blog20100729-2.txt

root@mini2440:/dev/shm# time pngread small.png 
begin
Open
read
png_ptr
Init
info
jmpbuf
sig bytes
info2
read_image
Read done
real	0m 0.02s
user	0m 0.01s
sys	0m 0.01s

blog20100729.txt

root@mini2440:/dev/shm# ls -lh test.png
-rwxr-xr-x    1 root     root        79.7K Apr  5 14:01 test.png
root@mini2440:/dev/shm# time ./pngread test.png 
begin
Open
read
png_ptr
Init
info
jmpbuf
sig bytes
info2
read_image
Read done
Pixel at position [ 1679 - 1049 ] has the following RGBA values: 212 - 208 - 200 - 255
real	0m 1.16s
user	0m 1.06s
sys	0m 0.10s
root@mini2440:/dev/shm# 

pngread.c

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

#define PNG_DEBUG 3
#include <png.h>

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 abort_(const char * s, ...)
{
	va_list args;
	va_start(args, s);
	vfprintf(stderr, s, args);
	fprintf(stderr, "\n");
	va_end(args);
	abort();
}

void read_png_file(char* file_name)
{
	char header[8];	// 8 is the maximum size that can be checked
        printf("begin\n");
	/* 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);
        else printf("Open\n");
	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);
	else printf("read\n");


	/* initialize stuff */
	png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
	printf("png_ptr\n");
	
	if (!png_ptr)
		abort_("[read_png_file] png_create_read_struct failed");
	else printf("Init\n");

	info_ptr = png_create_info_struct(png_ptr);
	if (!info_ptr)
		abort_("[read_png_file] png_create_info_struct failed");
	else printf("info\n");

	if (setjmp(png_jmpbuf(png_ptr)))
		abort_("[read_png_file] Error during init_io");
	else printf("jmpbuf\n");

	png_init_io(png_ptr, fp);
	png_set_sig_bytes(png_ptr, 8);
        printf("sig bytes\n");
        
	png_read_info(png_ptr, info_ptr);
        printf("info2\n");
	width = info_ptr->width;
	height = info_ptr->height;
	color_type = info_ptr->color_type;
	bit_depth = info_ptr->bit_depth;

	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");
	else printf("read_image\n");

	row_pointers = (png_bytep*) malloc(sizeof(png_bytep) * height);
	for (y=0; y<height; y++)
		row_pointers[y] = (png_byte*) malloc(info_ptr->rowbytes);

	png_read_image(png_ptr, row_pointers);

        fclose(fp);
}

void process_file(void)
{
	if (info_ptr->color_type != PNG_COLOR_TYPE_RGBA)
		abort_("[process_file] color_type of input file must be PNG_COLOR_TYPE_RGBA (is %d)",
                       info_ptr->color_type);
        x=1679;
        y=1049;
        png_byte* row = row_pointers[y];
	png_byte* ptr = &(row[x*4]);
	printf("Pixel at position [ %d - %d ] has the following RGBA values: %d - %d - %d - %d\n",
			       x, y, ptr[0], ptr[1], ptr[2], ptr[3]);
}

int main(int argc, char **argv)
{
        if (argc != 2)
		abort_("Usage: program_name <file_in>");
	read_png_file(argv[1]);
	printf("Read done\n");
	process_file();
        return 0;
}