stbライブラリでtruetypeフォントをラスタライズ（１）

//参考
//https://gist.github.com/cloudwu/766bccc60c254f9cc2abfa397bcff2ea

#include<windows.h>
#include <tchar.h>

#include <cstdio>

#define STB_TRUETYPE_IMPLEMENTATION
#define STBTT_STATIC

#include "stb_truetype.h"

// utf8文字コードをコードポイントに変換
unsigned int utf8_decode(const char* o);

// フォントで文字を描画した画像を作成
void drawfontchar(unsigned int f);

unsigned char* pixels = nullptr; //画素の格納先
int iwidth; //画像サイズの格納先
int iheight;


LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
  HDC hdc;
  PAINTSTRUCT ps;
  switch (msg) {
  case WM_DESTROY:
    //画像の破棄
    delete [] pixels;

    PostQuitMessage(0);
    return 0;
  case WM_PAINT://画面に表示
    hdc = BeginPaint(hwnd, &ps);
    if (pixels != nullptr) {
      for (int y = 0; y < iheight; y++) {
        for (int x = 0; x < iwidth; x++) {
          unsigned char r = pixels[y * iwidth + x];
          if (r == 0)
            SetPixel(hdc, x, y, RGB(255, 255, 255));
          else
            SetPixel(hdc, x, y, RGB(0, 0, 0));
        }
      }
    }

    EndPaint(hwnd, &ps);
    break;
  }
  return DefWindowProc(hwnd, msg, wp, lp);
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
  PSTR lpCmdLine, int nCmdShow) {
  HWND hwnd;
  WNDCLASS winc;
  MSG msg;

  winc.style = CS_HREDRAW | CS_VREDRAW;
  winc.lpfnWndProc = WndProc;
  winc.cbClsExtra = winc.cbWndExtra = 0;
  winc.hInstance = hInstance;
  winc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
  winc.hCursor = LoadCursor(NULL, IDC_ARROW);
  winc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
  winc.lpszMenuName = NULL;
  winc.lpszClassName = TEXT("NEW_WINDOW");

  if (!RegisterClass(&winc)) return 0;

  //指定した文字のunicodeのコードポイントを取得
  unsigned int c = utf8_decode(u8"ぬ");

  //一文字書かれた画像を作成(pixelsに格納)
  drawfontchar(c);


  hwnd = CreateWindow(
    TEXT("NEW_WINDOW"), TEXT("example : stb_truetype.h"),
    WS_OVERLAPPEDWINDOW | WS_VISIBLE,
    CW_USEDEFAULT, CW_USEDEFAULT,
    200, 100,
    NULL, NULL,
    hInstance, NULL
  );

  if (hwnd == NULL) return 0;


  while (GetMessage(&msg, NULL, 0, 0)) DispatchMessage(&msg);
  return msg.wParam;
}


unsigned char ttf_buffer[1 << 25];


//一文字書かれた画像を作成
void drawfontchar(unsigned int codepoint) {

  const int HEIGHT = 60;

  stbtt_fontinfo font;

  //フォントファイルを開く
  FILE* pf_font = fopen(R"(C:\fonts\GenShinGothic-Bold.ttf)", "rb");
  fread(ttf_buffer, 1, 1 << 25, pf_font);

  stbtt_InitFont(&font, ttf_buffer, stbtt_GetFontOffsetForIndex(ttf_buffer, 0));
  float scale = stbtt_ScaleForPixelHeight(&font, HEIGHT);
  int ascent, baseline, decent;
  stbtt_GetFontVMetrics(&font, &ascent, &decent, 0);
  baseline = (int)(ascent * scale);
  int height = (int)((ascent - decent) * scale);

  int x0, y0, x1, y1;
  stbtt_GetCodepointBitmapBox(&font, codepoint, scale, scale, &x0, &y0, &x1, &y1);

  //出力先のメモリ確保
  iwidth = x1 - x0;
  iheight = y1 - y0;
  pixels = new unsigned char[iheight * iwidth];

  stbtt_MakeCodepointBitmap(
    &font,  //フォント情報
    pixels, //描画先
    iwidth, //描画先の幅
    iheight,//描画先の高さ
    iwidth, //描画先の画像の一行のバイト数（?）
    scale,  //X方向倍率 
    scale,  //Y方向倍率
    codepoint);

}

//utf8一文字をコードポイントに変換
unsigned int utf8_decode(const char* o) {
  const unsigned int MAXUNICODE = 0x10FFFF;

  static const unsigned int limits[] = { 0xFF, 0x7F, 0x7FF, 0xFFFF };
  const unsigned char* s = (const unsigned char*)o;
  unsigned int c = s[0];
  unsigned int res = 0;  /* final result */
  if (c < 0x80)  /* ascii? */
    res = c;
  else {
    int count = 0;  /* to count number of continuation bytes */
    while (c & 0x40) {  /* still have continuation bytes? */
      int cc = s[++count];  /* read next byte */
      if ((cc & 0xC0) != 0x80)  /* not a continuation byte? */
        return -1;  /* invalid byte sequence */
      res = (res << 6) | (cc & 0x3F);  /* add lower 6 bits from cont. byte */
      c <<= 1;  /* to test next bit */
    }
    res |= ((c & 0x7F) << (count * 5));  /* add first byte */
    if (count > 3 || res > MAXUNICODE || res <= limits[count])
      return -1;  /* invalid byte sequence */
    s += count;  /* skip continuation bytes read */
  }
  return res;
}