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[doc] Revise font variation API.

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Werner Lemberg 2025-06-29 07:45:09 +02:00
parent 636aede94b
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161
include/freetype/ftmm.h
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@ -37,24 +37,79 @@ FT_BEGIN_HEADER
* multiple_masters
*
* @title:
* Multiple Masters
* OpenType Font Variations, TrueType GX, and Adobe MM Fonts
*
* @abstract:
* How to manage Multiple Masters fonts.
* How to manage variable fonts with multiple design axes.
*
* @description:
* The following types and functions are used to manage Multiple Master
* fonts, i.e., the selection of specific design instances by setting
* design axis coordinates.
* The following types and functions manage OpenType Font Variations,
* Adobe Multiple Master (MM) fonts, and Apple TrueType GX fonts. These
* formats have in common that they allow the selection of specific
* design instances by setting design coordinates for one or more axes
* like font weight or width.
*
* Besides Adobe MM fonts, the interface supports Apple's TrueType GX and
* OpenType Font Variations. Some of the routines only work with Adobe
* MM fonts, others will work with all three types. They are similar
* enough that a consistent interface makes sense.
* For historical reasons there are two interfaces. The first, older one
* can be used with Adobe MM fonts only, and the second, newer one is a
* unified interface that handles all three font formats. However, some
* differences remain and are documented accordingly; in particular,
* Adobe MM fonts don't have named instances (see below).
*
* For Adobe MM fonts, macro @FT_IS_SFNT returns false. For TrueType GX
* and OpenType Font Variations, it returns true.
*
* We use mostly the terminology of the OpenType standard. Here are some
* important technical terms.
*
* * A 'named instance' is a tuple of design coordinates that has a
* string ID (i.e., an index into the font's 'name' table) associated
* with it. The font can tell the user that, for example,
* [Weight=700,Width=110] is 'Bold'. Another name for 'named instance'
* is 'named style'.
*
* Adobe MM fonts don't have named instances.
*
* * The 'default instance' of a variation font is that instance for
* which the nth axis coordinate is equal to the nth default axis
* coordinate (i.e., `axis[n].def` as specified in the @FT_MM_Var
* structure), with~n covering all axes. In TrueType GX and OpenType
* Font Variations, the default instance is explicitly given. In Adobe
* MM fonts, the `WeightVector` entry as found in the font file is
* taken as the default instance.
*
* For TrueType GX and OpenType Font Variations, FreeType synthesizes
* a named instance for the default instance if the font does not
* contain such an entry.
*
* * 'Design coordinates' are the axis values found in a variation font
* file. Their meaning is specified by the font designer and the
* values are rather arbitrary.
*
* For example, the 'weight' axis in design coordinates might vary
* between 100 (thin) and 900 (heavy) in font~A, while font~B
* contains values between 400 (normal) and 800 (extra bold).
*
* * 'Normalized coordinates' are design coordinates mapped to a standard
* range; they are also called 'blend coordinates'.
*
* For TrueType GX and OpenType Font Variations, the range is [-1;1],
* with the minimum mapped to value~-1, the default mapped to
* value~0, and the maximum mapped to value~1, and all other
* coordinates mapped to intervening points. Please look up the
* [OpenType
* specification](https://learn.microsoft.com/en-us/typography/opentype/spec/otvaroverview)
* on how this mapping works in detail.
*
* For Adobe MM fonts, this standard range is [0;1], with the minimum
* mapped to value~0 and the maximum mapped to value~1, and all other
* coordinates mapped to intervening points. Please look up [Adobe
* TechNote
* #5015](https://adobe-type-tools.github.io/font-tech-notes/pdfs/5015.Type1_Supp.pdf)
* on how this mapping works in detail.
*
* Assuming that the two fonts in the previous example are OpenType
* Font Variations, both font~A's [100;900] and font~B's [400;800]
* coordinate ranges get mapped to [-1;1].
*/
@ -90,7 +145,7 @@ FT_BEGIN_HEADER
* @description:
* A structure to model a given axis in design space for Adobe MM fonts.
*
* This structure can't be used for TrueType GX or OpenType Font
* This structure can't be used with TrueType GX or OpenType Font
* Variations.
*
* @fields:
@ -120,7 +175,7 @@ FT_BEGIN_HEADER
* @description:
* A structure to model the axes and space of an Adobe MM font.
