Function which returns the pixel color for a XBM (X BitMap) Image

get-xbm-pixel.c

/*
 * returns the pixel color for a XBM (X BitMap) image.
 *
 * XBM is a plain text monochrome image format that takes the form of 
 * C source code. images can directly be compiled into an application.
 *
 */
static int is_black(int x, int y, int img_width, int img_height) 
{
	char byte_containing_px;
	
	/* byte, containing pixel bit */
	int	byte_containing_px_position = ((y * img_width) + x) / 8;
	
	
	if (img_width % 8 > 0)
		byte_containing_px_position += (y / 8) + 1;
	
	/* pointer in ranges? */
	if (byte_containing_px_position >=  ((img_width * img_height) / 8))
		return 0;
	
	byte_containing_px = *(earth_bits + byte_containing_px_position);
	
	/* searched byte has to be on rightmost position */
	byte_containing_px >>= x;
	
	return (byte_containing_px & 0x1) ? 1 : 0;
}


/* 
 * checks whether there is a bigger black then white part
 * in the square "width * height"
 */
static Bool is_more_col(int x, int y, float point_width_on_map, 
				float point_height_on_map) 
{
	int x1, y1;
	int pixels_in_color = 0;
	int treshold = (point_width_on_map * point_height_on_map) / 2;
	
	for (x1 = 0; x1 < point_width_on_map; x1++) {
		for (y1 = 0; y1 < point_height_on_map; y1++) {
			
			if (is_black(x + x1, y + y1, EARTH_WIDTH, EARTH_HEIGHT) == 1)
				pixelsInColor++;
			
		}
	}
	
	if (pixels_in_color >= treshold)
		return True;
	else
		return False;
}

I've found out that this function doesn't work when bitmap is wider than 8 pixels - it happens because of that bit shift in line 24. Program tries to shift byte more than 8 bits right, therefore returns always zero. It can be fixed easily by replacing x with x%8 :)