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PHP Server Side Scripting Forum

    
GD color limit is too low
Skier88




msg:4291614
 10:11 pm on Apr 3, 2011 (gmt 0)

I just started learning about the GD library and made a small script to test a few features. I quickly found out that I was limited to 256 colors, but that creating the image with imagecreatetruecolor() instead of imagecreate() all but eliminates the issue. However, after a few modifications I saw the 256 color limit reappear, even though I am still using imagecreatetruecolor(). What is causing this and how do I fix it?

Here's my testing / playing around script. You'll notice that if you set the number of line segments (for loop's upper bound) to greater than 256 the line appears increasingly disjointed. This is because the rest of the line in being drawn in the background color, black.

<?php
$s=500;
$im=imagecreatetruecolor($s,$s);
$tmp=rand(1,$s-1);
$x=rand(0,1)?(rand(0,1)?0:$s-1):$tmp;
$y=$x&&$x<$s-1?(rand(0,1)?0:$s-1):$tmp;
$vx=0;
$vy=0;
for($i=0;$i<1000;$i++) {
$_x=$x;
$_y=$y;
$x=$x+(rand(0,1)?1:-1)+$vx;
$y=$y+(rand(0,1)?1:-1)+$vy;
if($x<0) {
$x=0;
$vx=-$vx;
}
if($x>=$s) {
$x=$s-1;
$vx=-$vx;
}
if($y<0) {
$y=0;
$vy=-$vy;
}
if($y>=$s) {
$y=$s-1;
$vy=-$vy;
}
$vx=$vx*.8+($x-$_x)*.2;
$vy=$vy*.8+($y-$_y)*.2;
$c=imagecolorallocate($im,0,(255-$i)%255,$i%255);
imageline($im,$_x,$_y,$x,$y,$c);
}
imagefilter($im,IMG_FILTER_GAUSSIAN_BLUR);
header('Content-type:image/png;');
imagepng($im);
imagedestroy($im);
?>

 

coopster




msg:4305476
 4:28 pm on Apr 28, 2011 (gmt 0)

The RGB components cannot go beyond that value:

These parameters are integers between 0 and 255 or hexadecimals between 0x00 and 0xFF.


[php.net...]

Little_G




msg:4305633
 9:59 pm on Apr 28, 2011 (gmt 0)

Hi,

Your script is, in some instances, calling
imagecolorallocate with the third argument (green) as a negative number, this causes imagecolorallocate to return false, which imageline is interpreting as black.
Also you're always setting the red argument to 0 which will limit you range of colours.

Andrew

Skier88




msg:4306082
 7:36 pm on Apr 29, 2011 (gmt 0)

Well that's embarrassing. Thanks for your replies though, and hopefully next time I'll wait a little longer before asking for help.

Little_G




msg:4306127
 9:28 pm on Apr 29, 2011 (gmt 0)

Don't worry can't-see-the-wood-for-the-trees syndrome is common amongst programmers, I know because I sometimes struggle with the condition myself.

Andrew

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