SAME_OR_POSITIVE ambiguity

[simoncozens]

Consider these two simple glyph flag bits:

Mask	Name	Description
0x02	X_SHORT_VECTOR	Bit 1: If set, the corresponding x-coordinate is 1 byte long. If not set, it is two bytes long. For the sign of this value, see the description of the X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR flag.
0x10	X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR	Bit 4: This flag has two meanings, depending on how the X_SHORT_VECTOR flag is set. If X_SHORT_VECTOR is set, this bit describes the sign of the value, with 1 equalling positive and 0 negative. If X_SHORT_VECTOR is not set and this bit is set, then the current x-coordinate is the same as the previous x-coordinate. If X_SHORT_VECTOR is not set and this bit is also not set, the current x-coordinate is a signed 16-bit delta vector.

If X_SHORT_VECTOR is not set, and X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR is set, how many bytes should be read from the xCoordinates array?

• One interpretation: X_SHORT_VECTOR is not set, so "the corresponding x-coordinate... is two bytes long".
• Another interpretation: X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR is set, so "the current x-coordinate is the same as the previous x-coordinate". Therefore, no bytes need to be read.

Apparently the correct answer is that no bytes should be read. The problem is that "the corresponding x-coordinate is two bytes long" is explicitly stated in the text, and "no bytes should be read" is not explicitly stated in the text (the fact that the current x-coordinate is the same is the previous does not imply anything about storage), so a correct implementation should discard the explicit meaning of the specification in preference for the implicit meaning.

Please make the correct interpretation "no bytes should be read" more explicit.

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• ID: b2a9a6a1-a9b9-0864-8982-322da821389e
• Version Independent ID: a42612e3-dc1d-f980-fe4a-4880d1fd0086
• Content: glyf - Glyf data table (OpenType 1.8.4) - Typography
• Content Source: typographydocs/opentype/spec/glyf.md
• Product: typography
• GitHub Login: @PeterCon
• Microsoft Alias: alib

[PeterCon]

You're correct: in the case X_SHORT_VECTOR is not set and X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR is set, then no bytes are read. This should be clarified.

[PeterCon]

Proposed revised description for X_SHORT_VECTOR flag:

Bit 1: If set, the corresponding x-coordinate is 1 byte long, and the sign is determined by the X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR flag. If not set, its interpretation depends on the X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR flag: If that other flag is set, the x-coordinate is the same as the previous x-coordinate, and no element is added to the xCoordinates array. If both flags are not set, the corresponding element in the xCoordinates array is two bytes and interpreted as a signed integer. See the description of the X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR flag for additional information.

Analogous wording will be used for Y_SHORT_VECTOR.

[PeterCon]

Addressed in OpenType 1.9. Closing.