the the (original) segment's y2 is larger than its y1 */
typedef enum {DIR_UP, DIR_DOWN, DIR_UNKNOWN} segment_dir_t;
-#define DIR_INVERT(d) ((d)^=(DIR_UP^DIR_DOWN))
+#define DIR_INVERT(d) ((d)^(DIR_UP^DIR_DOWN))
typedef struct _edgestyle {
void*internal;