Spline or Bezier....that is the question

When making a CAD drawing, what is the proper tool for drawing sheers, chines, etc. , the Spline tool or the Bezier?

The bezier "makes the curve fit" but am I inducing error by using this?

Since I am basically a the novice level when it comes to drawings and especially in the use of CAD, I would appreciate advice from those more skilled in CAD drawings.

Thanks in advance.

Ray

 

Good Question

I have recently written a piece of software using bezier curves to draw a hull. This method is quite successful, using cubic bezier curves. To draw all hull lines.

Good Luck,

Tim B.

 

Bezier curves are themselves splines. I suspect your "Spline" tool passes the curve exactly through the given points, while the "Bezier" tool uses control points instead and only passes through the end points. You will probably get a fairer curve using the Bezier tool to draw a curve by hand. It's especially difficult to produce a really fair curve using a spline that goes through the points unless you use only the minimum number of points or generate the points using a mathematical formula.

No matter which tool you use, the curve IS the shape, and on that basis is just as accurate. Once you've laid the curve down, you should use the curve itself (or points on it) to define the parts of the boat that intersect it. For example, once you've drawn the sheer, it should be used as the edge of the deck and the edge of the hull surface.

 

Thanks for the advice.

My splines anchor at the end snaps and adjust themself on the intermediate points, while the bezier snaps to each designated point and adjust to the curve between the points.

My question is how do I remove the distortion that occurs when the control / snap points are fairly close together and the direction of the curve changes. What is happening is that I get a consistent "bump" in the curve at these locations.

For lack of a better way to describe it, it is like the line is under tension and these points are locations where the stress is relieved and the line is allowed to bulge.

Thanks again

Ray

 

I was short on time for my last post, here is a more detailed description.

There are various different types of splines, amongst which are B-Splines, C-Splines and T-Splines. Whilst they all use different methods to calculate the curve, they all essentially fit a curve through a number of points. There are various parameters which can be changed to obtain the best fit. A higher-order curve will be a tighter fit to the points but will probably induce error. A lower order curve will fit fewer of the points but the resultant curve will be smoother.

A Bezier curve, be it quadratic, cubic, quartic (or any other order), works by having two anchored end points, and (order-1) control points. The curve does not go through these control points, but they can be used to change the curve. Again, a higher order curve will have more 'bends' in it.

Therefore, if you are drawing any curve manually on CAD to some list of points, then a bezier curve is the way to go. If you want an automated fit, the spline is the best option.

Good Luck,

Tim B.

 

Try using fewer control points. A Bezier curve is attracted to the control points, and the way you produce a sharp kink is to put mulitple points there.

 

I know this is an old thread, but have you tried T-splines?
http://www.cadjunky.com/index.php/main/video/show?id=4140811

 

Have you tried plastic or wood splines with a pencil or pen?? Maybe some ducks or whales or whatever you want to call spline weights??

 

Turn off the "snap to" or turn the grid off. Maybe you can use the "snap to line" and the "snap to object" settings. For curved work grids do not seem to help me at all.

 

nice T splines Raggi, looks like only rhino but thanks mentioning

 

T-Splines are relatively recent. I'm yet to play around with them, but they look interesting. I managed to find some Fortran 77 code that implements T-Splines (or exponential tension splines more correctly), So I'll convert it to C++ and see what I get when I have time.

Tim B.

 

Raggi, now I have one more reason to upgrade to R4. T-splines looks like a great tool.
Gary

 

Check your software documentation. Some software (if not all?) allows you to manipulate the properties of the spline control points to get a more accurate curve. When looking to make a fair curve though, a bezier is easier to work with.

 

Whatever kind of spline you use in software, you have to check the curvature if you are going to build the boat in a process that includes bending og stringers, frames or panels. In 1992 I wrote a lisp program in AutoCAD that calculated longitudinal stringers as natural cubic splines. The argument was that this spline has the same shape as you get in real life bending and with zero curvature in the ends. In rhino you have to make sure that curvature is close to zero in the ends of a stringer for example. One easy way to do this is to have three control points in a straight line, or just look at the curvature graph while nudging.

 

Don't know if this is the right thread, but FreeDimension just received an award: http://www.tenlinks.com/news/PR/cadwire/041607_freedimension.htm

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SPOKANE, Washington, April 16, 2007 - CADwire.net, the leading information site for the CAD, AEC, GIS, EDA and PLM industries, today announced FreeDimension as the winner of its 2006 Tech Innovator Award.

FreeDesign introduced a unique 3D surface generating software called FreeDimension to the market last year. The FreeDimension application represents a radically new approach to surface generation in 3D models. The new technology enables designers to stylize 3D models intuitively, unburdened by the need for complex tools to create curved surfaces. With a remarkably easy user interface, the software quickly expedites what used to be the most difficult task in 3D models: the molding of natural-looking organic shapes, ergonomic surfaces, and rich three-dimensional textures.

This new freedom in design is possible because FreeDimension departs from the usual method of CAD surfacing, which uses NURBS (Non-Uniform Rational B-Splines). FreeDimension instead employs a new flexible technique of curve-creation called "n-sided" surfacing.
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