I rewrote the code given below to skip calculations and pixel setting when not needed (full opaque or full transparent pixels), as the content of my overlays is generally mostly transparent. Reduced processing time from ~0.17s to ~0.06s on 216x145px images.
function alphaOverlay($destImg, $overlayImg, $imgW, $imgH)
{
for($y=0;$y<$imgH;$y++)
{
for($x=0;$x<$imgW;$x++)
{
$ovrARGB = imagecolorat($overlayImg, $x, $y);
$ovrA = ($ovrARGB >> 24) << 1;
$ovrR = $ovrARGB >> 16 & 0xFF;
$ovrG = $ovrARGB >> 8 & 0xFF;
$ovrB = $ovrARGB & 0xFF;
$change = false;
if($ovrA == 0)
{
$dstR = $ovrR;
$dstG = $ovrG;
$dstB = $ovrB;
$change = true;
}
elseif($ovrA < 254)
{
$dstARGB = imagecolorat($destImg, $x, $y);
$dstR = $dstARGB >> 16 & 0xFF;
$dstG = $dstARGB >> 8 & 0xFF;
$dstB = $dstARGB & 0xFF;
$dstR = (($ovrR * (0xFF-$ovrA)) >> 8) + (($dstR * $ovrA) >> 8);
$dstG = (($ovrG * (0xFF-$ovrA)) >> 8) + (($dstG * $ovrA) >> 8);
$dstB = (($ovrB * (0xFF-$ovrA)) >> 8) + (($dstB * $ovrA) >> 8);
$change = true;
}
if($change)
{
$dstRGB = imagecolorallocatealpha($destImg, $dstR, $dstG, $dstB, 0);
imagesetpixel($destImg, $x, $y, $dstRGB);
}
}
}
return $destImg;
}