*
* This structure can't be used for TrueType GX or OpenType Font
* This structure can't be used with TrueType GX or OpenType Font
* Variations.
*
* @fields:
@ -128,7 +183,7 @@ FT_BEGIN_HEADER
* Number of axes. Cannot exceed~4.
*
* num_designs ::
* Number of designs; should be normally 2^num_axis even though the
* Number of designs; should be normally `2^num_axis` even though the
* Type~1 specification strangely allows for intermediate designs to be
* present. This number cannot exceed~16.
*
@ -214,11 +269,11 @@ FT_BEGIN_HEADER
* entry for each axis.
*
* strid ::
* The entry in 'name' table identifying this instance.
* An index into the 'name' table identifying this instance.
*
* psid ::
* The entry in 'name' table identifying a PostScript name for this
* instance. Value 0xFFFF indicates a missing entry.
* An index into the 'name' table identifying a PostScript name for
* this instance. Value 0xFFFF indicates a missing entry.
*/
typedef struct FT_Var_Named_Style_
{
@ -235,8 +290,8 @@ FT_BEGIN_HEADER
* FT_MM_Var
*
* @description:
* A structure to model the axes and space of an Adobe MM, TrueType GX,
* or OpenType Font Variations.
* A structure to model the axes and space of Adobe MM fonts, TrueType
* GX, or OpenType Font Variations.
*
* Some fields are specific to one format and not to the others.
*
@ -246,19 +301,13 @@ FT_BEGIN_HEADER
* limit in TrueType GX or OpenType Font Variations.
*
* num_designs ::
* The number of designs; should be normally 2^num_axis for Adobe MM
* The number of designs; should be normally `2^num_axis` for Adobe MM
* fonts. Not meaningful for TrueType GX or OpenType Font Variations
* (where every glyph could have a different number of designs).
*
* num_namedstyles ::
* The number of named styles; a 'named style' is a tuple of design
* coordinates that has a string ID (in the 'name' table) associated
* with it. The font can tell the user that, for example,
* [Weight=1.5,Width=1.1] is 'Bold'. Another name for 'named style' is
* 'named instance'.
*
* For Adobe MM fonts, this value is always zero because the format
* does not support named styles.
* The number of named instances. For Adobe MM fonts, this value is
* always zero.
*
* axis ::
* An axis descriptor table. TrueType GX and OpenType Font Variations
@ -266,7 +315,7 @@ FT_BEGIN_HEADER
* of this pointer is done internally by FreeType.
*
* namedstyle ::
* A named style (instance) table. Only meaningful for TrueType GX and
* An array of named instances. Only meaningful for TrueType GX and
* OpenType Font Variations. Memory management of this pointer is done
* internally by FreeType.
*/
@ -473,7 +522,7 @@ FT_BEGIN_HEADER
*
* @output:
* coords ::
* The design coordinates array.
* The design coordinates array, which must be allocated by the user.
*
* @return:
* FreeType error code. 0~means success.
@ -498,8 +547,7 @@ FT_BEGIN_HEADER
* FT_Set_MM_Blend_Coordinates
*
* @description:
* Choose an interpolated font design through normalized blend
* coordinates.
* Choose an interpolated font design through normalized coordinates.
*
* This function works with all supported variation formats.
*
@ -514,9 +562,10 @@ FT_BEGIN_HEADER
* the number of axes, use default values for the remaining axes.
*
* coords ::
* The design coordinates array. Each element is a 16.16 fractional
* value and must be between 0 and 1.0 for Adobe MM fonts, and between
* -1.0 and 1.0 for TrueType GX and OpenType Font Variations.
* The normalized coordinates array. Each element is a 16.16
* fractional value and must be between 0 and 1.0 for Adobe MM fonts,
* and between -1.0 and 1.0 for TrueType GX and OpenType Font
* Variations.
*
* @return:
* FreeType error code. 0~means success.
@ -550,8 +599,8 @@ FT_BEGIN_HEADER
* FT_Get_MM_Blend_Coordinates
*
* @description:
* Get the normalized blend coordinates of the currently selected
* interpolated font.
* Get the normalized coordinates of the currently selected interpolated
* font.
*
* This function works with all supported variation formats.
