
	Spline Projection

Spline projection can be used in two ways. The first is to draw a spline curve that is projected onto the mesh to create smooth curved slices (see Figure 259). The second is to use the drawn spline curve as a path for a series of straight slices (see Figure 260).

Figure 259. Spline as Slice

Figure 260. Spline as Path

To access the spline projection tool, click on the white right arrow after the Spline Projection label, and the Spline Projection window will appear (see Figure 261).

Figure 261. Spline Projection Window

Drawing Splines

To begin drawing a spline, click on the Edit button towards the top of the Spline Projection window. Whenever the Edit button is active, the left, middle and right mouse buttons are used to add, move and delete control points. If you want to change your view, or draw slices as described in previous chapters, you will need to disable the spline editing by selecting either Hide or Show.

To add a control point to the current spline, move the mouse pointer to the new control point location, and click on the left mouse button. The control point is added as soon as you depress the mouse button; you can move the new point around by moving the mouse around before letting go of the left mouse button.

Figure 262 shows the spline after two control points have been added. The control points are drawn as small hollow red squares, and the spline is drawn in a thick solid red line. A two control point spline is just a straight line, so to create a curved line a third or more control points need to be added.

Figure 262. A Two Control Point Spline

New control points are added to the ends of, or inserted in, the curve depending on where you initially position them. Figure 263 shows you what happens when the closest part of the existing spline is an end point; the new control point is just added to that end. Notice how the control line is drawn in a thin dashed red line.

Figure 263. Control Point Added To End

Figure 264 shows you that the new control point is inserted in the curve when the closest part of the spline isn't an end point.

Figure 264. Control Point Inserted

To change any control point's position after its been added, move the mouse pointer close to the control point, then press the left or middle mouse buttons, move the mouse about, and let go of the mouse button once you are happy with the new position.

To change the position of a pair of adjacent control points, move the mouse pointer close to control line between the points, then press the middle mouse button and move the mouse to position them.

To change the position of the entire curve, move the mouse pointer so its away from any control points or the control curve, then press the middle mouse button and move the mouse about.

To rotate the curve, hold down the <Ctrl> button and the left mouse button, and move the mouse from side to side.

To scale the curve, hold down the <Ctrl> button and the middle mouse button, and move the mouse up and down.

To delete any control point, move the mouse pointer until its fairly close, then click the right mouse button.

NOTE for users with 2 button mice: hold down the <Shift> key to turn your second (middle) mouse button into a third (right) mouse button.

To delete the entire curve, hit the red Reset button towards the bottom of the Spline Projection window.

Projecting Splines

Click on the green Project button and the 2D spline curve you have drawn is projected out into the 3D scene and is intersected with the visible faces.

The spline is projected out as a series of short line segments. You control the number of segments by changing the Prec field; this is the curve precision, and you increase or reduce it to increase or reduce the number of segments. You'll get a much smooth line with a higher precision, but the projection calculation time will be increased.

As soon as the projection is complete, the spline curve is hidden to allow you to see the new slices. These slices are just like any other slice; they can be picked, split, or even interpolated to create 3D splines that follow the mesh surface.

Extracting Splines

If you want to modify the shape of any slice, whether its been created from a projected spline or not, then the easiest way is to pick that slice then click on the right hand arrow after the Extract input field (see Figure 265). The slice will be projected back (i.e. extracted) from the 3D scene to create a 2D projection curve. You can then modify the projection curve using the control points, and finish off by clicking on Project to create a new slice with the modified shape.

Figure 265. Extracting Splines

You can adjust the number of control points in the extracted projection curve by typing a new number into the Extract input field (hit <Enter> to make the change), or by clicking on the left and right arrows to reduce or increase the value by one. The lower this number, the easier the projection curve will be to modify (i.e. fewer control points to move). The higher the number, the closer the projection curve will follow the original slice.

Projecting Ribs

As mentioned above, the spline can also be used as a path for a series of straight slices. Click on the Spine button, and the curve acts as a spine to a series of ribs.

You can change the number of ribs by editing the Ribs field. Hit <Enter> and the number you have entered in the field is applied to the current display.

Figure 266. Rib Widths

To change the width of the ribs, use the handles on each side of the first and last ribs (see Figure 266). Position the mouse pointer over the handle you want to manipulate, then hold down the middle mouse button to drag the handle in or out to change the width at that end, on that side.

You can easily change the shape of the spine by just adding, moving or deleting control points as described above.

When you are happy with the rib positioning and width, hit Project and the 2D ribs are projected out into the 3D scene to create slices as if you had drawn them by hand.

Drawing Circular Arcs

If you need to project a perfect circle, or project a series of lines radiating from a common point, then use the circular arc tool; select the On button after the Arc label. You only need two control points to define a circular arc; the center and a point on the arc itself (see Figure 267).

Figure 267. Circular Arc as Line and Spine

You manipulate these two control points in exactly the same way that you would spline control points.

If you want something other than a complete circle, change the text after the On button to the number of degrees you want; hit <Enter> to update the display with the new angle.

If you use the circular arc as a path for ribs, you'll have only two, not the usual four, width handles. You won't be able to move the inner one of these handles past the center of the circular arc; if you were able to you would have problems with slices overlapping each other.

Slicing Front Or Back

By default, spline projection cuts through both front and back facing surfaces; in effect its projecting all the way through the visible mesh.

By selecting either of the Front or Back buttons you can disable slicing through that set of faces. For example, select the Back button to disable it, and only front facing surfaces will be sliced when the spline or ribs are projected.

After the spline or ribs are projected, the disabled button is re-enabled. In testing it was found that users often forgot to re-enable the disabled button themselves, and didn't get the expected results in subsequent projections.

Keyboard Shortcuts

Hitting the <q> key is equivalent to selecting the Edit button towards the top of the Spline Projection window.

Hitting the <w> key is equivalent to selecting the Project button.

The <q> and <w> keys in combination allow you to draw a projection spline, project it through the mesh, then move it and modify it, project again, and so on to quickly create a series of curved projection slices.