[Webkit-unassigned] [Bug 186804] New: Variable fonts: FeatureVariations conditions on multiple axes seem to be ORed, not ANDed

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=186804

            Bug ID: 186804
           Summary: Variable fonts: FeatureVariations conditions on
                    multiple axes seem to be ORed, not ANDed
           Product: WebKit
           Version: Safari 11
          Hardware: Macintosh
                OS: macOS 10.13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: lorp at lorp.org

Created attachment 343053

  --> https://bugs.webkit.org/attachment.cgi?id=343053&action=review

A TrueType font with 2 variation axes and 2-dimensional FeatureVariations

FeatureVariations in variable fonts allow glyph substitution to happen based on location in the designspace, as specified by CSS font-variation-settings (and other methods of setting axis positions). The spec allows substitutions to be made in any "rectangular" region of the font's designspace, by means of sets of conditions. For example, a small rectangle within a 2-dimensional designspace can be defined by giving a min and max in axis 0 and a min and max in axis 1. In the language of the spec, this would be one ConditionSet containing two ConditionTables. From the spec:

https://docs.microsoft.com/en-us/typography/opentype/spec/chapter2#featurevariations-table
“For a given condition set, conditions are conjunctively related (boolean AND): all of the specified conditions must be met in order for the associated feature table substitution to be applied.”

As the spec states, for such multi-dimensional ConditionSets to resolve correctly, the ConditionTables must process the ConditionTables in a chain of AND relations. All of the ConditionTables need to resolve to true for the ConditionSet to resolve to true.

However, Safari seems to use a chain of OR relations, so that ConditionSets are resolveing to true too often.

This CodePen uses a test font that contains two glyphs: u and u.alt (a square looking like a missing glyph), to demonstrate the issue. When handled correctly, FeatureVariations in the font cause the "u" in parts of the 2-dimensional designspace to switch to "u.alt". The total region for the substitutions in this font are non-rectangular and non-contiguous, are are defined by multiple ConditionSets.

https://codepen.io/lorp/pen/RMmRVB

It works on Chrome, but on Safari we see no substitutions. See screengrabs and attached font.

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