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VDMX - Vertical Device Metrics (OpenType 1.8.1)

  • Artikkeli

The VDMX table relates to OpenType™ fonts with TrueType outlines. Under Windows, the usWinAscent and usWinDescent values from the 'OS/2' table will be used to determine the maximum black height for a font at any given size. Windows calls this distance the Font Height. Because TrueType instructions can lead to Font Heights that differ from the actual scaled and rounded values, basing the Font Height strictly on the yMax and yMin can result in “lost pixels.” Windows will clip any pixels that extend above the yMax or below the yMin. In order to avoid grid fitting the entire font to determine the correct height, the VDMX table has been defined.

The VDMX table consists of a header followed by groupings of VDMX records

VDMX Header:

Type Name Description
uint16 version Version number (0 or 1).
uint16 numRecs Number of VDMX groups present
uint16 numRatios Number of aspect ratio groupings
RatioRange ratRange[numRatios] Ratio record array.
Offset16 offset[numRatios] Offset from start of this table to the VDMXGroup table for a corresponding RatioRange record.

RatioRange Record:

Type Name Description
uint8 bCharSet Character set (see below).
uint8 xRatio Value to use for x-Ratio
uint8 yStartRatio Starting y-Ratio value.
uint8 yEndRatio Ending y-Ratio value.

Ratios are set up as follows:

For a 1:1 aspect ratio Ratios.xRatio = 1;
Ratios.yStartRatio = 1;
Ratios.yEndRatio = 1;
For 1:1 through 2:1 ratio Ratios.xRatio = 2;
Ratios.yStartRatio = 1;
Ratios.yEndRatio = 2;
For 1.33:1 ratio Ratios.xRatio = 4;
Ratios.yStartRatio = 3;
Ratios.yEndRatio = 3;
For all aspect ratios Ratio.xRatio = 0;
Ratio.yStartRatio = 0;
Ratio.yEndRatio = 0;

All values set to zero signal the default grouping to use; if present, this must be the last Ratio group in the table. Ratios of 2:2 are the same as 1:1.

Aspect ratios are matched against the target device by normalizing the entire ratio range record based on the current X resolution and performing a range check of Y resolutions for each record after normalization. Once a match is found, the search stops. If the 0,0,0 group is encountered during the search, it is used (therefore if this group is not at the end of the ratio groupings, no group that follows it will be used). If there is not a match and there is no 0,0,0 record, then there is no VDMX data for that aspect ratio.

Note that range checks are conceptually performed as follows:

(deviceXRatio == Ratio.xRatio) && (deviceYRatio >= Ratio.yStartRatio) && (deviceYRatio <= Ratio.yEndRatio)

Each ratio grouping refers to a specific VDMX record group; there must be at least 1 VDMX group in the table.

The bCharSet value is used to denote cases where the VDMX group was computed based on a subset of the glyphs present in the font file. The semantics of bCharSet is different based on the version of the VDMX table. It is recommended that VDMX version 1 be used. The currently defined values for character set are:

Character Set Values, Version 0:

Value Description
0 No subset; the VDMX group applies to all glyphs in the font. This is used for symbol or dingbat fonts.
1 Windows ANSI subset; the VDMX group was computed using only the glyphs required to complete the Windows ANSI character set. Windows will ignore any VDMX entries that are not for the ANSI subset (i.e. ANSI_CHARSET)

Character Set Values, Version 1:

Value Description
0 No subset; the VDMX group applies to all glyphs in the font. If adding new character sets to existing font, add this flag and the groups necessary to support it. This should only be used in conjunction with ANSI_CHARSET.
1 No subset; the VDMX group applies to all glyphs in the font. Used when creating a new font for Windows. No need to support SYMBOL_CHARSET.

VDMX groups immediately follow the table header. Each set of records (there need only be one set) has the following layout:

VDMXGroup Table:

Type Name Description
uint16 recs Number of height records in this group
uint8 startsz Starting yPelHeight
uint8 endsz Ending yPelHeight
vTable entry[recs] The VDMX records

vTable Record:

Type Name Description
uint16 yPelHeight yPelHeight to which values apply.
int16 yMax Maximum value (in pels) for this yPelHeight.
int16 yMin Minimum value (in pels) for this yPelHeight.

This table must appear in sorted order (sorted by yPelHeight), but need not be continous. It should have an entry for every pel height where the yMax and yMin do not scale linearly, where linearly scaled heights are defined as:

Hinted yMax and yMin are identical to scaled/rounded yMax and yMin

It is assumed that once yPelHeight reaches 255, all heights will be linear, or at least close enough to linear that it no longer matters. Please note that while the Ratios structure can only support ppem sizes up to 255, the vTable structure can support much larger pel heights (up to 65535). The choice of int16 and uint16 for the vTable record is dictated by the requirement that yMax and yMin be signed values (and 127 to -128 is too small a range) and the desire to word-align the vTable elements.