From 8354c99fbee2887cb71440f4e25ad1e6f46b2592 Mon Sep 17 00:00:00 2001 From: Nathan Willis Date: Tue, 30 Apr 2019 17:22:08 +0100 Subject: [PATCH] [Docs] Usermanual; add Fonts And Faces chapter. --- docs/usermanual-fonts-and-faces.xml | 447 +++++++++++++++++++++++++++- 1 file changed, 438 insertions(+), 9 deletions(-) diff --git a/docs/usermanual-fonts-and-faces.xml b/docs/usermanual-fonts-and-faces.xml index 553600439..c1787bddf 100644 --- a/docs/usermanual-fonts-and-faces.xml +++ b/docs/usermanual-fonts-and-faces.xml @@ -5,20 +5,449 @@ ]> - Fonts and faces -
+ Fonts, faces, and output + + In the previous chapter, we saw how to set up a buffer and fill + it with text as Unicode code points. In order to shape this + buffer text with HarfBuzz, you will need also need a font + object. + + + HarfBuzz provides abstractions to help you cache and reuse the + heavier parts of working with binary fonts, so we will look at + how to do that. We will also look at how to work with the + FreeType font-rendering library and at how you can customize + HarfBuzz to work with other libraries. + + + Finally, we will look at how to work with OpenType variable + fonts, the latest update to the OpenType font format, and at + some other recent additions to OpenType. + + +
+ Font and face objects + + The outcome of shaping a run of text depends on the contents of + a specific font file (such as the substitutions and positioning + moves in the 'GSUB' and 'GPOS' tables), so HarfBuzz makes + accessing those internals fast. + + + An hb_face_t represents a face + in HarfBuzz. This data type is a wrapper around an + hb_blob_t blob that holds the contents of a binary + fotn file. Since HarfBuzz supports TrueType Collections and + OpenType Collections (each of which can include multiple + typefaces), a HarfBuzz face also requires an index number + specifying which typeface in the file you want to use. Most of + the font files you will encounter in the wild include just a + single face, however, so most of the time you would pass in + 0 as the index when you create a face: + + + hb_blob_t* blob = hb_blob_create_from_file(file); + ... + hb_face_t* face = hb_face_create(blob, 0); + + + On its own, a face object is not quite ready to use for + shaping. The typeface must be set to a specific point size in + order for some details (such as hinting) to work. In addition, + if the font file in question is an OpenType Variable Font, then + you may need to specify one or variation-axis settings (or a + named instance) in order to get the output you need. + + + In HarfBuzz, you do this by creating a font + object from your face. + + + Font objects also have the advantage of being considerably + lighter-weight than face objects (remember that a face contains + the contents of a binary font file mapped into memory). As a + result, you can cache and reuse a font object, but you could + also create a new one for each additional size you needed. + Creating new fonts incurs some additional overhead, of course, + but whether or not it is excessive is your call in the end. In + contrast, face objects are substantially larger, and you really + should cache them and reuse them whenever possible. + + + You can create a font object from a face object: + + + hb_font_t* hb_font = hb_font_create(hb_face); + + + After creating a font, there are a few properties you should + set. Many fonts enable and disable hints based on the size it + is used at, so setting this is important for font + objects. hb_font_set_ppem(font, x_ppem, + y_ppem) sets the pixels-per-EM value of the font. You + can also set the point size of the font with + hb_font_set_ppem(font, ptem). HarfBuzz uses the + industry standard 72 points per inch. + + + HarfBuzz lets you specify the degree subpixel precision you want + through a scaling factor. You can set horizontal and + vertical scaling factors on the + font by calling hb_font_set_scale(font, x_scale, + y_scale). + + + There may be times when you are handed a font object and need to + access the face object that it comes from. For that, you can call + + + hb_face = hb_font_get_face(hb_font); + + + You can also create a font object from an existing font object + using the hb_font_create_sub_font() + function. This creates a child font object that is initiated + with the same attributes as its parent; it can be used to + quickly set up a new font for the purpose of overriding a specific + font-functions method. + + + All face objects and font objects are lifecycle-managed by + HarfBuzz. After creating a face, you increase its reference + count with hb_face_reference(face) and + decrease it with + hb_face_destroy(face). Likewise, you + increase the reference count on a font with + hb_font_reference(font) and decrease it + with hb_font_destroy(font). + + + You can also attach user data to face objects and font objects. + +
