Arcs.-
The object Arc is a quite sophisticated object as a great effort has been made to give it all the capabilities included in Java, possible to translate to Metapost. Although it can be used in its simplest way, the most usual, it is also possible to take advantage of its advanced possibilities.
Everything related with the object Arc can be applied to its closed variations.
An arc is created in the same way as all the other objects: click & drag to create the first two point. A third click in some other point will define totally the arc, and it will be visible. In what follows it will be explained what are these points:
The first two points created by clicking and dragging the mouse will be the center of the circle from which the arc is a part (where the button has been clicked the first time) and the origin of the arc (where the mouse is released after dragging). The line connecting these two point, will be the radius of the theoretical circle.
The third and last point clicked afterwards, will define the direction of the second radius that will finish the arc. It can be clicked anywhere as it will serve only to establish the theoretical line trough this point and the center. The intersection of this line with the “father circle”, will define the third point.
The arc will begin always at the first point and will continue in a counterclockwise way until the the last point is attained. Once created any one of the three points used to its definition, can be edited (selected and moved) in the usual double way (mouse/arrow-keys). Nevertheless, there is a small difference according to the point selected. Changing the position of any one of the first two points will produce an automatic change of the third one. If the edited point is the third, the position of the first two will not change. Only the angle can be changed. So, the movements of the third points are constrained.
When creating or selecting an arc, a couple of values will appear on the text window at bottom, signaling the angle between the two extreme radius, as first value, and the angle between the first radius and the horizontal.
The arc can be used to create half circles or ellipses. It will support text placed on it, and all type of arrows.
The arc of circle, can be transformed into an arc of ellipse. When in mode “Ctrl-ON”, selecting the arc will not allow for any kind of rotation or scaling. Instead, it will be seen only a vertical short line finished in a small circle. Clicking on this circle and dragging the mouse cursor upwards, will make visible a new arc line appearing under the circular one, now in color blue. This is the transformed ellipse arc produced from the circular one according with the methods used by Java.
For understanding how this elliptical arc is created it should be known that the new extreme points of the arc are placed on the same vertical as the original on the circular one. One “surprising” consequence of this is that the angle between the lines connecting the center this the extreme points, will change, but Mg5 will refer always to the original angle corresponding to the circular one. And this is true also for the values shown on the text windows. Anyway, knowing this it is simply to accommodate everything.
For rotating or scaling the object arc, it should be used the “keyboard” method explained before. The center of the figure is the center of the theoretical circle. Rotating an arc will just rotate the sector of the circle used, inside this circle. If the arc as been transformed to an arc of ellipse, rotating it will permit to visualize better how the sector is moving inside an ellipse.
Madrid, January 2006 |
|