png_loader.h

//	Taken from the OpenGL Wikibook: http://en.wikibooks.org/wiki/OpenGL_Programming/Intermediate/Textures#A_simple_libpng_example

#include <cstdio>
#include <string>

#ifdef __APPLE__
#  include <OpenGL/gl.h>
#  include <OpenGL/glu.h>
#  include "/usr/X11/include/png.h"
#else
#  include <GL/gl.h>
#  include <GL/glu.h>
#  include <png.h>
#endif

#define TEXTURE_LOAD_ERROR 0

using namespace std;
/** loadTexture
 *     loads a png file into an opengl texture object, using cstdio , libpng, and opengl.
 * 
 *     \param filename : the png file to be loaded
 *     \param width : width of png, to be updated as a side effect of this function
 *     \param height : height of png, to be updated as a side effect of this function
 * 
 *     \return GLuint : an opengl texture id.  Will be 0 if there is a major error,
 *                                     should be validated by the client of this function.
 * 
 */
GLuint loadTexture(const string filename, int &width, int &height) 
{
	//header for testing if it is a png
	png_byte header[8];

	//open file as binary
	FILE *fp = fopen(filename.c_str(), "rb");
	if (!fp) {
		return TEXTURE_LOAD_ERROR;
	}

	//read the header
	fread(header, 1, 8, fp);

	//test if png
	int is_png = !png_sig_cmp(header, 0, 8);
	if (!is_png) {
		fclose(fp);
		return TEXTURE_LOAD_ERROR;
	}

	//create png struct
	png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL,
			NULL, NULL);
	if (!png_ptr) {
		fclose(fp);
		return (TEXTURE_LOAD_ERROR);
	}

	//create png info struct
	png_infop info_ptr = png_create_info_struct(png_ptr);
	if (!info_ptr) {
		png_destroy_read_struct(&png_ptr, (png_infopp) NULL, (png_infopp) NULL);
		fclose(fp);
		return (TEXTURE_LOAD_ERROR);
	}

	//create png info struct
	png_infop end_info = png_create_info_struct(png_ptr);
	if (!end_info) {
		png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp) NULL);
		fclose(fp);
		return (TEXTURE_LOAD_ERROR);
	}

	//png error stuff, not sure libpng man suggests this.
	if (setjmp(png_jmpbuf(png_ptr))) {
		png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
		fclose(fp);
		return (TEXTURE_LOAD_ERROR);
	}

	//init png reading
	png_init_io(png_ptr, fp);

	//let libpng know you already read the first 8 bytes
	png_set_sig_bytes(png_ptr, 8);

	// read all the info up to the image data
	png_read_info(png_ptr, info_ptr);

	//variables to pass to get info
	int bit_depth, color_type;
	png_uint_32 twidth, theight;

	// get info about png
	png_get_IHDR(png_ptr, info_ptr, &twidth, &theight, &bit_depth, &color_type,
			NULL, NULL, NULL);

	//update width and height based on png info
	width = twidth;
	height = theight;

	// Update the png info struct.
	png_read_update_info(png_ptr, info_ptr);

	// Row size in bytes.
	int rowbytes = png_get_rowbytes(png_ptr, info_ptr);

	// Allocate the image_data as a big block, to be given to opengl
	png_byte *image_data = new png_byte[rowbytes * height];
	if (!image_data) {
		//clean up memory and close stuff
		png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
		fclose(fp);
		return TEXTURE_LOAD_ERROR;
	}

	//row_pointers is for pointing to image_data for reading the png with libpng
	png_bytep *row_pointers = new png_bytep[height];
	if (!row_pointers) {
		//clean up memory and close stuff
		png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
		delete[] image_data;
		fclose(fp);
		return TEXTURE_LOAD_ERROR;
	}
	// set the individual row_pointers to point at the correct offsets of image_data
	for (int i = 0; i < height; ++i)
		row_pointers[height - 1 - i] = image_data + i * rowbytes;

	//read the png into image_data through row_pointers
	png_read_image(png_ptr, row_pointers);

	//Now generate the OpenGL texture object
	GLuint texture;
	glGenTextures(1, &texture);
	glBindTexture(GL_TEXTURE_2D, texture);
	glTexImage2D(GL_TEXTURE_2D,0, GL_RGBA, width, height, 0,
			GL_RGBA, GL_UNSIGNED_BYTE, (GLvoid*) image_data);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

	//clean up memory and close stuff
	png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
	delete[] image_data;
	delete[] row_pointers;
	fclose(fp);

	return texture;
}