background-blending-alpha-ref.html (1518B)
1 <!-- 2 Any copyright is dedicated to the Public Domain. 3 http://creativecommons.org/publicdomain/zero/1.0/ 4 --> 5 <html> 6 <style> 7 .reftest { 8 background-color: rgba(0, 255, 0, 0.5); 9 } 10 11 .child { 12 width: 100px; 13 height: 100px; 14 opacity: 0.5; 15 background-color: rgb(0, 127, 0); 16 } 17 18 .topleft { 19 float:left; 20 21 width: 100px; 22 height: 100px; 23 24 /*First blend black rect with green in place*/ 25 /* Cs = (1 - αb) x Cs + αb x B(Cb, Cs)*/ 26 /* B(Cb, Cs) = | Cb - Cs | = (0, 0, 0) */ 27 /* 0.5 * (0, 255, 0) + 0.5 * (0, 0, 0) = (0, 127.5, 0)*/ 28 29 /* Now, composite the resulting color with src-over; the alpha is the original alpha for the top layer 30 while the color is the blending result*/ 31 /* co = αs x Fa x Cs + αb x Fb x Cb - this is premultiplied */ 32 /* αo = αs + αb x (1 – αs) */ 33 /* Source over: Fa = 1; Fb = 1 – αs */ 34 /*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)*/ 35 /*ao = 0.5 + 0.5 * 0.5 = 0.75*/ 36 /* Co = co/ao = (0, 127.5, 0) / 0.75*/ 37 38 /* Now alpha composite on white background */ 39 /*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) */ 40 41 background-color: rgb(64, 191, 64); 42 } 43 44 .topright { 45 float:left; 46 width: 100px; 47 height: 100px; 48 } 49 50 .bottom { 51 width:200px; 52 height: 100px; 53 clear:both; 54 } 55 56 </style> 57 58 <div class="topleft"></div> 59 <div class="reftest topright"></div> 60 <div class="reftest bottom"></div> 61 62 </html>