Drawing text into a bitmap w/ stb_truetype

sdl2_stbtt.c

#include "SDL.h"

/*
   Compilation:
   ============
   gcc -g --std=c99 -o sdl2_stbtt sdl2_stbtt.c `sdl2-config --cflags --libs` -lm

   Usage:
   ======
   ./sdl2_stbtt /usr/share/fonts/TTF/LiberationMono-Regular.ttf


   By Samrat Man Singh (http://samrat.me)
*/

typedef struct sdl_buffer {
  SDL_Texture *texture;
  SDL_Window *window;
  SDL_Renderer *renderer;
  void *pixels;
  int texture_width;
  int texture_height;
  int bytes_per_pixel;
} sdl_buffer;


#define STB_TRUETYPE_IMPLEMENTATION
#include "stb_truetype.h"

static unsigned char ttf_buffer[1<<25];
static sdl_buffer backbuffer;

static unsigned char *
get_pixel(int x, int y) {
  int width = backbuffer.texture_width;
  int height = backbuffer.texture_height;

  int pitch = width * backbuffer.bytes_per_pixel;
  uint8_t *row = (uint8_t *)(backbuffer.pixels +
                             (y * width * backbuffer.bytes_per_pixel) +
                             (x * backbuffer.bytes_per_pixel));

  unsigned char *pixel = (unsigned char *)row;

  return pixel;
}


static FILE *
init_font(char *filename) {
  FILE *f = fopen(filename, "rb");
  fread(ttf_buffer, 1, 1<<25, f);

  return f;
}

static void
draw_text(int x, int y, unsigned char *text)
{
  stbtt_fontinfo font;
  unsigned char *bitmap;

  int s = 30;

  stbtt_InitFont(&font, ttf_buffer, stbtt_GetFontOffsetForIndex(ttf_buffer, 0));

  float scale = stbtt_ScaleForPixelHeight(&font, s);
  int ascent = 0;

  stbtt_GetFontVMetrics(&font, &ascent, 0, 0);
  int baseline = ascent*scale;

  int x_cursor = x;
  while (*text) {
    int advance, lsb, x0, y0, x1, y1;

    stbtt_GetCodepointHMetrics(&font, *text, &advance, &lsb);
    stbtt_GetCodepointBitmapBox(&font, *text, scale, scale, &x0, &y0, &x1, &y1);

    unsigned char *pixel = get_pixel(x_cursor+x0, y+baseline+y0);
    stbtt_MakeCodepointBitmap(&font,
                              pixel,
                              backbuffer.bytes_per_pixel*(x1-x0),
                              backbuffer.bytes_per_pixel*(y1-y0),
                              backbuffer.texture_width*backbuffer.bytes_per_pixel,
                              2*scale, scale,
                              *text);

    x_cursor += (advance*scale);
    if (*(text+1))
      x_cursor += (scale*stbtt_GetCodepointKernAdvance(&font, *text, *(text+1)));

    text++;
  }
}

/* from Handmade Penguin
   https://davidgow.net/handmadepenguin/ch3.html */
static void
SDLUpdateWindow(SDL_Window *window, SDL_Renderer *renderer) {
  SDL_UpdateTexture(backbuffer.texture,
                    0,
                    backbuffer.pixels,
                    backbuffer.texture_width * backbuffer.bytes_per_pixel);
  SDL_RenderCopy(renderer,
                 backbuffer.texture,
                 0,
                 0);

  SDL_RenderPresent(renderer);
}


int main(int argc, char **argv) {
  FILE *font_file = init_font((argc > 1) ? argv[1] : "DejaVuSans.ttf");
  if (SDL_Init(SDL_INIT_VIDEO) != 0) {
    fprintf(stderr, "\nUnable to initialize SDL: %s\n", SDL_GetError());
  }


  SDL_Window *window;
  window = SDL_CreateWindow("stb_truetype w/ SDL",
                            SDL_WINDOWPOS_UNDEFINED,
                            SDL_WINDOWPOS_UNDEFINED,
                            640,
                            480,
                            SDL_WINDOW_RESIZABLE);

  if (window)
    {
      SDL_Renderer *renderer = SDL_CreateRenderer(window,
                                                  -1,
                                                  0);

      if (renderer)
        {
          backbuffer.bytes_per_pixel = 4;
          backbuffer.window = window;
          backbuffer.renderer = renderer;

          int width, height;
          SDL_GetWindowSize(window, &width, &height);
          backbuffer.texture_width = width;
          backbuffer.texture_height = height;

          backbuffer.texture = SDL_CreateTexture(renderer,
                                                 SDL_PIXELFORMAT_ARGB8888,
                                                 SDL_TEXTUREACCESS_STREAMING,
                                                 width,
                                                 height);

          backbuffer.pixels = malloc(width * height * backbuffer.bytes_per_pixel);

          draw_text(50, 50, "Just some plain old text");
          SDLUpdateWindow(window, renderer);

          // wait for keypress
          SDL_Event event;
          while ((SDL_WaitEvent(&event)) && event.key.keysym.sym != SDLK_SPACE);

        }
    }

  SDL_Quit();
  free(backbuffer.pixels);
  fclose(font_file);

  return 0;
}