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What's wrong with transparency by GD?
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Subject Author Date
What's wrong with transparency by GD? Ela 04-30-2008
Posted by Ela on April 30, 2008, 4:05 am
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 After testing different parameters, I'm still unable to obtain 2 transparent
circles. Could anybody tell me what's wrong in the following codes?

#!/usr/bin/perl
use GD;

$im = new GD::Image(550,450);
$white = $im->colorAllocate(255,255,255);
$black = $im->colorAllocate(0,0,0);
$red = $im->colorAllocateAlpha(255,0,0,119);
$blue = $im->colorAllocateAlpha(0,0,255,119);
$im->transparent($white);
$im->alphaBlending(1);
$im->setStyle($black);

$im->filledArc(460,380,95,75,0,360,$red);
$im->filledArc(500,380,95,75,0,360,$blue);

open(IMG, ">test.png");
binmode IMG;
print IMG $im->png;
close IMG;



Posted by smallpond on April 30, 2008, 10:11 am
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> After testing different parameters, I'm still unable to obtain 2 transparent
> circles. Could anybody tell me what's wrong in the following codes?
>
> #!/usr/bin/perl
> use GD;
>
> $im = new GD::Image(550,450);
> $white = $im->colorAllocate(255,255,255);
> $black = $im->colorAllocate(0,0,0);
> $red = $im->colorAllocateAlpha(255,0,0,119);
> $blue = $im->colorAllocateAlpha(0,0,255,119);
> $im->transparent($white);
> $im->alphaBlending(1);
> $im->setStyle($black);
>
> $im->filledArc(460,380,95,75,0,360,$red);
> $im->filledArc(500,380,95,75,0,360,$blue);
>
> open(IMG, ">test.png");
> binmode IMG;
> print IMG $im->png;
> close IMG;

"Blending mode is not available when drawing on palette images."
You need to be in true color mode, the default is palette mode.
--S

Posted by Ela on April 30, 2008, 11:20 am
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> "Blending mode is not available when drawing on palette images."
> You need to be in true color mode, the default is palette mode.
> --S

You're correct. But why does color mode change lead to background
blackening?



Posted by smallpond on April 30, 2008, 12:05 pm
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> > "Blending mode is not available when drawing on palette images."
> > You need to be in true color mode, the default is palette mode.
> > --S
>
> You're correct. But why does color mode change lead to background
> blackening?

New questions should go in new threads.

You need to initialize the background.

# Draw a transparent background
$bkg = $im->colorAllocateAlpha(0,0,0,127);
$im->transparent($bkg);
$im->alphaBlending(0); # replace pixels
$im->filledRectangle(0,0,549,449,$bkg);

# Now draw with blending
$im->alphaBlending(1);

now draw your circles


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