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Editorial Team
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Asked: 2026-05-30T23:54:25+00:00

I was trying to code up some color manipulation code and was stuck for

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I was trying to code up some color manipulation code and was stuck for a long time with alpha, then I (2 hours later) realized browsers render rgba differently.

I created this test: http://jsbin.com/adekez/2/
add /edit to look at the code

Here are the results in 4 browsers in OSX Lion: https://i.stack.imgur.com/i9wSw.jpg

That is an element with a background of rgba(0,0,0,0.5) on top of white.

My hex calculator says: FF÷2 = 7F.8 which means

Safari at #808080 is correct
Firefox at #7F7F7F is off by 1
Opera at #7E7E7E is off by 2
Chrome at #929292 is way off

What’s going here, is this a bug in Chrome or do I just not know about colors (true)?

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1 Answer

  1. Editorial Team

    First off

    Well, actually Safari and Firefox are both correct:
    0xFF = 255, 255 / 2 = 127.5. So 128 may be correct (0x80), but also 127 (0x7F) – depending on the rounding convention of the browser.

    Explantion of the issue

    These are rounding issues. I don’t quite get why there are rounding issues, since 0.5 is representable in binary numbers without precision loss, but there are, in fact rounding issues:

    Opera:
    Opera Screenshot

    Chrome:
    Chrome Screenshot

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