#define bishop_width 32
#define bishop_height 32
-static char bishop_bits[] = {
+static unsigned char bishop_bits[] = {
0x00,0x00,0x00,0x00,0x00,0xc0,0x01,0x00,0x00,0x30,0x06,0x00,0x00,0x0c,0x18,
0x00,0x00,0x03,0x60,0x00,0xc0,0x00,0x80,0x01,0x20,0xe0,0x03,0x02,0x20,0x18,
0x03,0x06,0x20,0x80,0x01,0x06,0x20,0xfc,0x0f,0x06,0x10,0xcc,0x0c,0x04,0x10,