*
@ -560,14 +609,14 @@ FT_BEGIN_HEADER
* A handle to the source face.
*
* num_coords ::
* The number of normalized blend coordinates to retrieve. If it is
* larger than the number of axes, set the excess values to~0.5 for
* Adobe MM fonts, and to~0 for TrueType GX and OpenType Font
* Variations.
* The number of normalized coordinates to retrieve. If it is larger
* than the number of axes, set the excess values to~0.5 for Adobe MM
* fonts, and to~0 for TrueType GX and OpenType Font Variations.
*
* @output:
* coords ::
* The normalized blend coordinates array (as 16.16 fractional values).
* The normalized coordinates array (as 16.16 fractional values), which
* must be allocated by the user.
*
* @return:
* FreeType error code. 0~means success.
@ -641,8 +690,8 @@ FT_BEGIN_HEADER
* FreeType error code. 0~means success.
*
* @note:
* Adobe MM fonts limit the number of designs, and thus the length of
* the weight vector to 16~elements.
* Adobe MM fonts limit the number of designs, and thus the length of the
* weight vector, to 16~elements.
*
* If `len` is larger than zero, this function sets the
* @FT_FACE_FLAG_VARIATION bit in @FT_Face's `face_flags` field (i.e.,
@ -650,7 +699,7 @@ FT_BEGIN_HEADER
* is unset and the weight vector array is reset to the default values.
*
* The Adobe documentation also states that the values in the
* WeightVector array must total 1.0 +/-~0.001. In practice this does
* `WeightVector` array must total 1.0 +/-~0.001. In practice this does
* not seem to be enforced, so is not enforced here, either.
*
* @since:
@ -688,14 +737,14 @@ FT_BEGIN_HEADER
*
* @output:
* weightvector ::
* An array to be filled.
* An array to be filled; it must be allocated by the user.
*
* @return:
* FreeType error code. 0~means success.
*
* @note:
* Adobe MM fonts limit the number of designs, and thus the length of
* the WeightVector to~16.
* Adobe MM fonts limit the number of designs, and thus the length of the
* weight vector, to~16 elements.
*
* @since:
* 2.10
@ -771,8 +820,8 @@ FT_BEGIN_HEADER
* A handle to the source face.
*
* instance_index ::
* The index of the requested instance, starting with value 1. If set
* to value 0, FreeType switches to font access without a named
* The index of the requested instance, starting with value~1. If set
* to value~0, FreeType switches to font access without a named
* instance.
*
* @return:
@ -782,11 +831,11 @@ FT_BEGIN_HEADER
* The function uses the value of `instance_index` to set bits 16-30 of
* the face's `face_index` field. It also resets any variation applied
* to the font, and the @FT_FACE_FLAG_VARIATION bit of the face's
* `face_flags` field gets reset to zero (i.e., @FT_IS_VARIATION will
* return false).
* `face_flags` field gets reset to zero (i.e., @FT_IS_VARIATION returns
* false).
*
* For Adobe MM fonts (which don't have named instances) this function
* simply resets the current face to the default instance.
* For Adobe MM fonts, this function resets the current face to the
* default instance.
*
* @since:
* 2.9
@ -805,10 +854,6 @@ FT_BEGIN_HEADER
* Retrieve the index of the default named instance, to be used with
* @FT_Set_Named_Instance.
*
* The default instance of a variation font is that instance for which
* the nth axis coordinate is equal to `axis[n].def` (as specified in the
* @FT_MM_Var structure), with~n covering all axes.
*
* FreeType synthesizes a named instance for the default instance if the
* font does not contain such an entry.
*
@ -824,8 +869,8 @@ FT_BEGIN_HEADER
* FreeType error code. 0~means success.
*
* @note:
* For Adobe MM fonts (which don't have named instances) this function
* always returns zero for `instance_index`.
* For Adobe MM fonts, this function always returns zero for
* `instance_index`.
*
* @since:
* 2.13.1

2
src/truetype/ttgxvar.c
View file

@ -2810,7 +2810,7 @@
FT_UInt strid = ~0U;
/* The default instance is missing in array the */
/* The default instance is missing in the array */
/* of named instances; try to synthesize an entry. */
/* If this fails, `default_named_instance` remains */
/* at value zero, which doesn't do any harm. */