/* for some reason we have to flip the image vertically */
void flipVertical(unsigned char *data, int w, int h){
	int x, y, i1, i2;
	unsigned char temp;
	for (x=0;x<w;x++){
		for (y=0;y<h/2;y++){
			i1 = (y*w + x)*3; // this pixel
			i2 = ((h - y - 1)*w + x)*3; // its opposite (across x-axis)
			if (i1 == 226953) {
				int k = 5;
			}

			// swap pixels
			temp = data[i1];
			data[i1] = data[i2];
			data[i2] = temp;

			i1++; i2++;
			temp = data[i1];
			data[i1] = data[i2];
			data[i2] = temp;

			i1++; i2++;
			temp = data[i1];
			data[i1] = data[i2];
			data[i2] = temp;

		}
	}
}


/* write the current frame buffer to the file 'filename' */
void writeFrameBuffer(char *filename){
	FILE *fp = fopen(filename, "wb");
	int data_size = frame_width*frame_height*3;
	unsigned char *framebuffer = malloc(data_size * sizeof(unsigned char));
	glReadPixels(0, 0, frame_width, frame_height, GL_RGB, GL_UNSIGNED_BYTE, framebuffer);
	flipVertical(framebuffer, frame_width, frame_height);

    fprintf(fp, "P6\n%d %d\n%d\n", frame_width, frame_height, 255);
	fwrite(framebuffer, data_size, 1, fp);
	fclose(fp);
}