Bug 62141 – Incorrect rounding of color values during transitions

• Component: CSS
• Status: RESOLVED FIXED

[Boris Zbarsky]

Created attachment 96114 [details]
Testcase

BUILD: Current WebKit nightly (or current Chrome dev)

STEPS TO REPRODUCE:
1)  Load attached testcase.
2)  Examine the sum of the "r" and "b" values in the printed rgb() triplets.

EXPECTED RESULTS: Sum is always 255.

ACTUAL RESULTS: Sum is 254 for most of them.

ADDITIONAL NOTES: This is a regression from Safari 5.

To see why the expected results are expected, note that at time t/5s in the range [0, 1] in the transition the computed color should be:

  rgb(255*(1-t), 0, 255*t);

Now the color components will be rounded, but, for example, if 255*(1-t) is rounding to 254 then 255*t > 0.5.  And therefore 255*t should be rounding to 1, not to 0.

It looks like WebKit is just truncating the floating-point color values to get the interpolated color value instead of correctly rounding.

[Simon Fraser (smfr)]

Color values are stored as unsigned bytes, so rounding is unsurprising.

[Boris Zbarsky]

Rounding is unsurprising, but my point is that your rounding is wrong.  You're rounding down instead of rounding to nearest.

[Simon Fraser (smfr)]

Created attachment 96187 [details]
Better testcase

[Simon Fraser (smfr)]

Created attachment 96190 [details]
Patch

[Simon Fraser (smfr)]

http://trac.webkit.org/changeset/88214