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+ Customizing font functions + + During shaping, HarfBuzz frequently needs to query font objects + to get at the contents and parameters of the glyphs in a font + file. It includes a built-in set of functions that is tailored + to working with OpenType fonts. However, as was the case with + Unicode functions in the buffers chapter, HarfBuzz also wants to + make it easy for you to assign a substitute set of font + functions if you are developing a program to work with a library + or platform that provides its own font functions. + + + Therefore, the HarfBuzz API defines a set of virtual + methods for accessing font-object properties, and you can + replace the defaults with your own selections without + interfering with the shaping process. Each font object in + HarfBuzz includes a structure called + font_funcs that serves as a vtable for the + font object. The virtual methods in + font_funcs are: + + + + + hb_font_get_font_h_extents_func_t: returns + the extents of the font for horizontal text. + + + + + hb_font_get_font_v_extents_func_t: returns + the extents of the font for vertical text. + + + + + hb_font_get_nominal_glyph_func_t: returns + the font's nominal glyph for a given code point. + + + + + hb_font_get_variation_glyph_func_t: returns + the font's glyph for a given code point when it is followed by a + given Variation Selector. + + + + + hb_font_get_nominal_glyphs_func_t: returns + the font's nominal glyphs for a series of code points. + + + + + hb_font_get_glyph_advance_func_t: returns + the advance for a glyph. + + + + + hb_font_get_glyph_h_advance_func_t: returns + the advance for a glyph for horizontal text. + + + + + hb_font_get_glyph_v_advance_func_t:returns + the advance for a glyph for vertical text. + + + + + hb_font_get_glyph_advances_func_t: returns + the advances for a series of glyphs. + + + + + hb_font_get_glyph_h_advances_func_t: returns + the advances for a series of glyphs for horizontal text . + + + + + hb_font_get_glyph_v_advances_func_t: returns + the advances for a series of glyphs for vertical text. + + + + + hb_font_get_glyph_origin_func_t: returns + the origin coordinates of a glyph. + + + + + hb_font_get_glyph_h_origin_func_t: returns + the origin coordinates of a glyph for horizontal text. + + + + + hb_font_get_glyph_v_origin_func_t: returns + the origin coordinates of a glyph for vertical text. + + + + + hb_font_get_glyph_extents_func_t: returns + the extents for a glyph. + + + + + hb_font_get_glyph_contour_point_func_t: + returns the coordinates of a specific contour point from a glyph. + + + + + hb_font_get_glyph_name_func_t: returns the + name of a glyph (from its glyph index). + + + + + hb_font_get_glyph_from_name_func_t: returns + the glyph index that corresponds to a given glyph name. + + + + + You can fetch the font-functions configuration for a font object + by calling hb_font_get_font_funcs(): + + + hb_font_funcs_t *ffunctions; + ffunctions = hb_font_get_font_funcs (font); + + + The individual methods can each be replaced with their own setter + function, such as + hb_font_funcs_set_nominal_glyph_func(*ffunctions, + func, *user_data, destroy). + + + Font-functions structures can be reused for multiple font + objects, and can be reference counted with + hb_font_funcs_reference() and + hb_font_funcs_destroy(). Just like other + objects in HarfBuzz, you can set user-data for each + font-functions structure and assign a destroy callback for + it. + + + You can also mark a font-functions structure as immutable, + with hb_font_funcs_make_immutable(). This + is especially useful if your code is a library or framework that + will have its own client programs. By marking your + font-functions structures as immutable, you prevent your client + programs from changing the configuration and introducing + inconsistencies and errors downstream. + +
+ +
