Using gd to highlight parts of an image.

[gurubob]

Hi,

I'm using the gd routines to display a map on the screen. What I would like to be able to do is to dynamically draw a polygon over a particular area to highlight that area.

To do this I load the map image first and then I create another image the same height and width which I then draw my polygon onto. I then use the ImageCopyMerge function to generate the final result.

The problem is that when I merge the two images the part of the polygon image that has no polygon on it is black and it darkens the image. If I make it white then it lightens the image. I can't seem to find a halfway or "dont process" colour.

Anyone got any ideas on this one?

Cheers,

• Bob -

[[deleted]]

How about make the polygon image a transparent GIF or PNG file format instead of the polygon on a black or white background.
Cheers

[majik-sheff]

The easiest solution is to draw the polygon directly onto the map with an alpha component specified in the color. The following is a complete program that demonstrates:

<?PHP
header("Content-type: image/png");

$points = array(10, 10, 10, 40, 50, 20); //Just here for an example :)
$polygonColor = 0xff0000;


define("TRANS25", 0x5f000000); //I like to define alpha values in an include file so I don't
define("TRANS50", 0x40000000); //have to do the computations over and over.  These are a few
define("TRANS75", 0x20000000); //common ones, just remember that higer values are more transparent.

$map = imagecreatefrompng("map.png");
imagealphablending($map, TRUE);  //this is the key line, without this alpha blending is turned off.

// this line just demonstrates how to use the predefined transparencies, just OR the color with the trans.
imagefilledpolygon($map, $points, count($points) / 2, $polygonColor | TRANS25);

imagepng($map);

?>