How to render color emoji font with FreeType 2.5

clfontpng.cc

// = Requirements: freetype 2.5, libpng, libicu, libz, libzip2
// = How to compile:
//  % export CXXFLAGS=`pkg-config --cflags freetype2 libpng`
//  % export LDFLAGS=`pkg-config --libs freetype2 libpng`
//  % clang++ -o clfontpng -static $(CXXFLAGS) clfontpng.cc $(LDFLAGS) \
//    -licuuc -lz -lbz2
#include <cassert>
#include <cctype>
#include <iostream>
#include <memory>
#include <vector>
#include <string>

#include <stdio.h>

#include <unicode/umachine.h>
#include <unicode/utf.h>

#include <ft2build.h>
#include FT_FREETYPE_H
#include FT_TRUETYPE_TABLES_H

#define PNG_SKIP_SETJMP_CHECK
#include <png.h>

namespace {

const char* kDefaultOutputFile = "out.png";
const int kBytesPerPixel = 4; // RGBA
const int kDefaultPixelSize = 128;
const int kSpaceWidth = kDefaultPixelSize / 2;

FT_Library gFtLibrary;

// Only support horizontal direction.
class DrawContext {
 public:
  DrawContext()
      : pos_(0), width_(0), height_(0) {}
  uint8_t* Bitmap() { return &bitmap_[0]; }
  const uint32_t Width() const { return width_; }
  const uint32_t Height() const { return height_; }
  void SetSize(int width, int height) {
    width_ = width;
    height_ = height;
    int size = width * height * kBytesPerPixel;
    bitmap_.resize(size);
    bitmap_.assign(size, 0x00);
  }
  void Advance(int dx) { pos_ += dx; }
  uint8_t* GetDrawPosition(int row) {
    uint32_t index =(row * width_ + pos_) * kBytesPerPixel;
    assert(index < bitmap_.size());
    return &bitmap_[index];
  }
 private:
  DrawContext(const DrawContext&) = delete;
  DrawContext& operator=(const DrawContext&) = delete;

  uint32_t pos_;
  uint32_t width_;
  uint32_t height_;
  std::vector<uint8_t> bitmap_;
};

struct FaceOptions {
  int pixel_size;
  int load_flags;
  FT_Render_Mode render_mode;
  FaceOptions()
      : pixel_size(kDefaultPixelSize)
      , load_flags(0), render_mode(FT_RENDER_MODE_NORMAL) {}
};

class FreeTypeFace {
 public:
  FreeTypeFace(const std::string& font_file)
      : font_file_(font_file)
      , options_()
      , face_(nullptr)
  {
    error_ = FT_New_Face(gFtLibrary, font_file_.c_str(), 0, &face_);
    if (error_) {
      face_ = nullptr;
      return;
    }
    if (IsColorEmojiFont())
      SetupColorFont();
    else
      SetupNormalFont();
  }
  ~FreeTypeFace() {
    if (face_)
      FT_Done_Face(face_);
  }
  FreeTypeFace(FreeTypeFace&& rhs)
      : font_file_(rhs.font_file_)
      , options_(rhs.options_)
      , face_(rhs.face_)
      , error_(rhs.error_)
  {
    rhs.face_ = nullptr;
  }
  bool CalculateBox(uint32_t codepoint, uint32_t& width, uint32_t& height) {
    if (!RenderGlyph(codepoint))
      return false;
    width += (face_->glyph->advance.x >> 6);
    height = std::max(
        height, static_cast<uint32_t>(face_->glyph->metrics.height >> 6));
    return true;
  }
  bool DrawCodepoint(DrawContext& context, uint32_t codepoint) {
    if (!RenderGlyph(codepoint))
      return false;
    printf("U+%08X -> %s\n", codepoint, font_file_.c_str());
    return DrawBitmap(context, face_->glyph);
  }
  int Error() const { return error_; }

 private:
  FreeTypeFace(const FreeTypeFace&) = delete;
  FreeTypeFace& operator=(const FreeTypeFace&) = delete;

  bool RenderGlyph(uint32_t codepoint) {
    if (!face_)
      return false;
    uint32_t glyph_index = FT_Get_Char_Index(face_, codepoint);
    if (glyph_index == 0)
      return false;
    error_ = FT_Load_Glyph(face_, glyph_index, options_.load_flags);
    if (error_)
      return false;
    error_ = FT_Render_Glyph(face_->glyph, options_.render_mode);
    if (error_)
      return false;
    return true;
  }
  bool IsColorEmojiFont() {
    static const uint32_t tag = FT_MAKE_TAG('C', 'B', 'D', 'T');
    unsigned long length = 0;
    FT_Load_Sfnt_Table(face_, tag, 0, nullptr, &length);
    if (length) {
      std::cout << font_file_ << " is color font" << std::endl;
      return true;
    }
    return false;
  }
  void SetupNormalFont() {
    error_ = FT_Set_Pixel_Sizes(face_, 0, options_.pixel_size);
  }
  void SetupColorFont() {
    options_.load_flags |= FT_LOAD_COLOR;

