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feat: Support for kerning and ligatures (#873)

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## Summary

**What is the goal of this PR?**
Improved typesetting, including
[kerning](https://en.wikipedia.org/wiki/Kerning) and
[ligatures](https://en.wikipedia.org/wiki/Ligature_(writing)#Latin_alphabet).

**What changes are included?**
- The script to convert built-in fonts now adds kerning and ligature
information to the generated font headers.
- Epub page layout calculates proper kerning spaces and makes ligature
substitutions according to the selected font.


![3U1B1808](https://github.com/user-attachments/assets/1accb16f-2f1a-41e5-adca-89f1f1348494)

![3U1B1810](https://github.com/user-attachments/assets/2f6bd007-490e-420f-b774-3380b4add7ea)

![3U1B1815](https://github.com/user-attachments/assets/1986bb77-2db0-46e2-a5d6-8315dae9eb19)

## Additional Context

- I am not a typography expert. 
- The implementation has been reworked from the earlier version, so it
is no longer necessary to omit Open Dyslexic, and kerning data now
covers all fonts, styles, and codepoints for which we include bitmap
data.
- Claude Opus 4.6 helped with a lot of this.
- There's an included test epub document with lots of kerning and
ligature examples, shown in the photos.

**_After some time to mature, I think this change is in decent shape to
merge and get people testing._**

After opening this PR I came across #660, which overlaps in adding
ligature support.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES, Claude Opus 4.6**_

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
This commit is contained in:
Zach Nelson
2026-02-24 02:31:43 -06:00
committed by GitHub
parent 13592db50f
commit 0eb8a9346b
67 changed files with 80729 additions and 36388 deletions
Download Patch File Download Diff File Expand all files Collapse all files

