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TUTORIAL V

Using the Spline Editor for Creating Models


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Introducing the Spline Edior

The Spline Editor is essentially a tool for creating curves. Using these curves 3D objects can be created. The main use for the spline editor is the creation of 3D logos. True type fonts may be used to make outlines for characters or character strings that can then be extruded, with different types of bevelling, to generate 3D logos. A spline consists of an axis and, at its basic level, a single curve. The curve is defined by a set of points known as knots. Each knot consists of a point and two handles. The curve passes through the point of the knot. The alignment and size of the handles defines the shape of the curve.

Knots may be continuous or discontinuous. With a continuous knot the curve flows through the knot point and the alignment of each handle is dependant of the other. With a discontinuous knot the alignment of the knot's handles are independant allowing sharp corners and tighter curves to be modelled.

Looking at Figure 1 below. The top knot is continuous, the curve flows through the knot point and the line between the two handles forms a tangent to the curve. Notice how the longer handle on the right has made the curve more pronounced than the side with the shorter handle. The bottom knot is discontinuous, the curve does not flow through the knot point but a sharp corner is created by moving the two handles independantly of each other.

Figure for the Introduction
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Figure 1

Step 1 - Creating the Spline

Initially create an image of the profile you want to model. For this tutorial I created a simple outline of a artist's palette, outline.jpg. The image is shown below in Figure 2. Notice the colour scheme, a dark grey background with a dark green foreground. These colours have been used as they provide a good contrast to Imagine's colour scheme.

Run Imagine and pick Spline Editor from the Editor menu. Next, select the FRONT view and pick Load from Backdrop in the View menu. The standard Windows file select dialog box will be displayed. For this tutorial select outline.jpg (this can be found in the "tutor5" subdirectory if you have downloaded this tutorial). The image will then appear in the FRONT view.

Pick Add Axis from the Object menu. Pick the axis in Object Select mode and then pick Add Knots from the Mode menu. We are going to add the knots which will form the spline curve that follows the outline of the palette. Looking at Figure 3, click on point 1. You are then expected to place the handle for the first knot, move the mouse vertically up and click again. Next move the mouse pointer to point 2 and click again. Again, you a required to place the handle for the second knot. This time move mouse and watch the curve. Try to align the curve to the curve of the profile in the background image. Repeat this procedure for points 3,4 and 5. Once point 5 has been placed complete the curve by clicking again on point 1. Do not worry that the curve does not match the profile for the thumb cutout just yet.

Select the curve in Object Select Mode and pick Knot Control from the Mode menu. Click on point 1, notice that the handles for the knots are displayed. Now pick Make Discontinuous from the Functions menu. You can now move the lower handle, independantly of the other handle, by dragging the point at its end. Try and Align the curve to the curve of the thumb cutout, see Figure 4 below. Make point 5 discontinuous and move the upper handle to get the lower half of the thumb cutout curve. In Knot Control mode you can also move the knot points. So go round each point and move its knot point and knot handles so that the curve follows the profile of the background image accurately.

The hole for the thumb is added in the same way. This time add the points labelled a,b,c and d (in that order) shown in below in Figure 3. This time place the handles at random for each point. Then go to Knot Control in the Mode menu and edit each point so that the curve follow the thumb hole profile in the background image. This will help you understand how the knot points and knot handles affect the curve.

The spline curve is now complete, see Figure 5 below. Select it in object select mode and save it as palette.isp.

Figures for Step 1
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Figure 2
Figure 3
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Figure 4
Figure 5


Step 2 - Creating the Palette Object

If it is not already loaded, load palette.isp. We are going to extrude this spline so pick it in object select mode and pick Add Points for the Objects menu. The Bevel/Extrude Parameters dialog box is displayed. The parameters to use are Extrude Depth=20, Bevel Depth=5, Bevel Width=5 and Extra Width=0. Leave the Extrude it, Front Faces and Back Faces check boxes checked. Select Round for both the Front Bevel and the Back Bevel. Then click on OK to make the object. Notice that the curve for the thumb hole has been extruded as a hole. You should now have an object looking like the one shown below in Figure 6. Select it in Object Select mode then pick Save Points from the Object menu and save the points as palette.iob. It is very important to note that Save only saves the spline curve. To save the extruded object you must select Save Points. The object can then be loaded into the detail editor and worked on as normal.

Figures for Step 2
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Figure 6


Step 2 - And Finally.......

Using this method is very useful for creating objects, especially from a 2D image or design. Very intricate logos can be modelled with very little effort and without any artistic ability. This method is also very useful for modelling objects with rounded edges, such as a rounded box perhaps? Of course the spline editor can be used to make 3D fonts.

One important feature of the spline editor not highlighted in this tutorial is the Make Line function. This is found in the Functions menu and turns a curve into a straight line which is useful when modelling well, straight lines. Try this, load palette.isp and make points 2 and 3 discontinuous. Then select Pick Knots from the Object menu and select point 2. Then pick Make Line. The curve between points 2 and 3 is transformed into a straight edge, see Figure 7 below. You can still edit the knots to recreate the curve but remember this function when modelling a profile the includes straight edges as well as curves.

Figure for And Finally..
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Figure 7

Why not add some paint puddles. Simply model a curve and extrude it with a Depth equal to the values for the Front and Back Bevels added together. Remember to save the puddles using Save Points. Then load the palette into the detail editor and load then position the three paint puddles. Finally, colour the paint puddles red, green and blue.

For anyone thinking "Why not use the Image Convert function?". This method produces smooth curves irrespective of the image being used. Also there are the different bevel options which make this method much more useful.


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© Richard Jennings 1997, 1998