    if (face_->num_fixed_sizes == 0)
      return;
    int best_match = 0;
    int diff = std::abs(options_.pixel_size - face_->available_sizes[0].width);
    for (int i = 1; i < face_->num_fixed_sizes; ++i) {
      int ndiff =
          std::abs(options_.pixel_size - face_->available_sizes[i].width);
      if (ndiff < diff) {
        best_match = i;
        diff = ndiff;
      }
    }
    error_ = FT_Select_Size(face_, best_match);
  }
  bool DrawBitmap(DrawContext& context, FT_GlyphSlot slot) {
    int pixel_mode = slot->bitmap.pixel_mode;
    if (pixel_mode == FT_PIXEL_MODE_BGRA)
      DrawColorBitmap(context, slot);
    else
      DrawNormalBitmap(context, slot);
    context.Advance(slot->advance.x >> 6);
    return true;
  }
  void DrawColorBitmap(DrawContext& context, FT_GlyphSlot slot) {
    uint8_t* src = slot->bitmap.buffer;
    // FIXME: Should use metrics for drawing. (e.g. calculate baseline)
    int yoffset = context.Height() - slot->bitmap.rows;
    for (int y = 0; y < slot->bitmap.rows; ++y) {
      uint8_t* dest = context.GetDrawPosition(y + yoffset);
      for (int x = 0; x < slot->bitmap.width; ++x) {
        uint8_t b = *src++, g = *src++, r = *src++, a = *src++;
        *dest++ = r; *dest++ = g; *dest++ = b; *dest++ = a;
      }
    }
  }
  void DrawNormalBitmap(DrawContext& context, FT_GlyphSlot slot) {
    uint8_t* src = slot->bitmap.buffer;
    // FIXME: Same as DrawColorBitmap()
    int yoffset = context.Height() - slot->bitmap.rows;
    for (int y = 0; y < slot->bitmap.rows; ++y) {
      uint8_t* dest = context.GetDrawPosition(y + yoffset);
      for (int x = 0; x < slot->bitmap.width; ++x) {
        *dest++ = 255 - *src;
        *dest++ = 255 - *src;
        *dest++ = 255 - *src;
        *dest++ = *src; // Alpha
        ++src;
      }
    }
  }

  std::string font_file_;
  FaceOptions options_;
  FT_Face face_;
  int error_;
};

class FontList {
  typedef std::vector<std::unique_ptr<FreeTypeFace>> FaceList;
 public:
  FontList() {}

  void AddFont(const std::string& font_file) {
    auto face = std::unique_ptr<FreeTypeFace>(new FreeTypeFace(font_file));
    face_list_.push_back(std::move(face));
  }
  void CalculateBox(uint32_t codepoint, uint32_t& width, uint32_t& height) {
    static const uint32_t kSpace = 0x20;
    if (codepoint == kSpace) {
      width += kSpaceWidth;
    } else {
      for (auto& face : face_list_) {
        if (face->CalculateBox(codepoint, width, height))
          return;
      }
    }
  }
  void DrawCodepoint(DrawContext& context, uint32_t codepoint) {
    for (auto& face : face_list_) {
      if (face->DrawCodepoint(context, codepoint))
        return;
    }
    std::cerr << "Missing glyph for codepoint: " << codepoint << std::endl;
  }

 private:
  FontList(const FontList&) = delete;
  FontList& operator=(const FontList&) = delete;
  FaceList face_list_;
};

