PNG.h

/*
 * A simple libpng example program
 * Taken from https://gist.github.com/niw/5963798
 * http://zarb.org/~gc/html/libpng.html
 *
 * Modified by Yoshimasa Niwa to make it much simpler
 * and support all defined color_type.
 *
 * To build, use the next instruction on OS X.
 * $ brew install libpng
 * $ clang -lz -lpng16 libpng_test.c
 *
 * Copyright 2002-2010 Guillaume Cottenceau.
 *
 * This software may be freely redistributed under the terms
 * of the X11 license.
 *
 */

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

using namespace std;

class PNG_handler {
public:
  int width, height;
  png_byte color_type;
  png_byte bit_depth;
  png_bytep *row_pointers = NULL;

  PNG_handler() {}

  ~PNG_handler() {
    if (row_pointers != NULL) {
      for (int r_idx = 0; r_idx < height; r_idx++) {
        if (row_pointers[r_idx] != NULL) {
          free(row_pointers[r_idx]);
        }
      }
      free(row_pointers);
    }
  }

  void read_png_file(const char *filename) {
    FILE *fp = fopen(filename, "rb");

    png_structp png = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if(!png) abort();

    png_infop info = png_create_info_struct(png);
    if(!info) abort();

    if(setjmp(png_jmpbuf(png))) abort();

    png_init_io(png, fp);

    png_read_info(png, info);

    width      = png_get_image_width(png, info);
    height     = png_get_image_height(png, info);
    color_type = png_get_color_type(png, info);
    bit_depth  = png_get_bit_depth(png, info);

    // Read any color_type into 8bit depth, RGBA format.
    // See http://www.libpng.org/pub/png/libpng-manual.txt

    if(bit_depth == 16)
      png_set_strip_16(png);

    if(color_type == PNG_COLOR_TYPE_PALETTE)
      png_set_palette_to_rgb(png);

    // PNG_COLOR_TYPE_GRAY_ALPHA is always 8 or 16bit depth.
    if(color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
      png_set_expand_gray_1_2_4_to_8(png);

    if(png_get_valid(png, info, PNG_INFO_tRNS))
      png_set_tRNS_to_alpha(png);

    // These color_type don't have an alpha channel then fill it with 0xff.
    if(color_type == PNG_COLOR_TYPE_RGB ||
      color_type == PNG_COLOR_TYPE_GRAY ||
      color_type == PNG_COLOR_TYPE_PALETTE)
      png_set_filler(png, 0xFF, PNG_FILLER_AFTER);

    if(color_type == PNG_COLOR_TYPE_GRAY ||
      color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
      png_set_gray_to_rgb(png);

    png_read_update_info(png, info);

    if (row_pointers) abort();

    row_pointers = (png_bytep*)malloc(sizeof(png_bytep) * height);
    for(int y = 0; y < height; y++) {
      row_pointers[y] = (png_byte*)malloc(png_get_rowbytes(png,info));
    }

    png_read_image(png, row_pointers);

    fclose(fp);

    png_destroy_read_struct(&png, &info, NULL);
  }

  void write_png_file(const char *filename) {
    int y;

    FILE *fp = fopen(filename, "wb");
    if(!fp) abort();

    png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!png) abort();

    png_infop info = png_create_info_struct(png);
    if (!info) abort();

    if (setjmp(png_jmpbuf(png))) abort();

    png_init_io(png, fp);

    // Output is 8bit depth, RGBA format.
    png_set_IHDR(
      png,
      info,
      width, height,
      8,
      PNG_COLOR_TYPE_RGBA,
      PNG_INTERLACE_NONE,
      PNG_COMPRESSION_TYPE_DEFAULT,
      PNG_FILTER_TYPE_DEFAULT
    );
    png_write_info(png, info);

    // To remove the alpha channel for PNG_COLOR_TYPE_RGB format,
    // Use png_set_filler().
    //png_set_filler(png, 0, PNG_FILLER_AFTER);

    if (!row_pointers) abort();

    png_write_image(png, row_pointers);
    png_write_end(png, NULL);

    for(int y = 0; y < height; y++) {
      free(row_pointers[y]);
    }
    free(row_pointers);

    fclose(fp);

    png_destroy_write_struct(&png, &info);
  }

  void process_png_file() {
    for(int y = 0; y < height; y++) {
      png_bytep row = row_pointers[y];
      for(int x = 0; x < width; x++) {
        png_bytep px = &(row[x * 4]);
        // Do something awesome for each pixel here...
        //printf("%4d, %4d = RGBA(%3d, %3d, %3d, %3d)\n", x, y, px[0], px[1], px[2], px[3]);
      }
    }
  }
};