Check pixels pattern - php

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.
}
}
}