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Hixie's Natural Log

2008-03-29 02:12 UTC The antialiasing controversy in Acid3

The Acid3 test has a rendering subtest that checks the positioning of text in particular conditions (absolutely-positioned generated content with embedded fonts). To get precise results, I used a single glyph from the Ahem font, which has well-known metrics. My plan was to have the glyph set up so that a perfect white 20×20 pixel square glyph from the font would be overlaid exactly on a 20×20 pixel square red background, thus hiding everything when things lined up. I positioned this test in the upper right hand corner, snug against the black border of the test.

The problem with this test is that on some platforms, specifically Mac platforms with LCD antialiasing, the font rendering system actually renders the glyph using sub-pixel effects, which ends up overlapping the border and makes the test not look the same as the reference rendering.

This would affect any browser, but only on Mac. Unfortunately, the WebKit team "fixed" this problem by simply hard-coding the Ahem font and making it not antialias.

Now, I argue that this is a bug in the antialiasing, but sadly there's no real spec for the antialiasing and so other people argue that it shouldn't be in the test in the first place, whether I'm right or wrong. What we all agree on is that the font-specific hack is lame. (It's especially bad with this font because Ahem is supposed to be a testing font and we specifically don't want it going down different codepaths!)

So Hyatt and I came to a deal. I would move the test down and to the left one pixel, so it doesn't affect the border anymore, he would accept to remove the hack, and would fix one additional bug (a background-position rounding bug).

This doesn't affect Opera's news of their progress, as their build is currently only available on Windows.


The test will probably have a few more minor changes as people track down the last few remaining problems, in particular in the SVG subtests and on the performance part of test 26. (Test 26 is supposed to track the incremental speed of computer hardware, but it hasn't been really calibrated well yet, I just estimated what the numbers should be.)

It's great to see WebKit and Opera work so hard on interoperability issues such as those brought up by Acid3. The Microsoft and Mozilla teams are currently in the "crunch time" of their respective browsers' releases, so it's expected that they wouldn't be working on this at this time — at the end of a release cycle, stability, performance, and user experience are usually much more critical. Hopefully once IE8 and Firefox3 are released they will be able to turn once more to the world of standards and we'll see big improvements of the Web platform again.