Sketcher CreateEllipseByCenter/zh-cn

描述
本工具会根据用户拾取的三个点：中点、长半轴点与短半轴点来绘制一个椭圆. 在开启此工具时，鼠标指针会变为一个附有红色椭圆图标的白色十字. 同时还会将当前的指针坐标实时显现在侧.



如何使用

 * Invoke the command by clicking a toolbar button, picking the menu item, or by using keyboard shortcut (needs to be assigned first in Interface Customization).
 * First click in 3D view sets ellipse center. Second click sets the first radius and orientation of the ellipse. Third click sets the other radius (the distance from the line defined by first two clicks is the second radius).
 * After the third click, the ellipse is created, together with a set of construction geometry aligned to it (major diameter, minor diameter, two foci). The construction geometry can be manually deleted if not needed, and recreated later. See Internal Alignment Constraint and Sketcher Show Hide Internal Geometry.
 * Pressing or clicking the right mouse button cancels the function.

Peculiarities

 * Major and minor axes of ellipses are strict and cannot be swapped by resizing the ellipse. This is a consequence of the solver parametrization used (center (x,y), focus1 (x,y) and minor radius length (b)) and the same strict behavior of OpenCascade. The ellipse must be rotated to swap the axes.
 * Ellipse can function as a circle when its major and minor diameter lines are deleted, and one of the foci is constrained to coincide with the center. But radius constraint won't work on such a circle.
 * Moving the ellipse by edge is the same as moving ellipse's center.