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laurikar on 04/04/13

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Last Edited at 04/04/13 09:48pm

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Photoshop overlay blending with opacity in PHP

/ Published in: PHP

This function mimics Photoshop overlay blending by accepting two RGB arrays, one which will be overlayed and one which will overlay. Opacity is optional.

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/**
 * Overlay blending
 *
 * This function mimics Photoshop overlay blending mode by accepting two RGB arrays,
 * one which will be overlayed and one which will overlay. The exact equation for overlay
 * is not known, however Kevin Jensen has a pretty accurate equation available on his website.
 * Please see http://www.venture-ware.com/kevin/coding/lets-learn-math-photoshop-blend-modes/
 *
 * @param array $bottom Color to be overlayed formatted as RGB array, i.e. for red array(255, 0, 0)
 * @param array $top Color which overlays $bottom formatted as RGB array, i.e. for white array(255, 255, 255)
 * @param float $opacity Optional opacity to be applied to the overlayed color in relation to $bottom
 *
 * @return array Color resulted in overlaying formatted as RGB array
 */
function blend_overlay($bottom, $top, $opacity = NULL) {
	// Overlay $bottom with $top
	$overlay = array();
	foreach ($bottom as $i => $a) {
		$b = $top[$i];
		if ($a < 128) {
			$overlay[$i] = (int) (2 * $b * $a / 255);
		} else {
			$overlay[$i] = (int) (255 * (1 - 2 * (1 - $b / 255) * (1 - $a / 255)));
		}
	}
 
	// Apply opacity to $overlay in relation to $bottom
	if (isset($opacity)) {
		foreach ($overlay as $i => $b) {
			$a = $bottom[$i];
			$overlay[$i] = (int) ((1 - $opacity) * $a + $opacity * $b);
		}
	}
 
	return $overlay;
}