A simple libpng example program


Sometimes, even if Gimp can do wonderful things, one may want to iterate over the pixels of a file to compute things on it. One solution is to script gimp using scheme or perl. Another one is to use the libpng to read and write png files, computing anything necessary in the middle. Since I didn't find myself a "simple" example program using libpng, I had to more or less hack mine, so I publish it there after. It's short and simple - that's the point.

Here is the plain text version - check next paragraph for the beautified one.

The beautified code

 * Copyright 2002-2010 Guillaume Cottenceau.
 * This software may be freely redistributed under the terms
 * of the X11 license.

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>

#define PNG_DEBUG 3
#include <png.h>

void abort_(const char * s, ...)
        va_list args;
        va_start(args, s);
        vfprintf(stderr, s, args);
        fprintf(stderr, "\n");

int x, y;

int width, height;
png_byte color_type;
png_byte bit_depth;

png_structp png_ptr;
png_infop info_ptr;
int number_of_passes;
png_bytep * row_pointers;

void read_png_file(char* file_name)
        char header[8];    // 8 is the maximum size that can be checked

        /* open file and test for it being a png */
        FILE *fp = fopen(file_name, "rb");
        if (!fp)
                abort_("[read_png_file] File %s could not be opened for reading", file_name);
        fread(header, 1, 8, fp);
        if (png_sig_cmp(header, 0, 8))
                abort_("[read_png_file] File %s is not recognized as a PNG file", file_name);

        /* initialize stuff */
        png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);

        if (!png_ptr)
                abort_("[read_png_file] png_create_read_struct failed");

        info_ptr = png_create_info_struct(png_ptr);
        if (!info_ptr)
                abort_("[read_png_file] png_create_info_struct failed");

        if (setjmp(png_jmpbuf(png_ptr)))
                abort_("[read_png_file] Error during init_io");

        png_init_io(png_ptr, fp);
        png_set_sig_bytes(png_ptr, 8);

        png_read_info(png_ptr, info_ptr);

        width = png_get_image_width(png_ptr, info_ptr);
        height = png_get_image_height(png_ptr, info_ptr);
        color_type = png_get_color_type(png_ptr, info_ptr);
        bit_depth = png_get_bit_depth(png_ptr, info_ptr);

        number_of_passes = png_set_interlace_handling(png_ptr);
        png_read_update_info(png_ptr, info_ptr);

        /* read file */
        if (setjmp(png_jmpbuf(png_ptr)))
                abort_("[read_png_file] Error during read_image");

        row_pointers = (png_bytep*) malloc(sizeof(png_bytep) * height);
        for (y=0; y<height; y++)
                row_pointers[y] = (png_byte*) malloc(png_get_rowbytes(png_ptr,info_ptr));

        png_read_image(png_ptr, row_pointers);


void write_png_file(char* file_name)
        /* create file */
        FILE *fp = fopen(file_name, "wb");
        if (!fp)
                abort_("[write_png_file] File %s could not be opened for writing", file_name);

        /* initialize stuff */
        png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);

        if (!png_ptr)
                abort_("[write_png_file] png_create_write_struct failed");

        info_ptr = png_create_info_struct(png_ptr);
        if (!info_ptr)
                abort_("[write_png_file] png_create_info_struct failed");

        if (setjmp(png_jmpbuf(png_ptr)))
                abort_("[write_png_file] Error during init_io");

        png_init_io(png_ptr, fp);

        /* write header */
        if (setjmp(png_jmpbuf(png_ptr)))
                abort_("[write_png_file] Error during writing header");

        png_set_IHDR(png_ptr, info_ptr, width, height,
                     bit_depth, color_type, PNG_INTERLACE_NONE,

        png_write_info(png_ptr, info_ptr);

        /* write bytes */
        if (setjmp(png_jmpbuf(png_ptr)))
                abort_("[write_png_file] Error during writing bytes");

        png_write_image(png_ptr, row_pointers);

        /* end write */
        if (setjmp(png_jmpbuf(png_ptr)))
                abort_("[write_png_file] Error during end of write");

        png_write_end(png_ptr, NULL);

        /* cleanup heap allocation */
        for (y=0; y<height; y++)


void process_file(void)
        if (png_get_color_type(png_ptr, info_ptr) == PNG_COLOR_TYPE_RGB)
                abort_("[process_file] input file is PNG_COLOR_TYPE_RGB but must be PNG_COLOR_TYPE_RGBA "
                       "(lacks the alpha channel)");

        if (png_get_color_type(png_ptr, info_ptr) != PNG_COLOR_TYPE_RGBA)
                abort_("[process_file] color_type of input file must be PNG_COLOR_TYPE_RGBA (%d) (is %d)",
                       PNG_COLOR_TYPE_RGBA, png_get_color_type(png_ptr, info_ptr));

        for (y=0; y<height; y++) {
                png_byte* row = row_pointers[y];
                for (x=0; x<width; x++) {
                        png_byte* ptr = &(row[x*4]);
                        printf("Pixel at position [ %d - %d ] has RGBA values: %d - %d - %d - %d\n",
                               x, y, ptr[0], ptr[1], ptr[2], ptr[3]);

                        /* set red value to 0 and green value to the blue one */
                        ptr[0] = 0;
                        ptr[1] = ptr[2];

int main(int argc, char **argv)
        if (argc != 3)
                abort_("Usage: program_name <file_in> <file_out>");


        return 0;

Thanks to Sbi, Demur Rumed and Glenn Randers-Pehrson for contributions.

Last update: Wed Aug 3 14:05:04 2011