Surfaces and Curves (Introduction)

Surfaces and curves consist of one or more outlines. Each outline consists of an arbitrary sequence of the following sub-objects:

Line

Bézier curve

Circular Arc

A surface is closed by definition.

If you draw a rectangle using four separate lines, and ensure that all start and endpoint are identical, CAD6 would still consider this contour as four independent objects. By using trimming functions, it is possible to generate a compound surface from those contour elements, or to create a surface object in the first place (e.g. Draw > Polygon > Rectangle).

In addition to the outline, a surface has got a filling. The filling can consists of both a fill color and a hatching. If a surface consists of only one outline, simply the inside area of that outline will be filled:

If a surface consists of multiple outlines, they will be filled alternately, i.e. only areas overlapped by an odd number of outlines (1, 3, 5, ...) will be filled. If, e.g., a small outline lies inside a large one, this small round outline will be transparent, as its inside is overlapped by two outlines:

A curve, however, is open. The start and end point of an outline can have the same coordinate but it is not a must. A curve has never a filling.

Generate Surface

A rectangle is generated from four lines. The initial objects are shown below. For the sake of a better overview the marking points are displayed.

Generate Surface

By using Trim > Surface / Curve > Generate Surface all involved objects for the generation can be specified. After choosing the objects, several reference points can be entered, one after another. These points can be entered using the mouse, by clicking anywhere in the drawing. A reference point specified the area that is used for the surface. In the current example the correct reference point is a point somewhere in the middle of the closed area. By right clicking the command are terminated and the surface are generated. The following figure shows the result of a correct input.

Result Surface

Note that all lines which have been involved during the generation process must be deleted to get the result of the figure above. Use the command Modify > Delete Objects and identify all previous selected objects by using "F11" (Previous Identification).

The next example illustrates the positioning of the reference points and shows how a reference point affects the surface to be generate.

The position of a reference point determines how the surface are generated. Each reference point should be positioned next to the contour element that is involved by the generation. Is the reference point positioned outside of the closed contour, the surface is generated related to the surrounding frame of the drawing.

Generate Surface

A last example simply explains the reference point handling. Imagine emptying a can of paint at the reference point. The paint spreads out in all directions until it reaches an object. The result is one area which is covered in paint, and one area which is not. The boundary between these two areas is the outline of the newly-generated surface.

A further and often used application is the generation of a single surface based on a combination of several surfaces. The next example shows that kind of entity. The rectangle are created by using Draw > Polygon > Rectangle and the circles by using Draw > Circle > Center - Point on Circle. For a better overview each single surface has been assigned a fill color by changing the object properties.

Generate Surface

CAD6 has got a variety of methods to combine several surfaces to a single surface.

1st Method:

The command Trim > Surface / Curve > Difference (A=A-B) can be used to combine two surfaces. The result is the same as the difference of two surfaces according to the group theory. Choose for the first objects the surrounding surface and for the second object the enclosed object. The status bar shows the expected user input. Retry the command for each single surface.

2nd Method:

After choosing the command Trim > Surface / Curve > Generate Surface, all involved objects have to be identified, inclusive of the surrounding polygon. A reference point must be set on every single surface. The entry can be terminated by clicking the right mouse button. Although it seems that only one object is displayed the source objects are still exiting in the background. Erase these objects by using Modify > Delete Objects and identifying all previous selected objects ("F11").

3rd Method:

The command Trim > Surface / Curve > Combine Outlines combines the outlines of all identified surface to a single surface.

Independent of the used method the result is one surface. For a better illustration the surface is filled either.

Result Surface

Assemble Surface

Unclosed outlines can be closed by using Trim > Surface / Curve > Assemble Surface. This means that the start point of the outline will be connect with the end point by a straight line. The only thing to do is to identify the object and carry out the command by clicking the right mouse button.

Assemble Curve

Several curves can be combined to one continuous curve by using Trim > Surface / Curve > Assemble Curve. The identification order determines the result.