With PHP/GD, how do I trim an image?

Question

I want to mimic the trim behavior of Photoshop (crop the area that is covered by the same color on all sides) with GD and PHP but I'm lacking ideas on how to accomplish this.

If someone has any idea on how to do this I would very much like to hear it.

Thanks in advance.

Answer 1

The easiest solution I can think of is:

1. Find out the color of the top left pixel.
2. Go through the image line by line, horizontally and vertically, starting on the top, bottom , left hand, and right hand side respectively. If a line consists of ONLY the color encountered in the top left corner it is trimmable empty space. Omit that line from the resulting image by increasing the x or y coordinate of your imagecopy*() function.

Untested but might work.

BTW, ImageMagick can do this out of the box: http://www.imagemagick.org/Usage/crop/#trim

Answer 2

Here's a sample function (which also can add padding):

// Trims an image then optionally adds padding around it.
// $im  = Image link resource
// $bg  = The background color to trim from the image
// $pad = Amount of padding to add to the trimmed image
//        (acts simlar to the "padding" CSS property: "top [right [bottom [left]]]")
function imagetrim(&$im, $bg, $pad=null){

    // Calculate padding for each side.
    if (isset($pad)){
        $pp = explode(' ', $pad);
        if (isset($pp[3])){
            $p = array((int) $pp[0], (int) $pp[1], (int) $pp[2], (int) $pp[3]);
        }else if (isset($pp[2])){
            $p = array((int) $pp[0], (int) $pp[1], (int) $pp[2], (int) $pp[1]);
        }else if (isset($pp[1])){
            $p = array((int) $pp[0], (int) $pp[1], (int) $pp[0], (int) $pp[1]);
        }else{
            $p = array_fill(0, 4, (int) $pp[0]);
        }
    }else{
        $p = array_fill(0, 4, 0);
    }

    // Get the image width and height.
    $imw = imagesx($im);
    $imh = imagesy($im);

    // Set the X variables.
    $xmin = $imw;
    $xmax = 0;

    // Start scanning for the edges.
    for ($iy=0; $iy<$imh; $iy++){
        $first = true;
        for ($ix=0; $ix<$imw; $ix++){
            $ndx = imagecolorat($im, $ix, $iy);
            if ($ndx != $bg){
                if ($xmin > $ix){ $xmin = $ix; }
                if ($xmax < $ix){ $xmax = $ix; }
                if (!isset($ymin)){ $ymin = $iy; }
                $ymax = $iy;
                if ($first){ $ix = $xmax; $first = false; }
            }
        }
    }

    // The new width and height of the image. (not including padding)
    $imw = 1+$xmax-$xmin; // Image width in pixels
    $imh = 1+$ymax-$ymin; // Image height in pixels

    // Make another image to place the trimmed version in.
    $im2 = imagecreatetruecolor($imw+$p[1]+$p[3], $imh+$p[0]+$p[2]);

    // Make the background of the new image the same as the background of the old one.
    $bg2 = imagecolorallocate($im2, ($bg >> 16) & 0xFF, ($bg >> 8) & 0xFF, $bg & 0xFF);
    imagefill($im2, 0, 0, $bg2);

    // Copy it over to the new image.
    imagecopy($im2, $im, $p[3], $p[0], $xmin, $ymin, $imw, $imh);

    // To finish up, we replace the old image which is referenced.
    $im = $im2;
}