Rhino - Editing multiple surfaces

[Willallison]

I want to alter the bow shape in the model pictured below. The hull is a polysurface. The others are surfaces. But I want to be able to alter the shape of the bow and have the other surfaces alter at the same time... is this possible?
Indeed, how do you alter the shape of a polysurface - as you can't turn control points on?

[bhnautika]

Will I am having a guess here (the pic?). It depends on how messy the surfaces are and how they were created but you can do it. You first have to explode the surfaces then turn on the control points, if you select the double points in the corners or on the edges both surfaces move together. You can of course move the middle points separately on each surface. When you are happy with the result, use the join command to make them poly’s again. I created a quick file ( rhino 4) to show what I mean, top hull is a poly, and the bottom I changed and returned it to a poly after.

[Willallison]

Ooops!!!... the pic....

[bhnautika]

Will can you show a pic of the bow with the control points turned on. Also if you have any trimmed surfaces the control points will be for the original surface not what’s there and even though you can edit the surface it may not be accurate to the other surfaces. If you have a lot of control point in the area, use the “edit>control points>select control points section “ to select the points you don’t want ( the u v’s are good for this), then use the hide points command to make things clearer.

[Willallison]

Ok - here's a couple of pics.
I've exploded the hull polysurface and turned on the points. This is for the surface indicated with the blue arrow. This is a trimmed version of a larger surface, so as you can see, the corner control points (in the top right) lie beyond the extents of the hull topside surface itself.
The 2nd image shows the control points for just one of the bulwark surfaces - the one that the red arrow is pointing to. Once again this is a trimmed section of a much larger surface.
The bulwark is actually made up of 7 surfaces!....
What I would like to do is to be able to change the bow shape - add more / less rake - but be able to constrain the other surfaces so that they are altered to correspond with this new shape. The alternative is to have to re-model all of the others, which would be a huge job - and even more so given that I want to compare a number of different bow shapes....

[bhnautika]

Will I am sorry but I don’t think you could edit all those trimmed surfaces to a satisfactory standard just by their control points as there is no relationship between there points (different amounts on each surface). Some cheat’s on the rebuild would be to create edge lines for each of the existing surfaces then untrim some surfaces, change the surface then re trim using the lines, or delete the surfaces, adjust the lines then re make the surface. I am intrigued why the bulwark surface is so big and has so many control points for such a small flat surface.

[DCockey]

ShrinkTrimmedSrf and ShrinkTrimmedSrfToEdge can be used to eliminate most of the overhanging portion of the untrimmed surface.

Record History works with some but not all types of surfaces to enable automatic updating of surfaces as the input geometry used to create the surfaces is modified.

[DCockey]

I have found editing surface control points to generally be a very unsatisfactory method of changing surface shape,. Occasionally I edit surface control points for the final tweak to a surface, but even that is tedious at best.

My prefered work flow is to create surfaces using curves and simple surfaces. The surface shapes are modified by modifying the input curves and simple surfaces, and then creating a set of surfaces.

I've also had some success modifying curves and surfaces using CageEdit but that takes some experimentation. The key is the choice of the control surface; usually it is not a simple box or straight line.

[Joe Petrich]

I have used CagEdit at times for this type of editing. As DCockey said it takes some trial and error.

[DavidJ]

The stretch command can achieve this if used carefully. You can use it the same way as you would manipulating control points except it can be used on polysurfaces. I find CageEdit to be a little less intuitive to me but perhaps more transparent as you can see the controls.

Having said that I agree completely with DCockey. Moving control points (and likewise the stretch/cageedit commands) is messy except for the simplest or very linear changes. If you were making a parallel midbody longer you would be crazy not to use those commands for example.

I also try to use curves and simple surfaces. In this case with wanting to compare multiple bow shapes I don't believe remodeling the corresponding surfaces would be as arduous a task as you envision. If you set up your model in an efficient way you can modify only a few curves and then quickly rebuild the corresponding surfaces to match. For example if the sheer curve controlled the top edge of the hull and the bottom edge of the bulwark then when you change the curve both of the surfaces will change to match. This can happen automatically of course if you use history, but I usually prefer to just remake the surface. Selecting a few curves and rerunning the command only takes a couple of seconds and then I can maintain a copy of my previous surface for comparison.

Even if you started this design in a different way (ie; exported from a different program) it would be easy to convert it to a curves and surface model. Just duplicate your edges, rebuild them, and fair if necessary. You can use contour, section or extract isocurve to capture areas where the shape is changing. These curves should also be rebuilt to a manageable number of control points. The first time you may have to move some end points around to make sure your curves correspond with each other. Once your curves are set up then just build new surfaces through these curves. Then to make changes you move the curve control points around. Once you have them set them up to represent your alternate bow shape then rebuild the surfaces.

[DCockey]

Excellent advice. Keep the curves as simple as possible but still consistent with the desired shape.

I haven't used Stretch recently and did a quick experiment with an arbitrary surface. CageEdit using a line as the control element and moving one of the line in the direction of the line produces the same shape as Scale1D with the scaling in the same direction. Stretch produces a slightly different shape.

[Willallison]

Thanks guys... I'll have a play and see what I can come up with...

The bulward surface that I showed the points turned on for is the forward end of a surface that becomes much larger further aft. It is the result of a sweep1 command as the surface is curved in both directions

I don't do my hull surface modelling in Rhino. It is developed elsewhere and then imported to rhino, where I do all the rest of the modelling.

Thanks again... I'll let you know how I get on....