[Webkit-unassigned] [Bug 54347] New: Angled -webkit-linear-gradient with hard edges doesn't fit to pixel grid consistently

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=54347

           Summary: Angled -webkit-linear-gradient with hard edges doesn't
                    fit to pixel grid consistently
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Other
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: eevee.webkit at veekun.com


Created an attachment (id=82246)
 --> (https://bugs.webkit.org/attachment.cgi?id=82246&action=review)
testcase

See attached testcase, which uses -webkit-linear-gradient and background-size to draw diagonal stripes of various sizes along a square background.  There are two similar problems here, which I assume have the same root cause.

1. When using "top left" as the anchor for the gradient, current Chromium builds sometimes draw one or two pixels the wrong color at the left edge of each gradient box, resulting in a jarring effect like this:

#####
####-
###--
##---
-----

2. When using "-45deg", the effect is worse; at virtually any background-size, bands of the gradient fluctuate in width by a pixel or two, resulting in an odd studded appearance.  It's so bad that with a background-size of 2px, this /diagonal/ gradient renders as a pattern of /vertical/ lines.

The spec doesn't legislate precisely how to render gradients, but current Gecko draws smooth diagonal lines at any size and zoom level, which seems more correct (or at least more useful).

Both problems occur as long as two color stops collide within a pixel, even if they don't have exactly the same length.  If they're at least a pixel apart, the rendering is perfect.

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