81
lib/GfxRenderer/GfxRenderer.cpp
View File

@@ -153,12 +153,10 @@ static void renderCharImpl(const GfxRenderer& renderer, GfxRenderer::RenderMode
}
}
if (!utf8IsCombiningMark(cp)) {
if constexpr (rotation == TextRotation::Rotated90CW) {
*cursorY -= glyph->advanceX;
} else {
*cursorX += glyph->advanceX;
}
if constexpr (rotation == TextRotation::Rotated90CW) {
*cursorY -= glyph->advanceX;
} else {
*cursorX += glyph->advanceX;
}
}
@@ -209,12 +207,11 @@ void GfxRenderer::drawCenteredText(const int fontId, const int y, const char* te
void GfxRenderer::drawText(const int fontId, const int x, const int y, const char* text, const bool black,
const EpdFontFamily::Style style) const {
int yPos = y + getFontAscenderSize(fontId);
int xpos = x;
int xPos = x;
int lastBaseX = x;
int lastBaseY = yPos;
int lastBaseAdvance = 0;
int lastBaseTop = 0;
bool hasBaseGlyph = false;
// cannot draw a NULL / empty string
if (text == nullptr || *text == '\0') {
@@ -230,8 +227,9 @@ void GfxRenderer::drawText(const int fontId, const int x, const int y, const cha
constexpr int MIN_COMBINING_GAP_PX = 1;
uint32_t cp;
uint32_t prevCp = 0;
while ((cp = utf8NextCodepoint(reinterpret_cast<const uint8_t**>(&text)))) {
if (utf8IsCombiningMark(cp) && hasBaseGlyph) {
if (utf8IsCombiningMark(cp)) {
const EpdGlyph* combiningGlyph = font.getGlyph(cp, style);
int raiseBy = 0;
if (combiningGlyph) {
@@ -247,16 +245,20 @@ void GfxRenderer::drawText(const int fontId, const int x, const int y, const cha
continue;
}
const EpdGlyph* glyph = font.getGlyph(cp, style);
if (!utf8IsCombiningMark(cp)) {
lastBaseX = xpos;
lastBaseY = yPos;
lastBaseAdvance = glyph ? glyph->advanceX : 0;
lastBaseTop = glyph ? glyph->top : 0;
hasBaseGlyph = true;
cp = font.applyLigatures(cp, text, style);
if (prevCp != 0) {
xPos += font.getKerning(prevCp, cp, style);
}
renderChar(font, cp, &xpos, &yPos, black, style);
const EpdGlyph* glyph = font.getGlyph(cp, style);
lastBaseX = xPos;
lastBaseY = yPos;
lastBaseAdvance = glyph ? glyph->advanceX : 0;
lastBaseTop = glyph ? glyph->top : 0;
renderChar(font, cp, &xPos, &yPos, black, style);
prevCp = cp;
}
}
@@ -882,7 +884,22 @@ int GfxRenderer::getSpaceWidth(const int fontId, const EpdFontFamily::Style styl
return spaceGlyph ? spaceGlyph->advanceX : 0;
}
int GfxRenderer::getTextAdvanceX(const int fontId, const char* text, const EpdFontFamily::Style style) const {
int GfxRenderer::getSpaceKernAdjust(const int fontId, const uint32_t leftCp, const uint32_t rightCp,
const EpdFontFamily::Style style) const {
const auto fontIt = fontMap.find(fontId);
if (fontIt == fontMap.end()) return 0;
const auto& font = fontIt->second;
return font.getKerning(leftCp, ' ', style) + font.getKerning(' ', rightCp, style);
}
int GfxRenderer::getKerning(const int fontId, const uint32_t leftCp, const uint32_t rightCp,
const EpdFontFamily::Style style) const {
const auto fontIt = fontMap.find(fontId);
if (fontIt == fontMap.end()) return 0;
return fontIt->second.getKerning(leftCp, rightCp, style);
}
int GfxRenderer::getTextAdvanceX(const int fontId, const char* text, EpdFontFamily::Style style) const {
const auto fontIt = fontMap.find(fontId);
if (fontIt == fontMap.end()) {
LOG_ERR("GFX", "Font %d not found", fontId);
@@ -890,14 +907,20 @@ int GfxRenderer::getTextAdvanceX(const int fontId, const char* text, const EpdFo
}
uint32_t cp;
uint32_t prevCp = 0;
int width = 0;
const auto& font = fontIt->second;
while ((cp = utf8NextCodepoint(reinterpret_cast<const uint8_t**>(&text)))) {
if (utf8IsCombiningMark(cp)) {
continue;
}
cp = font.applyLigatures(cp, text, style);
if (prevCp != 0) {
width += font.getKerning(prevCp, cp, style);
}
const EpdGlyph* glyph = font.getGlyph(cp, style);
if (glyph) width += glyph->advanceX;
prevCp = cp;
}
return width;
}
@@ -952,12 +975,12 @@ void GfxRenderer::drawTextRotated90CW(const int fontId, const int x, const int y
int lastBaseY = y;
int lastBaseAdvance = 0;
int lastBaseTop = 0;
bool hasBaseGlyph = false;
constexpr int MIN_COMBINING_GAP_PX = 1;
uint32_t cp;
uint32_t prevCp = 0;
while ((cp = utf8NextCodepoint(reinterpret_cast<const uint8_t**>(&text)))) {
if (utf8IsCombiningMark(cp) && hasBaseGlyph) {
if (utf8IsCombiningMark(cp)) {
const EpdGlyph* combiningGlyph = font.getGlyph(cp, style);
int raiseBy = 0;
if (combiningGlyph) {
@@ -973,16 +996,20 @@ void GfxRenderer::drawTextRotated90CW(const int fontId, const int x, const int y
continue;
}
const EpdGlyph* glyph = font.getGlyph(cp, style);
if (!utf8IsCombiningMark(cp)) {
lastBaseX = xPos;
lastBaseY = yPos;
lastBaseAdvance = glyph ? glyph->advanceX : 0;
lastBaseTop = glyph ? glyph->top : 0;
hasBaseGlyph = true;
cp = font.applyLigatures(cp, text, style);
if (prevCp != 0) {
yPos -= font.getKerning(prevCp, cp, style);
}
const EpdGlyph* glyph = font.getGlyph(cp, style);
lastBaseX = xPos;
lastBaseY = yPos;
lastBaseAdvance = glyph ? glyph->advanceX : 0;
lastBaseTop = glyph ? glyph->top : 0;
renderCharImpl<TextRotation::Rotated90CW>(*this, renderMode, font, cp, &xPos, &yPos, black, style);
prevCp = cp;
}
}

5
lib/GfxRenderer/GfxRenderer.h
View File

@@ -110,6 +110,11 @@ class GfxRenderer {
void drawText(int fontId, int x, int y, const char* text, bool black = true,
EpdFontFamily::Style style = EpdFontFamily::REGULAR) const;
int getSpaceWidth(int fontId, EpdFontFamily::Style style = EpdFontFamily::REGULAR) const;
/// Returns the kerning adjustment for a space between two codepoints:
/// kern(leftCp, ' ') + kern(' ', rightCp). Returns 0 if kerning is unavailable.
int getSpaceKernAdjust(int fontId, uint32_t leftCp, uint32_t rightCp, EpdFontFamily::Style style) const;
/// Returns the kerning adjustment between two adjacent codepoints.
int getKerning(int fontId, uint32_t leftCp, uint32_t rightCp, EpdFontFamily::Style style) const;
int getTextAdvanceX(int fontId, const char* text, EpdFontFamily::Style style) const;
int getFontAscenderSize(int fontId) const;
int getLineHeight(int fontId) const;