+ Font objects and HarfBuzz's native OpenType implementation + + By default, whenever HarfBuzz creates a font object, it will + configure the font to use a built-in set of font functions that + supports contemporary OpenType font internals. If you want to + work with OpenType or TrueType fonts, you should be able to use + these functions without difficulty. + + + Many of the methods in the font-functions structure deal with + the fundamental properties of glyphs that are required for + shaping text: extents (the maximums and minimums on each axis), + origins (the (0,0) coordinate point which + glyphs are drawn in reference to), and advances (the amount that + the cursor needs to be moved after drawing each glyph, including + any empty space for the glyph's side bearings). + + + As you can see in the list of functions, there are separate "horizontal" + and "vertical" variants depending on whether the text is set in + the horizontal or vertical direction. For some scripts, fonts + that are designed to support text set horizontally or vertically (for + example, in Japanese) may include metrics for both text + directions. When fonts don't include this information, HarfBuzz + does its best to transform what the font provides. + + + In addition to the direction-specific functions, HarfBuzz + provides some higher-level functions for fetching information + like extents and advances for a glyph. If you call + + + hb_font_get_glyph_advance_for_direction(font, direction, extents); + + + then you can provide any hb_direction_t as the + direction parameter, and HarfBuzz will + use the correct function variant for the text direction. There + are similar higher-level versions of the functions for fetching + extents, origin coordinates, and contour-point + coordinates. There are also addition and subtraction functions + for moving points with respect to the origin. + + + There are also methods for fetching the glyph ID that + corresponds to a Unicode code point (possibly when followed by a + variation-selector code point), fetching the glyph name from the + font, and fetching the glyph ID that corresponds to a glyph name + you already have. + + + HarfBuzz also provides functions for converting between glyph + names and string + variables. hb_font_glyph_to_string(font, glyph, s, + size) retrieves the name for the glyph ID + glyph from the font object. It generates a + generic name of the form gidDDD (where DDD is + the glyph index) if there is no name for the glyph in the + font. The hb_font_glyph_from_string(font, s, len, + glyph) takes an input string s + and looks for a glyph with that name in the font, returning its + glyph ID in the glyph + output parameter. It automatically parses + gidDDD and uniUUUU strings. + +
+ + + + + + +
+ Working with OpenType Variable Fonts + If you are working with OpenType Variable Fonts, there are a few + additional functions you should use to specify the + variation-axis settings of your font object. Without doing so, + your variable font's font object can still be used, but only at + the default setting for every axis (which, of course, is + sometimes what you want, but does not cover general usage). + + + HarfBuzz manages variation settings in the + hb_variation_t data type, which holds a tag for the + variation-axis identifier tag and a value for its + setting. You can retrieve the list of variation axes in a font + binary from the face object (not from a font object, notably) by + calling hb_ot_var_get_axis_count(face) to + find the number of axes, then using + hb_ot_var_get_axis_infos() to collect the + axis structures: + + + axes = hb_ot_var_get_axis_count(face); + ... + hb_ot_var_get_axis_infos(face, 0, axes, axes_array); + + + For each axis returned in the array, you can can access the + identifier in its tag. HarfBuzz also has + tag definitions predefined for the five standard axes specified + in OpenType (ital for italic, + opsz for optical size, + slnt for slant, wdth for + width, and wght for weight). Each axis also + has a min_value, a + default_value, and a max_value. + + + To set your font object's variation settings, you call the + hb_font_set_variations() function with an + array of hb_variation_t variation settings. Let's + say our font has weight and width axes. We need to specify each + of the axes by tag and assign a value on the axis: + + + unsigned int variation_count = 2; + hb_variation_t variation_data[variation_count]; + variation_data[0].tag = HB_OT_TAG_VAR_AXIS_WIDTH; + variation_data[1].tag = HB_OT_TAG_VAR_AXIS_WEIGHT; + variation_data[0].value = 80; + variation_data[1].value = 750; + ... + hb_font_set_variations(font, variation_data, variation_count); + + + That should give us a slightly condensed font ("normal" on the + wdth axis is 100) at a noticeably bolder + weight ("regular" is 400 on the wght axis). + + + In practice, though, you should always check that the value you + want to set on the axis is within the + [min_value,max_value] + range actually implemented in the font's variation axis. After + all, a font might only provide lighter-than-regular weights, and + setting a heavier value on the wght axis will + not change that. + + + Once your variation settings are specified on your font object, + however, shaping with a variable font is just like shaping a + static font.
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