Why is Rect empty when bottom <= top?

[Aglargil]

Clipper2/CPP/Clipper2Lib/include/clipper2/clipper.core.h

Line 351 in ff37866

bool IsEmpty() const { return bottom <= top || right <= left; };

[miemoe]

That is a good question. The question is actually what means left, right, bottom, and top in this context.

A hint might be in the documentation of isPositive(): https://www.angusj.com/clipper2/Docs/Units/Clipper/Functions/IsPositive.htm;
there a description can be found.
And other hints are in https://www.angusj.com/clipper2/Docs/Units/Clipper/Types/Path64.htm where outer contours and holes are defined by the rotation/orientation (clock-wise or counter-clock-wise).

From the implementation one would deduce, that the implementer is relating to a rect that has some area, if

• its left border has a smaller (x-) value than the right border, and
• the bottom border has a higher (y-) value than the top border.
  So, we are "looking" at a coordinate system, where the x-axis is pointing to rightwards and the y-axis is pointing downwards.
  It seems, that the implementer is fixed in this (very common) axis orientation in computer graphics (at least in 2-D).

@AngusJohnson Maybe it would be time to fill the documentation of:
https://www.angusj.com/clipper2/Docs/Units/Clipper/Types/Rect64.htm

[Aglargil]

Thank you for your answer.
But according to the IsPositive documentation, the Cartesian coordinate system is supposed to be the one currently used by Clipper2Lib, so I assume that theoretically it should be “top >= bottom”, is there anything wrong with thinking this way?

Positive winding paths will be oriented in an anti-clockwise direction in Cartesian coordinates (where x coordinate values increase toward the right and y coordinate values increase upward). However, in graphics display libraries that use an inverted Y-axis, Positive winding paths will be oriented clockwise.

[miemoe]

Thank you for your answer. But according to the IsPositive documentation, the Cartesian coordinate system is supposed to be the one currently used by Clipper2Lib, so I assume that theoretically it should be “top >= bottom”, is there anything wrong with thinking this way?

Positive winding paths will be oriented in an anti-clockwise direction in Cartesian coordinates (where x coordinate values increase toward the right and y coordinate values increase upward). However, in graphics display libraries that use an inverted Y-axis, Positive winding paths will be oriented clockwise.

There are right-handed Cartesian coordinate systems and left-handed systems. Maybe have a look
at https://en.wikipedia.org/wiki/Cartesian_coordinate_system in the section
Orientation and handedness..
As long as you do not speak about left, right or top, bottom you just have two ortogonal axes in a system. First by introducing orientations you have to decide and be consistent...
Maybe the documentation should note the Orientation idea.
In OpenGL the winding is used to decide if you look at the front or back of a polygons shape...