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

[Terry Hancock]

TK,

> I have seen many cases too where lines suddenly showing up and
> disappearing are distracting, so I see the point of your post.
> 
> Concerning crease lines, they are independent of the view point [...]

Actually I knew that. Or should have remembered! (I've read the paper about 
the Freestyle algorithms). So that's not what I was seeing, of course.

> Changing the thickness of a crease line depending on the angle between
> the two faces on both sides of the line is technically possible. I just
> did a proof-of-concept implementation using the Python Scripting mode of
> Freestyle in Blender. See https://vimeo.com/91306354 for a quick test
> result. Implementing this crease angle dependent line thickness in the
> Parameter Editor mode needs further coding (that could be a short term
> goal).

That was pretty neat, though! I may have to look into how you did that. That 
would matter when it's the model that's changing rather than the camera.

> Contours, on the other hand, are view dependent lines. 

Clearly that has to be the source of the problem I was talking about.

It is the "suggestive contour" algorithm that I think offers a chance for 
improvement here.

As I understand it, these are "almost contours", meaning that they do not 
quite undercut, but have a very steep angle relative to the line of sight.

Freestyle currently must just have some cutoff point for this. (?)

So then my request would amount to extracting the steepness -- "how close to a 
contour is it?" -- as a numerical value and making that available for 
modifiers.

Typically, when a contour disappears, it should be a "suggestive contour" for 
a short time after. So if you have a modifier with a rapid fall-off in thickness 
and/or alpha, it would implement a softer disappearance.

> Maintaining
> inter-frame properties of lines (such as stability and gradually varying
> thickness) is a problem called temporal coherence. 

Yep. That would get complicated.

But I think the suggestive contour modifier idea would only depend on the 
instantaneous geometry of the current frame, so it doesn't fall into that 
category.

> For now it is unlikely that the
> temporal coherence problem is solved in Freestyle for Blender.

Yeah, that's understandable, and even this contour issue is something I expect 
I'll just have to accept as a flaw for the present. Just thinking about ways to 
improve it. :-) 

Cheers,
Terry

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