STRETCH

Purpose:

To enable a user to change the shape of geometric objects by various 'stretching' operations.

Prerequisites:

The drawing must be open and currently viewed on the screen. The display must contain the geometric elements that are to be stretched. It is the responsibility of the user to determine if the stretching of an object produces a meaningful shape and the system does not contain any logic and/or limitations.

Instructions:

1. In the Modify menu, click Stretch.

2. On entering this function the user is presented with a multiple choice dialog box, containing six options, from which he must make a selection before proceeding any further. The usage of each option is described in some detail below. The first two options relate to the local redefinition of a region of a contour, and the last four relate to any geometric entity or collection of entities.

Options:

Option 0. Node

This option allows the user to move a contour node with the automatic updating of the bounding/neighbor segments.

Option 1. Elbow

This option allows the user to replace a single contour segment by two adjacent straight-line segments, meeting at a user defined 'crack' point

Options 2 to 5

These four options require the same graphics screen input from the user, however the methods whereby the system accomplishes the 'stretching' operation itself, and then creates the corresponding result, differ.

Basically the user moves a selected region, in an orthogonal manner, to a new location on the screen and if required rotates it in any direction and by any amount, either before or after translation. Although these simple manipulations will suffice for many design purposes the user can actually invoke all of the Transformation functions that are available within the system, as made available within the Geometry function. The system then stretches the connecting geometry, outside of the selected region, in order to meet with the transformed region.

The user positions the cursor at an appropriate point and clicks the Left-hand button once. As he then moves the cursor the system continuously draws an orthogonal rectangular box with the previously selected point as one corner and the current position of the cursor as the diagonally opposite corner. When a box has been created that covers the region to be transformed then the user clicks the left-hand button again.

It is most important to note that the lines and other geometric entities that cross into, and out of, the box are the entities that are to be eventually stretched by this function, in a manner that also depends on the type of geometric entity. Hence the user needs to ensure that geometric entities that are to be transformed within the box are completely within the box. This applies to, for example, arcs and circles that may pass out of and then re-enter the box, as the parts that are outside of the box will be held fixed and will not be moved as the other parts of the same element are transformed within the box.

After the user has accepted the position and size of the box, and has clicked the left-hand button the second time, then a vertical tool bar appears, offering all of the transformation icons.

After the transformation activity has been completed, the transformed contents of the box will remain on the screen, together with the original position of the box and its original contents. The user should then select the 'operation complete' icon and the resulting fully-connected 'stretched' structure will then appear on the screen.

The stretching will be according to the option selected. However while the stretching operation may be fairly predictable for straight lines that cross the boundary of the user-defined box, the stretching of curved members and other complex elements requires some more detailed understanding.

Straight lines, whether single discrete lines or parts of a polyline, have two end points. The stretching operation, subject to the following options, is based on these two end points, one will be within the box and one will be outside of the box. However complex shapes such as arcs of circles and splines are treated differently and the external end point for the stretching operation is based on the initial intersection point between the curve and the box. Thus, in some such cases the following options will not be meaningful.

Smooth

Basically, straight lines will be created between the points at each end of the original line. Thus the line outside of the box is changed as well as the line inside of the box.

Smooth Inside

A straight line will be created between the line end point within the box and the point that is the original intersection point between the line and the non-transformed box. Thus the original element of the line outside of the box will be unchanged.

Smooth Outside

A straight line is created between the line external end point and the point on the transformed box that is created by the intersection between the internal line element and the box. Thus the element that is within the box is unchanged except for the transformation itself.

Cracked

A straight line is created between the original intersection point between the line and the non-transformed box and the corresponding intersection point on the transformed box. Thus the structure that is external to the box in its original position is totally unchanged and the structure that is within the box is only changed according to the transformation itself, and with straight lines connecting the pairs of non-transformed and transformed points at the box intersection points.

Result:

The geometry will be modified, both as displayed on the screen and as stored in the drawing file.