<!-- Any copyright is dedicated to the Public Domain. http://creativecommons.org/publicdomain/zero/1.0/ --> <html> <style> .reftest { background-color: rgba(0, 255, 0, 0.5); } .child { width: 100px; height: 100px; opacity: 0.5; background-color: rgb(0, 127, 0); } .topleft { float:left; width: 100px; height: 100px; /*First blend black rect with green in place*/ /* Cs = (1 - αb) x Cs + αb x B(Cb, Cs)*/ /* B(Cb, Cs) = | Cb - Cs | = (0, 0, 0) */ /* 0.5 * (0, 255, 0) + 0.5 * (0, 0, 0) = (0, 127.5, 0)*/ /* Now, composite the resulting color with src-over; the alpha is the original alpha for the top layer while the color is the blending result*/ /* co = αs x Fa x Cs + αb x Fb x Cb - this is premultiplied */ /* αo = αs + αb x (1 – αs) */ /* Source over: Fa = 1; Fb = 1 – αs */ /*co = 0.5 * 1 * (0, 127.5, 0) + 0.5 * 0.5 * (0, 255, 0) = (0, 63.5, 0) + 0.25 * (0, 255, 0) = (0, 127.5, 0)*/ /*ao = 0.5 + 0.5 * 0.5 = 0.75*/ /* Co = co/ao = (0, 127.5, 0) / 0.75*/ /* Now alpha composite on white background */ /*co = 0.75 * 1 * (0, 127.5, 0) / 0.75 + 1 * 0.25 * (255, 255, 255) = (0, 127.5, 0) + (63.75, 63.75, 63.75) = (63.75, 159, 63.75) = (64, 191, 64) */ background-color: rgb(64, 191, 64); } .topright { float:left; width: 100px; height: 100px; } .bottom { width:200px; height: 100px; clear:both; } </style> <div class="topleft"></div> <div class="reftest topright"></div> <div class="reftest bottom"></div> </html>