default value when foreground and background both in color key range
COLORKEY_DEFAULT_B | default B channle value of color key |
COLORKEY_DEFAULT_G | default G channle value of color key |
COLORKEY_DEFAULT_R | default R channle value of color key |
COLORKEY_FG_BG_REVERSE | when pixel in bg ck range but not in fg ck range, 0: the result is bg, 1: the result is fg |