It will calculate the centre of this and this but not this
For polygons of less than 100 sides, the centre of area is indicated by the black dot, which you can see on the first two polygons above.
Tip: The dot is only visible while the polygon is selected. To mark the centre of area, you can snap on to the centre of the dot (you must have "center" selected in the Snap pallet) and draw a line or circle-by-center.
The length and area data are shown in the Edit pallet as follows:
L: Gives the length of all the sides combined. For a closed polygon this will be the same as the periphery.
P: Gives the length of the periphery. For an open polygon this will include the length from the two open end points.
S: Gives the area of the polygon. For an open polygon, it assumes it is closed with an imaginary line joining the two open end points.
For polygons of 100 or more sides, because of the complexity of the calculation, it slows the program down too much to calculate on the fly. Thus you need select the polygon and then click the "Length, area, center" menu item.
The image on the right is a frame drawing from a sailboat design. The area in the middle has been selected and the centre of area is indicated by the circle with a cross through it. Unlike polygons with less that 100 sides, the centre mark stays there after the polygon is deselected.
The length and area data are shown with the centre icon and are also shown on the Edit pallet as described above for polygons under 100 sides.
With the example shown, the frame is of constant thickness. So the center of area is also the centre of gravity (CoG). And the weight is the area multiplied by the thickness multiplied by the density. With this frame, to find the weight of the frame, we can take the outer shape and find its center and area and then deduct the area of the inside aperture (the polygon selected in the image). To calculate the vertical CoG of the frame we can use moment of the distances of the two centres from a common zero point and their weights, which is a classic "see-saw" calculation.
Tip: If you don't get a center of area with a complex polygon, it usually means that two of the lines making it up are overlapping or crossed. You will still get a length and periphery but area (S) in the Edit pallet will show as NAN (Not A Number). This is actually a useful check if you are supplying CNC cutting files – which must usually be complete, with no crossed lines etc., in order to cut correctly.
Tip: Again, with a complex polygon if you get an area OK but the length (L) is different from the periphery (P) then you know there is a gap somewhere in your polygon. This is also usually undesirable with files for CNC cutting.
As with many functions in RealCADD, the center of area calculations are contributing to the engineering of a project as well as simply drawing components.
Apply attributes (keyboard shortcut ⌥A) applies the attributes (as shown in the Attributes pallet) of a selected object to subsequently selected objects.
To apply attributes, select the object that has the attributes you want to transfer. Then, holding the shift key down, select the objects you want to transfer the attributes to. Hit ⌥A. The second and subsequently selected objects will take on the attributes of the first object you selected.
All the attributes in the Attributes pallet will be transferred. If the object is text, then the attributes in the Text pallet will also be transferred.
Tip: In the Cocoa version the keyboard shortcut (⌥A) may not work the first time it is used when RealCADD is first opened. If you do it manually (i.e. using the menu), the keyboard shortcut will work normally thereafter. This seems to be a bug in all shortcuts using the ⌥ key and a single operator.
Tip: The Action menu can also be accessed directly on the screen: select an object or objects and right-click or ⌃-left-click (control-left-click).