How to calculate thickness of an object in PNG file using PHP GD library

Question

I have a PNG frame and I want to know its thickness. I am able to calculate the width/height of the image itself.

$frame = imagecreatefrompng('frame.png');
// get frame dimentions
$frame_width = imagesx($frame);
$frame_height = imagesy($frame);

But can't figure out a way to calculate thickness of frame, please see image below so see what I mean.

Any suggestions?

Answer 1

From the last answer it shows that there's no objects in a raster image file. However, you can do it by searching the first occurrence of transparent colour and the first occurrence of the non-transparent colour and calculate the distance of them (assumes that your image's blank area are all transparent).

Example code:

<?php
$img = imagecreatefrompng('./frame.png');//open the image
$w = imagesx($img);//the width
$h = imagesy($img);//the height

$nonTransparentPos = null;//the first non-transparent pixel's position
$transparentPos = null;//the first transparent pixel's position

//loop through each pixel
for($x = 0; $x < $w; $x++){
   for($y = 0; $y < $h; $y++){
        $color = imagecolorsforindex($img,imagecolorat($img,$x,$y));
        if($color['alpha'] < 127 && $nonTransparentPos === null){
            $nonTransparentPos = array($x,$y);
        }
        if($color['alpha'] === 127 && $transparentPos === null){
            $transparentPos = array($x,$y);
        }
   }
   //leave the loop if we have finished finding the two values.
   if($transparentPos !== null && $nonTransparentPos !== null){
        break;
   }
}
$length = $transparentPos[0]-$nonTransparentPos[0];//calculate the two point's x-axis distance
echo $length;
?>

Answer 2

There aren't any objects in a PNG file. You can only get a color (with a transparency) by coordinates with imagecolorat() & imagecolorsforindex().