A simple wrapper for libpng

png.c

/* gcc -O2 -Werror -I/usr/include/libpng16 -lpng png.c -o png */

#include <stdio.h>
#include <stdlib.h>
#include <png.h>

typedef struct PNG {
  int width;
  int height;
  int depth;
  int pixel_bytes;
  png_bytep color;
  png_structp pngp;
  png_byte color_type;
  png_infop infop;
  png_byte **data;
} PNG;

int put_pixel(PNG *png, int x, int y) {
  if (x < 0 || y < 0 || x >= png->width || y >= png->height) {
    return -1;
  }
  png_bytep pixel = &(png->data[y][x*png->pixel_bytes]);
  pixel[0] = png->color[0];
  pixel[1] = png->color[1];
  pixel[2] = png->color[2];
  if (png->pixel_bytes == 4) {
    pixel[3] = 255;
  }
  return 0;
}

void write_png(PNG *png, char *fname) {
  FILE *fp = fopen(fname, "wb");
  png->pngp = png_create_write_struct(PNG_LIBPNG_VER_STRING,
      NULL, NULL, NULL);
  png->infop = png_create_info_struct(png->pngp);
  png_init_io(png->pngp, fp);
  png_set_IHDR(png->pngp, png->infop, png->width, png->height,
      png->depth, png->color_type, PNG_INTERLACE_NONE,
      PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_DEFAULT);
  png_write_info(png->pngp, png->infop);
  png_write_image(png->pngp, png->data);
  png_write_end(png->pngp, NULL);
  fclose(fp);
}

void set_color(PNG *png, png_byte r, png_byte g, png_byte b) {
  png->color[0] = r;
  png->color[1] = g;
  png->color[2] = b;
  if (png->pixel_bytes == 4) {
    png->color[3] = 255;
  }
}

int read_png(PNG *png, char *fname) {
  FILE *fp = fopen(fname, "rb");
  char header[8];
  fread(header, 1, 8, fp);
  if (png_sig_cmp((unsigned char*) header, 0, 8)) {
    return -1;
  }
  png->pngp = png_create_read_struct(PNG_LIBPNG_VER_STRING,
      NULL, NULL, NULL);
  png->infop = png_create_info_struct(png->pngp);
  png_init_io(png->pngp, fp);
  png->pixel_bytes = 3;
  png_set_sig_bytes(png->pngp, 8);
  png_read_info(png->pngp, png->infop);
  png->width = png_get_image_width(png->pngp, png->infop);
  png->height = png_get_image_height(png->pngp, png->infop);
  png->color_type = png_get_color_type(png->pngp, png->infop);
  png->depth = png_get_bit_depth(png->pngp, png->infop);
  png_read_update_info(png->pngp, png->infop);
  png->data = malloc(png->height * sizeof(png_bytep));
  png->color = malloc(3 * sizeof(png_byte));
  set_color(png, 255, 255, 255);
  for (int y = 0; y < png->height; y++) {
    png->data[y] = malloc(png->pixel_bytes * png->width * sizeof(png_byte));
  }
  png_read_image(png->pngp, png->data);
  fclose(fp);
  return 0;
}

void create_png(PNG *png, int width, int height, png_byte color_mode) {
  png->width = width;
  png->height = height;
  png->pngp = png_create_write_struct(PNG_LIBPNG_VER_STRING,
      NULL, NULL, NULL);
  png->infop = png_create_info_struct(png->pngp);
  png->depth = 8;
  png->color_type = color_mode;
  switch (png->color_type) {
    case PNG_COLOR_TYPE_RGBA:
      png->pixel_bytes = 4;
      break;
    case PNG_COLOR_TYPE_RGB:
      png->pixel_bytes = 3;
      break;
  }
  png_set_IHDR(png->pngp, png->infop, width, height,
      png->depth, png->color_type, PNG_INTERLACE_NONE,
      PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_DEFAULT);
  png->data = malloc(height * sizeof(png_bytep));
  png->color = malloc(3 * sizeof(png_byte));
  set_color(png, 255, 255, 255);
  for (int y = 0; y < height; y++) {
    png->data[y] = malloc(png->pixel_bytes * width * sizeof(png_byte));
  }
}

void draw_rect(PNG *png, int x, int y, int width, int height) {
  for (int yy = y; yy <= height; yy++) {
    for (int xx = x; xx <= width; xx++) {
      put_pixel(png, xx, yy);
    }
  }
}

int main(int argc, char **argv) {
  PNG png;
  /* read_png(&png, "input.png"); */
  create_png(&png, 1000, 1000, PNG_COLOR_TYPE_RGB);
  draw_rect(&png, 100, 100, 400, 400);
  set_color(&png, 255, 0, 0);
  draw_rect(&png, 200, 200, 300, 300);
  write_png(&png, "output.png");
  return 0;
}