dtk_create_arrow man page
dtk_create_arrow — Create and modify arrow shape
dtk_hshape dtk_create_arrow(dtk_hshape shp,
float cx, float cy, float width, float height,
int filled, const float *color);
dtk_create_arrow() creates an arrow pointing to the left with the pointed pointed located at (cx,cy) with a total width of width, a hat of length width/3 , a total height of height and the width of the body being height/2.
shp, filled and color have the same usage and meaning as for other shape creation function:
shp can be used to modify a previously created shape. If it is non-null, the handle will be used to modify the shape referenced by shp: no new shape is created and the returned value is ensured to be shp in case of success, NULL otherwise. If shp is NULL, the function will attempt to create a new shape.
filled is set to 0 if the described shape should be a continuous line or a non-null value if the shape is supposed to be filled polygon.
color should point to a 4 float array containing a normalized RGBA value (i.e. each component is between 0.0 and 1.0).
In case of success these functions return the handle to the newly created or modified shape. If the shp argument is non-null, the handle returned is the same value. In case of error, NULL is returned.