# Learn the Shape Maker. Surface editor.

The process of editing the shape of a surface patches is similar to editing a curve. The difference is that the surface case is a two-dimensional case. The points of the control polyhedron affect a certain region of surface variation and are somewhat farther from the surface than in the case of the line. In the case when there is a sufficiently large number of control points on the surface, a change in the position of one control point leads to a local change in the shape of the surface.

To edit a surface, just click inside the surface contour formed by the boundary lines. After selecting the surface for editing, the points of the control polyhedron will appear. Modification of the position of the points of the control polyhedron is the main method of smoothing the surface. Only internal points of the control polyhedron are available for editing. The corner points of the control polyhedron can only be modified by changing the position of the corner point of the surface contour. The same can be said about the boundary points of the control polyhedron. Their position depends on the shape of the boundary curve and changes only when the boundary curve changes.

**
Change the position of the corner points of the surface.**

As was shown in the case of editing the line, the points of the line and the surface form a single topological system. Therefore, as in the case of editing the end point of a line, editing the corner point of the surface will change the shape of the boundary lines and the surface. Changing the shape of the surface in this case depends on the mode of changing the shape. So the figure below shows the change in shape in the mode:

In this case, only the position of the end point of the lines and the corner point of the surface changes.

If the following form editing mode is selected:

The position of all control points adjacent to a given point of the lines and all control points of the surface, except for opposing boundaries, will be recalculated according to a linear law.

In the case of the following editing mode:

The tangents at the end points of the boundary curves and the tangents at the modified surface boundaries will be unchanged. All intermediate points of the curves and the surface area will be recalculated linearly.

This mode is convenient to use if you need to change the position of the corner point, but at the same time keep the tangents of the surfaces in connection to this point.

Also, as in the case of line modification for a surface, you can set the modification mode of part of the surface.

The following settings will result in partial surface modification.

As can be seen from this example, only part of the control points of the surface changes.

**Changing the shape of the boundary line of the surface.**

A change in the shape of the boundary curve affects the change in the position of the points of the control surface polyhedron similarly to the change in the corner point. So with the same editing mode settings as in the previous case, the result will be like this.

It is important to note that when changing the shape of the boundary curve, all surfaces based on this curve will be changed in accordance with the selected editing mode. In this case gaps at the boundary of two surfaces does not occur.

**Changing the position of the points of the control surface polyhedron.**

In order to start editing the surface, just click by the mouse inside the contour formed by the boundary lines of the surface. Changing the position of the points of the control polyhedron will lead to a change in the shape of the surface.

The control surface polyhedron is similar to the control line polygon and has similar properties. Let's consider some of them:

- Locality of surface changes. In the case of a sufficiently large number of points of the control polyhedron, a change in the position of one of them will lead to a local change in the surface shape.

- Tangent to surface boundaries. The surface shape near the boundary, the tangents, and the slope angles are determined by the points of the control polyhedron closest to the boundary.

- The planes. If all points of the control polyhedron belong to the same plane, then the surface is a plane.

- Cylindrical and conical surfaces. And if all the rows of control points in one direction are on a straight line, then the surface will be a cylinder or a cone depending on the shape of the opposite borders.

- Properties of the convexity of the control polyhedron. The convex control polyhedron defines a convex surface area without dents or kinks.

As well as for the points of the control line polygon, for the surface there are several different ways to modify the control polyhedron. The simplest is to change the position of one control point. In this case, the editing mode should be set as follows: