Drawing complex shapes - fl_arc

[pvr...@btinternet.com]

From the documentation:

void fl_arc(double x, double y, double r, double start, double end)

Add a series of points to the current path on the arc of a circle; you can get elliptical paths by using scale and rotate before calling fl_arc(). The center of the circle is given by x and y, and r is its radius. fl_arc() takes start and end angles that are measured in degrees counter-clockwise from 3 o'clock. If end is less than start then it draws the arc in a clockwise direction.

This infers that the two calls:

fl_arc(0, 0, radius, 90, 180);

fl_arc(0, 0, radius, 180, 90);

result in the same drawing - the arc selected is from 9 o'clock clockwise to 12 o'clock. And indeed they do.

I coded:

fl_arc(0, 0, radius, 180, 450);

to achieve what I wanted which was to draw an arc clockwise from 12 o'clock to 9 o'clock.

My concern is that this is not properly documented. Should I have drawn two separate arcs from 180 to 360 and 0 to 90?

I also noticed that the code

fl_start_polygon();

fl_vector(0,0);

fl_arc(0,0, radius, 180, 450);

fl_end_polygon();

actually worked to give me the 3/4 of the pie even though it is a concave shape.

Phil.

[Matthias Melcher]

The API is from 1998 or earlier. I would implement things differently today, but we need to live with the behavior of `fl_arc` as it is for back compatibility. If you have an idea how to document the peculiarities of `fl_arc` in a better way, we would appreciate text suggestions here or as a PR on GitHub to improve documentation.

[pvrose]

On 13 April 2024, at 11:59, "'Matthias Melcher' via fltk.general" <fltkg...@googlegroups.com> wrote:

>The API is from 1998 or earlier. I would implement things differently today, but we need to live with the behavior of `fl_arc` as it is for back compatibility. If you have an idea how to document the peculiarities of `fl_arc` in a better way, we would appreciate text suggestions here or as a PR on GitHub to improve documentation.
>

Thanks Matthias.

My suggestion would be to add a statement or image on how to draw an arc counter-clockwise through 3 o'clock. Something on the lines of: if the second angle is less than the first, then add 360° to the second angle to maintain ccw drawing.

If that is indeed the supported method.

Regards Phil.
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[Matthias Melcher]

Thanks. I updated the docs with some different wording. I hope that the intention is clear now.