Question

How can I check for a pixel pattern in PHP?

I mean I wanna use as condition that pixel A has xxx value and the following pixel B has another value yyy.

This is what I wrote:

$img = imagecreatefrompng("myimage.png"); 


$w = imagesx($img);
$h = imagesy($img);

for($y=0;$y<$h;$y++) {
    for($x=0;$x<$w;$x++) {
        $rgb = imagecolorat($img, $x, $y);
        $r = ($rgb >> 16) & 0xFF;
        $g = ($rgb >> 8) & 0xFF;
        $b = $rgb & 0xFF;        
        echo "#".$r.$g.$b.",";
        $pixel = $r.$g.$b;
        if ($pixel == "481023" and $pixel+1???
    }
    echo "<br />\r\n";
} 

I’d like to ask also if I can speed up the whole thing by incrementing the $x value by 2 every for cycle. This because I have a pattern of 2 pixels, maybe I can use something like:

for($x=0;$x<$w;$x+2) {
    //...
    if ($pixel == "xxx") {//check the following pixel}
    else if ($pixel == "yyy") {//check the previous pixel}
}

Answer 1

You might want to define a function like:

function getpixelat($img,$x,$y) {
    $rgb = imagecolorat($img,$x,$y);
    $r = dechex(($rgb >> 16) & 0xFF);
    $g = dechex(($rgb >> 8) & 0xFF);
    $b = dechex($rgb & 0xFF);
    return $r.$g.$b;
}

Notice the dechex – you need this if you want it to look like an HTML colour code. Otherwise “white” would be 255255255 instead of ffffff and you’d also get ambiguous colours – is 202020 a dark gray (20,20,20) or “red with a slight hint of blue” (202,0,20)?

Once you have this, it should be a simple matter:

for( $y=0; $y<$h; $y++) {
   for( $x=0; $x<$w; $x++) {
       $pixel = getpixelat($img,$x,$y);
       if( $pixel == "481023" && getpixelat($img,$x+1,$y) == "998877") {
           // pattern! Do something here.
           $x++; // increment X so we don't bother checking the next pixel again.
       }
   }
}