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    <p>The correct formula is something like this:</p> <pre><code>unsigned short blend(unsigned short fg, unsigned short bg, unsigned char alpha) { // Split foreground into components unsigned fg_r = fg &gt;&gt; 11; unsigned fg_g = (fg &gt;&gt; 5) &amp; ((1u &lt;&lt; 6) - 1); unsigned fg_b = fg &amp; ((1u &lt;&lt; 5) - 1); // Split background into components unsigned bg_r = bg &gt;&gt; 11; unsigned bg_g = (bg &gt;&gt; 5) &amp; ((1u &lt;&lt; 6) - 1); unsigned bg_b = bg &amp; ((1u &lt;&lt; 5) - 1); // Alpha blend components unsigned out_r = (fg_r * alpha + bg_r * (255 - alpha)) / 255; unsigned out_g = (fg_g * alpha + bg_g * (255 - alpha)) / 255; unsigned out_b = (fg_b * alpha + bg_b * (255 - alpha)) / 255; // Pack result return (unsigned short) ((out_r &lt;&lt; 11) | (out_g &lt;&lt; 5) | out_b); } </code></pre> <p>There is a shortcut you can use for dividing by 255. The compiler should be able to provide some strength reduction, but you might be able to do better by using the following formula instead:</p> <pre><code>// Alpha blend components unsigned out_r = fg_r * a + bg_r * (255 - alpha); unsigned out_g = fg_g * a + bg_g * (255 - alpha); unsigned out_b = fg_b * a + bg_b * (255 - alpha); out_r = (out_r + 1 + (out_r &gt;&gt; 8)) &gt;&gt; 8; out_g = (out_g + 1 + (out_g &gt;&gt; 8)) &gt;&gt; 8; out_b = (out_b + 1 + (out_b &gt;&gt; 8)) &gt;&gt; 8; </code></pre> <p>Note the large number of variables in the function... this is okay. If you try to "optimize" the code by rewriting the equations so that it creates fewer temporary variables, you are only doing work that the compiler already does for you. Unless you have a really bad compiler.</p> <p>If this is not fast enough, there are a few options for how to proceed. However, choosing the correct option depends on the results of profiling, how the code is used, and the target architecture.</p>
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    1. POCombining two 16 bits RGB colors with alpha blending
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    1. COThanks a LOT, I tested and this is working as should, (at least for white on black with half opacity, it gives a grey ;). I guess it doesn't give 0x8410 but 0x7BEF because of precision loss compared to 32bits colors?) Once again, thanks, I will study the code and try to understand it!
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    2. CONo, 0x7BEF is exactly correct, even if you use 32 bits. In binary it is 01111 011111 01111, as you can see, each component is exactly equal to 50% full scale, rounding down.
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    3. COIf you set the alpha to 132 instead of 127, then you will get 10000 100000 10000, which is slightly brighter than 50% gray. It is somewhat unspecified which direction you would like to round, but it rarely makes any visual difference.
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