Yesterday I thought about visualizing photo geodata. While tinkering with different ideas, I thought about making it look like a real paper photograph placed on a real map.

Of course it had to be automated somehow. So I spent a few hours with PHP's GD library functions. It's really cool what you can do with it.

Let's start with the result:

The image above was created by my MapDecorate class. Download, use and modify it as you like. It's licensed under the Creative Commons Attribution-Share Alike 3.0 license.

Here is how it works:

1. fetch a map image from the Google Static Maps API for given coordinates
2. convert the map colors to sepia
3. place an X on the center
4. put the compass on top
5. add a paper texture
6. do an address lookup on the coordinates and if found write it next to the X
7. write a custom text at the bottom
8. fetch an image from a given URL
9. optionally grayscale it
10. apply some grain texture to the photo
11. rotate the photo and place it on the map
12. apply a mask to cut out the map border
13. cache the map image for later reuse

The techniques I used are quite similar to the ones in my MonsterID script. It is basically overlaying different PNG images with alpha transparency. However there are a few things in it that took me a while to get right.

One thing was to rotate the photo but retain a transparent background. The trick is to pass a -1 as background color to imagerotate:

$img = imagerotate($img, 5, -1);

The other question was how to apply the alpha mask for cutting the border. There is no builtin function libGD so I had to do it on my own. Here is what I came up with:

function imagealphamask(&$img, &$mask){
    $width  = imagesx($img);
    $height = imagesy($img);
 
    imagealphablending($img, false);
    for ($x = 0; $x < $width; $x++) {
        for ($y = 0; $y < $height; $y++) {
 
            // current color
            $rgb = imagecolorat($img, $x, $y);
            $r   = ($rgb >> 16) & 0xFF;
            $g   = ($rgb >> 8) & 0xFF;
            $b   = $rgb & 0xFF;
 
            // red channel of mask
            $rgb   = imagecolorat($mask, $x, $y);
            $alpha = ($rgb >> 16) & 0xFF;
            $alpha = floor($alpha/2);
 
            $color = imagecolorallocatealpha($img,$r,$g,$b,$alpha);
            imagesetpixel($img,$x,$y,$color);
        }   
    }   
    imageSaveAlpha($img, true);
}

Be sure both passed images have the same dimensions! The function surely can be improved. It currently only uses the red part of the given mask image ¹⁾ and it's really slow.

Here's another example with a color photo and the reverse geocoding in action:

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