Problem on overlaying translucent images (GD)

[davidpage]

Hi all,

 

In php if I draw two translucent shapes with some parts of the one overlaying on the other, the overlaid parts will have a darker colour:

 

i.e. the following code...

<?php
$im = ImageCreateTrueColor(150,150);
$blue = ImageColorAllocate($im,150,150,255);
ImageFilledRectangle($im,0,0,150,150,$blue);
$gray = ImageColorResolveAlpha($im,70,70,70,63);
ImageFilledRectangle($im,10,10,80,80,$gray);
ImageFilledRectangle($im,60,60,120,120,$gray);
header('Content-Type: image/png');
ImagePNG($im);
?>

... will give an image as follows:

 

However, what should I do in order to eliminate the discrepancy in the colour of the two areas (the overlaid parts have the same colour as the non-overlaid parts)? i.e. I'm planning to create a image like the following (I've added a red circle to show that the gray area is really translucent)...

...which I made using the following code:

 

<?php
$im = ImageCreateTrueColor(150,150);
$blue = ImageColorAllocate($im,150,150,255);
$red = ImageColorAllocate($im,255,0,0);
ImageFilledRectangle($im,0,0,150,150,$blue);
$gray = ImageColorResolveAlpha($im,70,70,70,63);
ImageFilledEllipse($im,50,90,40,40,$red);
ImageFilledPolygon($im,array(10,10,10,80,60,80,60,120,120,120,120,60,80,60,80,10),8,$gray);
header('Content-Type: image/png');
ImagePNG($im);
?>

 

In my actual project, the areas are of course more complex than this simple rectangle problem, and so I need an easier way to achieve this, e.g. by adding a suitable command while I draw the rectangles (or ellipses or polygons).

 

Thanks!

[davidpage]

*bump*