[Bf-blender-npr] Feature/improvement idea for Freestyle

[Terry Hancock]

> I agree that what we like to have is a number quantifying how close to a
> contour.
> 
> [...]
> 
> Reference:
> http://gfx.cs.princeton.edu/proj/sg05lines/course7-5-linetypes.pdf

(Yeah, that's the paper I remember reading. :-)).

 > Radial curvatures are positive in convex areas, negative in concave
> areas, and zero at inflection point. So the magnitude (absolute value)
> of radial curvature could represent the closeness of a contour to an
> inflection point.

Yeah, it's from a dot product between the camera vector and the normal vector, 
I suppose.

> In Freestyle, radial curvatures are computed at individual input mesh
> vertices, only when the Face Smoothness option is enabled and mesh faces
> are flagged as smooth shading. Radial curvatures are only available
> through the Freestyle Python API at the moment.

OK. I can see I should probably learn more about this API.

> I did a quick test again: https://vimeo.com/91383521 .
> 
> In the video clip, line thickness is a function of the magnitude of
> radial curvatures. Since curvatures are computed at input mesh vertices,
> the input mesh have to be very fine (I used subsurface division at the
> maximum level 6). The high mesh resolution translates to a long view map
> computation time. That is a downside of this solution. The upside is a
> smooth transition of line thickness when the lines appear and disappear.
> 
> What do you think?

I think: "Hot damn! That's it!" :-)

The smooth surface issue may not be such a big deal for me. At least in the 
case I was thinking of the line was already on a mesh edge. I could get it to 
stay by making the crease angle extremely shallow, but then too many lines 
would show up, so I was picking it up as a contour. (This can happen when
the model has bevels -- not so good for Freestyle).

I can see how it might be a problem in other situations.

> Suggestive contours as in Freestyle for Blender tend to produce
> unpredictable and noisy line results as you may have noticed. By noisy I
> mean jagged, discontinuous pieces of line segments. I did not rely on
> them this time, although these lines might be a starting point toward
> what we like to have.

Yeah, I've seen that problem all right. So this is not the same thing as a 
suggestive contour, then. Interesting, because I thought it had to be.

Well, I don't know what your dev  priorities are exactly, but it would be very 
useful if this became available as a style modifier.

In the meantime, I guess I'll have to learn how this is done in Python. Can 
one write a new modifer in the Python API?  If so, I might attempt it.

Cheers,
Terry

--
Terry Hancock
Director/Producer - "Lunatics" Project - http://lunatics.tv
Anansi Spaceworks