GIMP_TOOL_CONSTRAIN_15_DEGREES

#define GIMP_TOOL_CONSTRAIN_15_DEGREES 12

GIMP_TOOL_CONSTRAIN_45_DEGREES

#define GIMP_TOOL_CONSTRAIN_45_DEGREES 4

GIMP_TOOL_CONSTRAIN_90_DEGREES

#define GIMP_TOOL_CONSTRAIN_90_DEGREES 2

gimp_tool_motion_constrain ()

void                gimp_tool_motion_constrain          (gdouble start_x,
                                                         gdouble start_y,
                                                         gdouble *end_x,
                                                         gdouble *end_y,
                                                         gint n_snap_lines);

Projects a line onto the specified subset of evenly radially distributed lines. n_lines of 2 makes the line snap horizontally or vertically. n_lines of 4 snaps on 45 degree steps. n_lines of 12 on 15 degree steps. etc.