Abstract font backend for text rendering operations. More...

#include "donner/svg/text/TextBackend.h"

Inheritance diagram for donner::svg::TextBackend:

Classes
struct	BitmapGlyph
	Bitmap glyph data from color fonts (CBDT/CBLC). More...
struct	ShapedGlyph
	A single shaped glyph with advance and cluster info. More...
struct	ShapedRun
	Result of shaping a text range. More...

Public Member Functions
virtual FontVMetrics	fontVMetrics (FontHandle font) const =0
	Vertical metrics (ascent, descent, lineGap) in font design units.
virtual float	scaleForPixelHeight (FontHandle font, float pixelHeight) const =0
	Scale factor: font design units → pixels, normalized to ascent−descent height. This is stbtt_ScaleForPixelHeight semantics.
virtual float	scaleForEmToPixels (FontHandle font, float pixelHeight) const =0
	Scale factor: em units → pixels. Differs from scaleForPixelHeight when ascent−descent != unitsPerEm. This is the correct scale for CSS font-size.
virtual std::optional< UnderlineMetrics >	underlineMetrics (FontHandle font) const =0
	Underline position/thickness from the 'post' table, in font design units. Returns std::nullopt if the table is missing.
virtual std::optional< UnderlineMetrics >	strikeoutMetrics (FontHandle font) const =0
	Strikeout position/thickness from the OS/2 table, in font design units. Returns std::nullopt if the table is missing.
virtual std::optional< SubSuperMetrics >	subSuperMetrics (FontHandle font) const =0
	Sub/superscript Y offsets from the OS/2 table, in font design units. Returns std::nullopt if the table is missing.
virtual Path	glyphOutline (FontHandle font, int glyphIndex, float scale) const =0
	Extract a glyph outline as a Path. Coordinates are in font units scaled by `scale`, with Y flipped for SVG's y-down convention.
virtual bool	isBitmapOnly (FontHandle font) const =0
	Returns true if the font is bitmap-only (no vector outlines).
virtual bool	isCursive (uint32_t codepoint) const =0
	Returns true if letter-spacing should be suppressed for this codepoint (cursive/connected scripts like Arabic). Simple backends return false.
virtual bool	hasSmallCapsFeature (FontHandle font) const =0
	Returns true if the font has a native OpenType small-caps feature (smcp). If false, the engine synthesizes small-caps via uppercase + reduced size.
virtual std::optional< BitmapGlyph >	bitmapGlyph (FontHandle font, int glyphIndex, float scale) const =0
	Extract a bitmap glyph (CBDT/CBLC color emoji). Returns std::nullopt if the glyph is not a bitmap. Simple backends always return std::nullopt.
virtual ShapedRun	shapeRun (FontHandle font, float fontSizePx, std::string_view spanText, size_t byteOffset, size_t byteLength, bool isVertical, FontVariant fontVariant, bool forceLogicalOrder) const =0
	Shape a range of text, producing glyph IDs with advances and kerning.
virtual double	crossSpanKern (FontHandle prevFont, float prevSizePx, FontHandle curFont, float curSizePx, uint32_t prevCodepoint, uint32_t curCodepoint, bool isVertical) const =0
	Compute cross-span kerning between the last codepoint of the previous span and the first codepoint of the current span.

Detailed Description

Abstract font backend for text rendering operations.

Abstracts all font-engine-specific calls (stb_truetype, HarfBuzz+FreeType) behind a uniform interface. Used by TextEngine for layout and by renderers for glyph outlines and font metrics.

Implementations:

TextBackendSimple: stb_truetype (no GSUB/GPOS, no bitmap glyphs)
TextBackendFull: HarfBuzz + FreeType (full OpenType shaping)

Class Documentation

◆ donner::svg::TextBackend::BitmapGlyph

struct donner::svg::TextBackend::BitmapGlyph

Bitmap glyph data from color fonts (CBDT/CBLC).

Collaboration diagram for donner::svg::TextBackend::BitmapGlyph:

[legend]

Class Members
double	bearingX = 0	Horizontal offset from glyph origin.
double	bearingY = 0	Vertical offset from baseline (positive = up).
int	height = 0	Bitmap height in pixels.
vector< uint8_t >	rgbaPixels	RGBA pixel data (premultiplied alpha).
double	scale = 1.0	Scale factor to apply.
int	width = 0	Bitmap width in pixels.

◆ donner::svg::TextBackend::ShapedGlyph

struct donner::svg::TextBackend::ShapedGlyph

A single shaped glyph with advance and cluster info.

Class Members
uint32_t	cluster = 0	Byte offset into the shaped text range.
float	fontSizeScale = 1.0f	< 1.0 for synthesized small-caps glyphs.
int	glyphIndex = 0	Backend-specific glyph ID.
double	xAdvance = 0	Nominal horizontal advance (without kerning).
double	xKern = 0	Kerning adjustment to apply before this glyph.
double	xOffset = 0	Horizontal offset (GPOS mark positioning; 0 for simple).
double	yAdvance = 0	Nominal vertical advance (without kerning).
double	yKern = 0	Kerning adjustment (vertical mode).
double	yOffset = 0	Vertical offset (GPOS mark positioning; 0 for simple).

◆ donner::svg::TextBackend::ShapedRun

struct donner::svg::TextBackend::ShapedRun

Result of shaping a text range.

