# On Surface

Create and edit a blend curve directly on one or more surfaces.

This tool is useful in the following situations:
• It is difficult to project a curve onto a surface for a trim operation because the curve is asymmetrical.
• It is difficult to project multiple curves onto a surface for a trim operation because the curves are oriented in different directions.
• You want to automatically maintain tangency and curvature while creating a curve directly on multiple surfaces.

## Create a Blend Curve on a Surface

Create a blend curve directly on one or more surfaces.

1. On the ribbon, click the Curves tab.
2. Click the On Surface icon.

3. Click one or more surfaces to place the blend points.
You can place points on different objects, as long as the objects share an edge or the objects are combined.
Tip: Avoid creating one curve with lots of points; instead, create multiple curves and combine them. This will improve performance and simplify editing.
4. Right-click and mouse through the check mark to exit, or double-right-click.

## Edit a Blend Curve

Reposition and reshape a blend curve.

These parameters are only available in the creation phase.

1. Double-click the blend curve to enter edit mode.
2. Edit the blend curve.
To Do this
Reposition a point
• Drag the point.
• Click the point, click the chevron, and then enter the x, y, and z coordinates.
1. Hover midway between two points, or at either end of the curve.
2. Click to add a point. Or drag to add a point in a different position.
Delete a point Click the point, and then press Delete.
Change the curve degree The curve degree controls the placement of the points to determine the shape of the blend curve.

In the guide bar, select one of the following options:

• Cubic (3)
• Quintic (5): In general, select this option to produce a smoother curve.
Change the continuity The continuity is used to constrain the shape of the curve at a selected point.

Click a point, and then select one of the following Continuity options:

• Positional (G0) (default): The blend curve is merely attached to the curve or surface at the blend point.
• Tangent (G1): The blend curve is made tangent to the curve or surface at the blend point.
• Curvature (G2): The blend curve follows the curvature of the attached curve or surface. The curvature is the same on both sides across the blend curve's boundaries.
• Double curvature (G3): Only available if you selected Quintic (5) for the Degree. The blend curve follows the curvature of the attached curve or surface. The curvature's rate of change is the same on both sides across the blend curve's boundaries.
Change the tangent direction of a point Click a point, click the chevron, and then select one of the following Tangent options:
• Auto (default): Automatically calculate the best tangent direction.
• Custom UV: You can define the custom tangent direction by clicking the four yellow dots on either side of the point.
• Free 3D: No tangent direction. You need to enter the 3D tangent direction coordinates. The continuity is automatically set to Positional.
Change the tangent magnitude of a point
• Drag the blue square handles.
• Click a point, click the chevron, and then enter a Tangent magnitude value.
Trim at a start or end point
• Click the start or end point, click the chevron, and then select Trim here.
• In the Control Panel, select Trim at start, Trim at end, or both.
Invert the trim at a start or end point
• Click the start or end point, click the chevron, and then select Invert trim side.
• In the Control Panel, select Invert start or Invert end.
Change the curve tension This controls the smoothness of the curve.

In the Control Panel, enter a Curve tension value. You need to choose a value from 0.01 to 1. Higher values correspond to smoother curves.

3. Right-click and mouse through the check mark to exit, or double-right-click.