Draw on a Surface with a line

 From:  Michael Gibson
976.2 In reply to 976.1 
Hi Nuri, I'm glad that you are enjoying MoI!

There have been quite a few requests for direct drawing on surfaces, so it is something that I am looking at doing in the future. But unfortunately it is a quite difficult area of work. The difficulty will make it likely that I will have to push off working on that area for some time yet...

> Is it technically possible to draw with any line tool directly on spline surfaces as
> if the line were restricted to motion only on the surface contour.

There are different methods that make this possible, but each of them tend to have drawbacks.

Each spline surface has a UV "parameter space" in it, kind of like a texture space. One method is to draw the line in this parameter space and then push that parameter space curve up into 3D. But this has some difficulties, like it doesn't behave well when you cross the seam of a closed shape like a cylinder, and it is also limited to only working on a single spline surface at a time, not across edges of an assembly of joined surfaces.

There are other methods that are possible to try, but each method that I know of tends to have some difficult calculation or some other type of drawback to it.

> but I know from Rhino3d and Alias that you can project lines on to surfaces
> and then trim and cut those surfaces into new shapes.

Yup, this also works in a similar way in MoI. Trimming has a projection method built into it when you trim surfaces with curves, and you can also use Construct / Curve / Project to manually project a curve on to a surface to use for some other construction.

> I would love to just take my tablet and draw on the model with the pen, the surface
> would automatically split into two distinct surfaces and then I could push and pull
> the thing into new shapes.

One problem is that this method of splitting into different pieces and then push/pulling them will tend to create blocky, hard-edged type shapes like the way Sketchup works.

To make something that would be able to sketch on a smooth surface, and then also push-pull it with the result being another smoothly connected surface is a really difficult problem to solve...

Really the whole underlying structure of trimmed NURBS technology is just not good at that kind of very "squishy" type of manipulation.

There are a new set of sculpting type applications out there now, like ZBrush, Mudbox, and Modo - these are polygon based and kind of fundamentally more squishable than a CAD NURBS model is. Those programs have a big focus on deforming the shape by drawing on it, so something like that may be more in line with what you are mentioning here...

- Michael