text antialiasing issue with opacity < 1

[paulrouget]

I guess it's an antialiasing issue. The top text here has a dark outline. That makes text opacity animations very ugly.

<style>
  body {
    padding: 20px;
    background: rgb(65, 149, 250);
  }
</style>

<div style="opacity: 0.8; color: white">FOOBAR</div>
<div style="opacity: 1.0; color: white">FOOBAR</div>

[pcwalton]

$10 says it's one of those annoying glBlendFunc()/premultiplied alpha issues.

[glennw]

Yep, this is occurring on Linux as well, so probably an issue like @pcwalton mentioned above rather than an issue with the platform font rasterizing.

[glennw]

@pcwalton Changing https://github.com/glennw/webrender/blob/master/src/renderer.rs#L988 from 0,0,0,0 -> 1,1,1,0 fixes this issue. However, it introduces artifacts into the box shadows in the ref tests.

I think this is a correct fix - any thoughts on what effect that would have on box shadow and if it can be worked around? I'll investigate further - just posting this in case you know straight away what the issue might be?

[glennw]

Interestingly the blur on text_shadow_blur_a.html remains the same with that change (which is a text-shadow), but the blur on text_shadow_blur_ref.html (which is a box shadow) is what starts to show artifacts.

[pcwalton]

@glennw Hmm, but I think our blend mode is wrong if we're relying on (1,1,1,0) instead of (0,0,0,0).

[glennw]

I'm not sure what blend mode you would use.

A traditional alpha blend is:

Result.rgb = Dest.rgb*(1-Source.alpha) + Source.rgb * Source.alpha.

Pre-multipled alpha blend is:

Result.rgb = Dest.rgb*(1-Source.alpha) + Source.rgb.

In both cases, the destination color is going to be modulated by the inverse source alpha, so you will get black fringing if drawing text with alpha on a black FBO, which is the case for text with opacity at the moment.

Or is there another alternative?

[glennw]

Hmmm, perhaps we could make of separate blend func to store the alpha but just use the color. I'll try that.

[glennw]

I have a fix for this, but it exposes an unrelated issue in the texture cache - working on fixing that now.