def check_collision(obj1,obj2):
    """checks if two objects have collided, using hitmasks"""
    try:rect1, rect2, hm1, hm2 = obj1.rect, obj2.rect, obj1.hitmask, obj2.hitmask
    except AttributeError:return False
    rect=rect1.clip(rect2)
    if rect.width==0 or rect.height==0:
        return False
    x1,y1,x2,y2 = rect.x-rect1.x,rect.y-rect1.y,rect.x-rect2.x,rect.y-rect2.y
    for x in range(rect.width):
        for y in range(rect.height):
            if hm1[x1+x][y1+y] and hm2[x2+x][y2+y]:return True
            else:continue
    return False
 
def get_colorkey_hitmask(image, rect, key=None):
    """returns a hitmask using an image's colorkey.
       image->pygame Surface,
       rect->pygame Rect that fits image,
       key->an over-ride color, if not None will be used instead of the image's colorkey"""
    if key==None:colorkey=image.get_colorkey()
    else:colorkey=key
    mask=[]
    for x in range(rect.width):
        mask.append([])
        for y in range(rect.height):
            mask[x].append(not image.get_at((x,y)) == colorkey)
    return mask
 
def get_alpha_hitmask(image, rect, alpha=0):
    """returns a hitmask using an image's alpha.
       image->pygame Surface,
       rect->pygame Rect that fits image,
       alpha->the alpha amount that is invisible in collisions"""
    mask=[]
    for x in range(rect.width):
        mask.append([])
        for y in range(rect.height):
            mask[x].append(not image.get_at((x,y))[3]==alpha)
    return mask
 
def get_colorkey_and_alpha_hitmask(image, rect, key=None, alpha=0):
    """returns a hitmask using an image's colorkey and alpha."""
    mask=[]
    for x in range(rect.width):
        mask.append([])
        for y in range(rect.height):
            mask[x].append(not (image.get_at((x,y))[3]==alpha or\
                                image.get_at((x,y))==colorkey))
    return mask
 
def get_full_hitmask(image, rect):
    """returns a completely full hitmask that fits the image,
       without referencing the images colorkey or alpha."""
    mask=[]
    for x in range(rect.width):
        mask.append([])
        for y in range(rect.height):
            mask[x].append(True)
    return mask