class PngWriter {
 public:
  PngWriter(const std::string& outfile)
      : outfile_(outfile), png_(nullptr), info_(nullptr)
  {
    fp_ = fopen(outfile_.c_str(), "wb");
    if (!fp_) {
      std::cerr << "Failed to open: " << outfile_ << std::endl;
      Cleanup();
      return;
    }
    png_ = png_create_write_struct(
        PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
    if (!png_) {
      std::cerr << "Failed to create PNG file" << std::endl;
      Cleanup();
      return;
    }
    info_ = png_create_info_struct(png_);
    if (!info_) {
      std::cerr << "Failed to create PNG file" << std::endl;
      Cleanup();
      return;
    }
  }
  ~PngWriter() { Cleanup(); }
  bool Write(uint8_t* rgba, int width, int height) {
    static const int kDepth = 8;
    if (!png_) {
      std::cerr << "Writer is not initialized" << std::endl;
      return false;
    }
    if (setjmp(png_jmpbuf(png_))) {
      std::cerr << "Failed to write PNG" << std::endl;
      Cleanup();
      return false;
    }
    png_set_IHDR(png_, info_, width, height, kDepth,
                 PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE,
                 PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
    png_init_io(png_, fp_);
    png_byte** row_pointers =
        static_cast<png_byte**>(png_malloc(png_, height * sizeof(png_byte*)));
    uint8_t* src = rgba;
    for (int y = 0; y < height; ++y) {
      png_byte* row =
          static_cast<png_byte*>(png_malloc(png_, width * kBytesPerPixel));
      row_pointers[y] = row;
      for (int x = 0; x < width; ++x) {
        *row++ = *src++;
        *row++ = *src++;
        *row++ = *src++;
        *row++ = *src++;
      }
      assert(row - row_pointers[y] == width * kBytesPerPixel);
    }
    assert(src - rgba == width * height * kBytesPerPixel);
    png_set_rows(png_, info_, row_pointers);
    png_write_png(png_, info_, PNG_TRANSFORM_IDENTITY, 0);
    for (int y = 0; y < height; y++)
      png_free(png_, row_pointers[y]);
    png_free(png_, row_pointers);
    Cleanup();
    return true;
  }
 private:
  PngWriter(const PngWriter&) = delete;
  PngWriter operator=(const PngWriter&) = delete;
  void Cleanup() {
    if (fp_) { fclose(fp_); }
    if (png_) png_destroy_write_struct(&png_, &info_);
    fp_ = nullptr; png_ = nullptr; info_ = nullptr;
  }

  std::string outfile_;
  FILE* fp_;
  png_structp png_;
  png_infop info_;
  char* rgba_;
  uint32_t width_;
  uint32_t height_;
};

class App {
 public:
  void AddFont(const std::string& font_file) { font_list_.AddFont(font_file); }
  bool SetText(const char* text) { return UTF8ToCodepoint(text); }
  bool Execute() {
    CalculateImageSize();
    Draw();
    return Output();
  }
 private:
  bool UTF8ToCodepoint(const char* text) {
    int32_t i = 0, length = strlen(text), c;
    while (i < length) {
      U8_NEXT(text, i, length, c);
      if (c < 0) {
        std::cerr << "Invalid input text" << std::endl;
        return false;
      }
      codepoints_.push_back(c);
    }
    return true;
  }
  void CalculateImageSize() {
    uint32_t width = 0, height = 0;
    for (auto c : codepoints_)
      font_list_.CalculateBox(c, width, height);
    printf("width: %u, height: %u\n", width, height);
    draw_context_.SetSize(width, height);
  }
  void Draw() {
    for (auto c : codepoints_)
      font_list_.DrawCodepoint(draw_context_, c);
  }
  bool Output() {
    PngWriter writer(kDefaultOutputFile);
    return writer.Write(draw_context_.Bitmap(),
                        draw_context_.Width(),
                        draw_context_.Height());
  }

  std::vector<uint32_t> codepoints_;
  FontList font_list_;
  DrawContext draw_context_;
};

bool Init() {
  int error = FT_Init_FreeType(&gFtLibrary);
  if (error) {
    std::cerr << "Failed to initialize freetype" << std::endl;
    return error;
  }
  return error == 0;
}

void Finish() {
  FT_Done_FreeType(gFtLibrary);
}

void Usage() {
  std::cout
      << "Usage: clfontpng font1.ttf [font2.ttf ...] text"
      << std::endl;
  std::exit(1);
}

bool ParseArgs(App& app, int argc, char** argv) {
  if (argc < 2)
    return false;
  for (int i = 1; i < argc - 1; ++i)
    app.AddFont(argv[i]);
  return app.SetText(argv[argc - 1]);
}

bool Start(int argc, char** argv) {
  App app;
  if (!ParseArgs(app, argc, argv))
    Usage();
  return app.Execute();
}

} // namespace

int main(int argc, char** argv) {
  if (!Init())
    std::exit(1);
  bool success = Start(argc, argv);
  Finish();
  return success ? 0 : 1;
}

I fixed the trailing EOL spacing and adjusted the glyph placement to use a calculated baseline from font metrics.

https://gist.github.com/brimston3/998330f32594bb00af50fe9cc37b74c6

However, by fixing the baseline, the glyphs take up a lot more space if you want to use it for say, generating a glyph atlas.