php gd read raw pixel data

I have the following php script:

<?php
class GimFile {
    public $data = '';
    public $header = '';
    public $rgba = '';
    public $width = 0;
    public $height = 0;
    public function __construct($imagedata) {
        $this->data = $imagedata;
        if (substr($this->data, 1, 3) != 'GIM') {
            exit("This data is not in GIM format");
        }

        $this->header = substr($this->data, 0, 128);
        $this->rgba = substr($this->data, 128);

        // PHP can't unpack signed short from big-endian
        // so we unpack it as unsigned, and subtract 2^16 if >2^15
        $dimensions = array_values(unpack("n2", substr($this->header, 72, 76)));
        for($i = 0; $i < count($dimensions); $i++) {
            if($dimensions[$i] >= pow(2, 15)) {
                $dimensions[$i] -= pow(2, 16);
            }
        }

        list($this->width, $this->height) = $dimensions;
    }
    public function save($dest) {
        //create image
        $img = imagecreatetruecolor($this->width, $this->height);
        //fill by iterating through your raw pixel data
        for($x = 0; $x < $this->width; $x++) {
            for($y = 0; $y < $this->height; $y++) {
                $pos = ($y * $this->width + $x) * 4;
                list($red, $green, $blue, $alpha) = array_values(unpack("C4", substr($this->rgba, $pos, $pos+4)));
                $alpha >>= 1; // alpha transprancy is saved as 8bit, we need 7 bit
                $color = imagecolorallocatealpha ( $img, $red, $green, $blue, $alpha );
                imagesetpixel($img, $x, $y, $color);
            }
        }
        imagepng($img, $dest);
        imagedestroy($img);
    }
}

header('Content-type: image/png');
$contents = file_get_contents('gim');
$gim = new GimFile($contents);
$gim->save('lol.png');

?>

It is supposed to read get raw binary data as an argument, and save this as a .png file. This data, contains the following:

first 128 byte are the headers, of which offset 72 and 74 contain width and height respectiv开发者_如何学运维ely (signed short big endian). This data is parsed in the constructor. The rest of the data is raw RGBA data.

The save function, goes through the raw RGBA data (iterating through every pixel), finds the colour, and writes to a new image of same width and height. and then saves it to a file.

however, the results are wrong for some reason.

The following file is used that contains the data needed: http://2971.a.hostable.me/gim/gim

The following png is expected: http://2971.a.hostable.me/gim/expected.png

The following png is generated: http://2971.a.hostable.me/gim/output.png

To closest i came to the expected result, was by setting the $alpha to 0, but that resulted in this: http://2971.a.hostable.me/gim/noalpha.png

Can anyone help to fix the script to produce the expected result?

Thanks in advance.

Hosh

Instead of image I've got message 'The image "http://localhost/gim/gim.php" cannot be displayed, because it contains errors' written on PNG canvas. I have used gim file (65,393 bytes) and tested your script on localhost (has GD Library support). Besides, output file lol.png looks like rainbow (width:128px, height:128px, bitdepth:24). Are you sure this is right code (not some your modification that doesn't work)? I want to help, but can't understand that 'gim' format.

P.S. gim.php is my php script where I put your code in folder ./gim at localhost.

Tell me something, if you use 4-bytes to code every pixel in 128x128 image... raw data MUST be at least 65,536 bytes + header... so another question, in case this code is right, what's wrong with 'gim' file, because it's 65,393 bytes only?