CAD - body curves from a scanned image

[Rodent]

This was a sub-topic within another thread here, so it's probably best to shift the discussion into a topic of its own vs. continuing to derail the other thread ...

Here's how I would approach capturing the contour of a bass body from an internet image. I'm intentionally using an image here that isn't aligned perfectly on centerline

I first established the centerline down the neck, and then set curves at the nut, 12th fret, 24th fret, and G-string saddle. These are the red lines in the image above. These allowed me to scale the image and average in the scale length so I had a full scale image to work against.

Next I created a curve from control points and roughly placed points around the outline. This is the cyan curve and control points. Once you've done a few of these you'll get a hang for where you need to place the initial control points.

From there it was time spent massaging the point locations until the curve as at an acceptable accuracy level to capture the body outline. It's ok if the image is a little bit fuzzy so long as your final curve is smooth and as close as you need it. This would have been a nightmare to work with downstream if I was obsessed with ensuring I placed points ON those fuzzy edges.

Once I have the curve close enough, I'll analyze the curve for smoothness. There are a couple places where I'd want to work this curve further, but I'm sure you get the concept.

Note that I'm using a familiar body outline here for reference. I do NOT have this outline as any part of my design libraries. I'm using the familiar body shape purely as reference, and using this instead of one of my own body shapes since my shapes would not include a scanned image as part of their design workflow.

An additional image on curve analysis, comparing the smooth curve created with the above workflow, and a fillet that was placed manually. Notice the drastic vectorin change that occurs right where the fillet is tangent to the smooth curve. If you're utilizing this curve with any kind of CNC, you're going to hear that during the machining process when the tool path comes to this location on the curve.

And similar to the workflow used to trace an image and develop smooth curves, here's a peek at the upper horn curve control points

I'm using Rhino as the CAD tool here, but the process is similar with most any CAD tool that utilizes relational geometry, so you would take a similar approach with CATIA, SolidWorks, AutoCAD, ProE, DraftSight, etc ... The exact command and mouse click will be in-step with your CAD flavor of choice.

 

[Will_White]

Very nice write-up, wonder if someone who knows InkScape, Adobe Illustrator, etc will come along and post how to do it in those programs. I would do one for Art of Illusion but I didn't think anyone else uses that program on here. If that happens I would think this would be sticky worthy.

There's one thing that I would say, and it's not a criticism of your post but an addendum, is that if someone is going to do a copy of a bass, tweak it and try and make it your own. A great example of that is @mapleglo's Ric style builds.

 

[Rodent]

Very nice write-up, wonder if someone who knows InkScape, Adobe Illustrator, etc will come along and post how to do it in those programs. I would do one for Art of Illusion but I didn't think anyone else uses that program on here. If that happens I would think this would be sticky worthy.

There's one thing that I would say, and it's not a criticism of your post but an addendum, is that if someone is going to do a copy of a bass, tweak it and try and make it your own. A great example of that is @mapleglo's Ric style builds.

Thanks. I totally agree on tweaking the design to make your own. My Pbass style body looks like a Pbass, until you set it next to a real Pbass and then the differences are very obvious. I took all that I like, and mashed it with all I wanted to 'fix' to suit my playing preferences. Same for my take on a Jbass - the differences are subtle, but significant when the two designs are next to each other

 

[D-Smith]

Good explanation. The one thing I do different is to chop up the big outline into smaller pieces so I can work on one segment at a time without it changing an adjacent one. Also lets me simplify every line down to two endpoints and two control points in between. The pics below are the steps I use:
-draw basic outline



-place points where line changes direction and at every point where the line is vertical or horizontal, then split line into segments:



-this is outline split into multiple segments with all of the control points the computer uses to make the lines:



-this is the section to work on:



-remove all extra control points, which are all the points except the ones right next to the endpoints:



Where you split the line on horizontal and vertical points, rotate the control points on both lines adjacent to the end point, using the end point as the center of rotation so they are perfectly horizontal or vertical. Where you split the line due to a change in direction that is not on a horizontal or vertical, draw a two sided line from the endpoint to the adjacent point, then extend that line a few inches on each end:



Now just move the two middle control points either along the horizontal and vertical axis or along the two sided line you drew until it matches the picture (turn the background image on and off) or until you think it "looks right". You can also rotate the control points and line to change the angle where the two lines meet by selecting the control points on either side of the end points, the double ended line and then rotating all of them using the end point as the center of rotation.



I use a few different ways other than the method above to get to the same point, but getting the lines in that state is my goal so I can work on adjacent lines and moving control points on one does not change the other line. The transition between curve direction will stay smooth as long as you move the control points along the vertical, horizontal or on the double ended line. Disclaimer is, I taught myself CAD, so I know just enough to be dangerous. There may be much easier ways to accomplish exactly what I'm doing.

 

[kris pung]

I'm a CAD manger by profession and the OP is exactly how I do it too.

 

[Rodent]

@smithcreek - that's a pretty unusual approach. Do you ever use your outline geometry for more than just printing onto a paper template?

 

[D-Smith]

@smithcreek - that's a pretty unusual approach. Do you ever use your outline geometry for more than just printing onto a paper template?

Yes. "Double-Bass Guitar", Archtop Acoustic/Electric Hybrid Build Thread

Once the line is completely done I just join all the segments. Then I can explode it later if I want to change any single part. Just to reiterate, the reason I go through all of that is it gives each line segment a discreet start and end point that does not move when you adjust a control point next to it or even several points downstream from it.

For example, you want an body that it 16" wide at the waist, and that widest point occurs at 8" from the tail. Using a single curve with control points that go through many changes in directions, if you move a control point anywhere near that 16" measurement there is a good chance the line will move off the 16" mark and off the 8" mark, either under or over and you will have to go back and fudge it to 16" and 8".

Using multiple line segments, for example the segment of an outline that starts at the centerline of the tail and goes to the widest point of the lower bout on the bass side. Let's say the start point is at the strap button (x=0, y=0) and going to the widest point (x=8, y=8), if that line starts and ends at those two points, it does not matter how you move the control points around in between the start and end, it will always start and stop on the same x,y coordinates.

 

[Dadagoboi]

I think Dave's (@smithcreek) programming skills are the best example I've seen of translating actual luthiery to CAD.