www.cit.gu.edu.au/~anthony/