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#26 arrow - head size
The head radius and shaft radius should be independently specifiable in the program. No set default can be perfect in all cases.
I believe that the default "radius" value should specify the shaft radius, since the shaft is usually the major part of the arrow. The arrow is like a cylinder with an ornament on the end.
Discussion
I second this. It is perhaps noteworthy that all drawing programs make the shaft and its thickness primary, with the head considered a decoration. I don't find compelling the argument that the "bounding box" is the important concept.
I've replaced "radius" with "shaftwidth". Because width is twice radius and the shaft width is one half the head width, "radius" and "shaftwidth" are equivalent.
Later there will be a "headwidth" and perhaps even a "headlength".
Because an arrow is like a cylinder, I think it should be specified by a shaft radius, not a diameter. It's confusing to try to remember different conventions for two very similar objects.
How is an arrow like a cylinder? It's composed of a box and a pyramid. Is the radius of the shaft the radius of a circle inscribing or circumscribing it?
There are now three separate attributes:
headlength
headwidth
shaftwidth
Assigning any of these attributes to 0 makes it use defaults based on the size of the arrow. It is the responsibility of the user to make the ratios between these three attributes reasonable, or to use defaults.
If the headlength becomes larger than half the length of the arrow, or the shaft becomes thinner than 1/50 the length, the entire arrow is scaled accordingly.