Collaboration diagram for donner::svg::TextBackend::ShapedRun:

[legend]

Class Members
vector< ShapedGlyph >	glyphs

Member Function Documentation

◆ bitmapGlyph()

virtual std::optional< BitmapGlyph > donner::svg::TextBackend::bitmapGlyph	(	FontHandle	font,
		int	glyphIndex,
		float	scale ) const

pure virtual

Extract a bitmap glyph (CBDT/CBLC color emoji). Returns std::nullopt if the glyph is not a bitmap. Simple backends always return std::nullopt.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ crossSpanKern()

virtual double donner::svg::TextBackend::crossSpanKern	(	FontHandle	prevFont,
		float	prevSizePx,
		FontHandle	curFont,
		float	curSizePx,
		uint32_t	prevCodepoint,
		uint32_t	curCodepoint,
		bool	isVertical ) const

pure virtual

Compute cross-span kerning between the last codepoint of the previous span and the first codepoint of the current span.

Returns: Kern adjustment in pixels (added to pen position).

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ fontVMetrics()

virtual FontVMetrics donner::svg::TextBackend::fontVMetrics ( FontHandle font ) const

pure virtual

Vertical metrics (ascent, descent, lineGap) in font design units.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ glyphOutline()

virtual Path donner::svg::TextBackend::glyphOutline	(	FontHandle	font,
		int	glyphIndex,
		float	scale ) const

pure virtual

Extract a glyph outline as a Path. Coordinates are in font units scaled by scale, with Y flipped for SVG's y-down convention.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ hasSmallCapsFeature()

virtual bool donner::svg::TextBackend::hasSmallCapsFeature ( FontHandle font ) const

pure virtual

Returns true if the font has a native OpenType small-caps feature (smcp). If false, the engine synthesizes small-caps via uppercase + reduced size.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ isBitmapOnly()

virtual bool donner::svg::TextBackend::isBitmapOnly ( FontHandle font ) const

pure virtual

Returns true if the font is bitmap-only (no vector outlines).

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ isCursive()

virtual bool donner::svg::TextBackend::isCursive ( uint32_t codepoint ) const

pure virtual

Returns true if letter-spacing should be suppressed for this codepoint (cursive/connected scripts like Arabic). Simple backends return false.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ scaleForEmToPixels()

virtual float donner::svg::TextBackend::scaleForEmToPixels	(	FontHandle	font,
		float	pixelHeight ) const

pure virtual

Scale factor: em units → pixels. Differs from scaleForPixelHeight when ascent−descent != unitsPerEm. This is the correct scale for CSS font-size.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ scaleForPixelHeight()

virtual float donner::svg::TextBackend::scaleForPixelHeight	(	FontHandle	font,
		float	pixelHeight ) const

pure virtual

Scale factor: font design units → pixels, normalized to ascent−descent height. This is stbtt_ScaleForPixelHeight semantics.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ shapeRun()

virtual ShapedRun donner::svg::TextBackend::shapeRun	(	FontHandle	font,
		float	fontSizePx,
		std::string_view	spanText,
		size_t	byteOffset,
		size_t	byteLength,
		bool	isVertical,
		FontVariant	fontVariant,
		bool	forceLogicalOrder ) const

pure virtual

Shape a range of text, producing glyph IDs with advances and kerning.

The full span text and a byte range within it are provided so context-aware backends (HarfBuzz) can use surrounding context for GSUB/GPOS.

Parameters

font	Font to shape with.
fontSizePx	Font size in pixels.
spanText	Full span text (UTF-8).
byteOffset	Start of the range to shape within `spanText`.
byteLength	Length of the range to shape.
isVertical	True for vertical writing mode.
fontVariant	Font variant (Normal or SmallCaps).
forceLogicalOrder	If true, return glyphs sorted by cluster (DOM order) rather than visual order. Used for per-character positioned RTL text.

Returns: Shaped glyphs with advances, kerning, and cluster mapping.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ strikeoutMetrics()

virtual std::optional< UnderlineMetrics > donner::svg::TextBackend::strikeoutMetrics ( FontHandle font ) const

pure virtual

Strikeout position/thickness from the OS/2 table, in font design units. Returns std::nullopt if the table is missing.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ subSuperMetrics()

virtual std::optional< SubSuperMetrics > donner::svg::TextBackend::subSuperMetrics ( FontHandle font ) const

pure virtual

Sub/superscript Y offsets from the OS/2 table, in font design units. Returns std::nullopt if the table is missing.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

◆ underlineMetrics()

virtual std::optional< UnderlineMetrics > donner::svg::TextBackend::underlineMetrics ( FontHandle font ) const

pure virtual

Underline position/thickness from the 'post' table, in font design units. Returns std::nullopt if the table is missing.

Implemented in donner::svg::TextBackendFull, and donner::svg::TextBackendSimple.

The documentation for this class was generated from the following file:

donner/svg/text/TextBackend.h

Classes

Public Member Functions

Detailed Description

Class Documentation

◆ donner::svg::TextBackend::BitmapGlyph

◆ donner::svg::TextBackend::ShapedGlyph

◆ donner::svg::TextBackend::ShapedRun

Member Function Documentation

◆ bitmapGlyph()

◆ crossSpanKern()

◆ fontVMetrics()

◆ glyphOutline()

◆ hasSmallCapsFeature()

◆ isBitmapOnly()

◆ isCursive()

◆ scaleForEmToPixels()

◆ scaleForPixelHeight()

◆ shapeRun()

◆ strikeoutMetrics()

◆ subSuperMetrics()

◆ underlineMetrics()