I'm glad you've at least tried Selba Ward; it is there for people to use!
I mentioned Spline only as a simple and easy way to draw a polygon's outline. Simply export Polygon's positions and set all Spline's positions from that export. Remember to close the Spline. You can also use a thickness for Spline so the outline is no longer infinitely thin (and drawn as a pixel thickness). You can
also round the corners. Note that Spline creates the entire polyline as a single object and draws it with a single draw call whereas drawing multiple sf::RectangleShapes (one for each edge) is one draw per edge. FYI, Selba Ward's Line can also draw a simple line with or without thickness and also used to use sf::RectangleShape internally but was replaced by custom vertex array. Still, Spline's better
Another thing to note is that if anything in Selba Ward isn't generic enough to be useful in multiple scenarios, I'd like to consider updates and improvements, and if anything is completely missing from Selba Ward, I consider adding it.
Selba Ward is still improving and Polygon is actually pretty new so somethings are yet to come.
My responses were to help with the creation of the object that you were having trouble with. Hope you at least got that shape made! Note that I didn't mention Selba Ward at first and tried to explain how to create the object yourself.
My point about any complex being able to be represented by triangles still stands. However, this is separate from being able to triangulate from a given set of points. Does the "butterfly shape" have holes? Polygon can handle holes but must be informed where they are; hole vertices must also be in opposite (clockwise) direction. In addition, the triangulation method must be set to "EarClip" rather than the more simple "BasicEarClip" to be able to handle the holes. Note that Polygon can only handle "simple polygons"; that is, a polygon that never crosses itself. Holes are technically not part of a "simple polygon" but Polygon converts it into a simple polygon first (as a bonus!).
If you could provide the vertices for the "butterfly shape" and an image of what it should look like, I'd like to do some tests.
It's worth noting that Polygon is designed to be a way to provide a filled polygonal area from a set of points representing its boundary. It's not, however, designed to have visual effects that manipulate those points (such as corner rounding). Those effects should be done separately and given to Polygon to create the shape. In fact, Spline can do this; it can handle Bezier curves and can export its interpolated position so a curved boundary could be created in Spline and then passed to Polygon to create the shape.
It was considered to have a base for both Spline and Polygon as they have similar concepts but decided against so that each Selba Ward drawable can be used independently.
Reading what you wrote about the "butterfly shape" again, I realise that you are creating a polygon where edges cross. This is not a simple polygon and cannot be handled automatically by Polygon. It breaks the rule that vertices must be passed in the same direction. The "butterfly" has vertices in
both clockwise
and anti-clockwise directions.
Probably better to not have allowed a polygon definition to do this than to try to fix it when the mistake is made.
This could be considered "ill-formed" as it has a "twisted" and not flat face. In fact, if that shape (like a bow-tie, I'm imagining) is required, it can be created; the point in the centre should be included since it exists. It would basically be 6 vertices (5 different ones) which would be the rectangle (in anti-clockwise direction) with the centre vertex between the two vertices of each opposing edge (either top and bottom or left and right).
It could be thought of this way: why is a shape with 2 triangles trying be defined without giving one of the points of each triangle?
Pre-processing can "fix" this. It can already be done before giving the points to Polygon (by specifying them correctly) but features that check and/or adjust for this could be included in Polygon at a later time.
As an aside, Polygon should, in the future, also be able to use monotone triangulation (uses scanline).
I hope I've addressed your questions and helped with your issues. If I missed anything, feel free to let me know.
Also, I'm open to ideas for Polygon and also, more generally, Selba Ward.
EDIT (2024/02/15):
Selba Ward's Polygon can now (as of v1.4), in fact, also work points that are in a clockwise order!
And more!
For more info, see this:
https://github.com/Hapaxia/SelbaWard/pull/46
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