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30485pb01 Windscreen 8 x 6 x 3 Curved Top Angled Canopy Pattern, how to finish? - Printable Version

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Re: 30485pb01 Windscreen 8 x 6 x 3 Curved Top Angled Canopy Pattern, how to start? - Daniel Goerner - 2012-03-24

Thank you for the answer, but it's like I'm trying to read chinese.


Conditional lines - was Re: 30485pb01 ,,, - Tim Gould - 2012-03-24

Conditional lines exist so that parts with curved surfaces, that would otherwise have no edge, can have an edge that shows only when the angle is right.

Think of a cylindercone made of 16 triangles with one point at the top, and the other two on a 16-agon at the bottom. At the side of each triangle you'd like an edge, but only if that edge will be at the side of the cylindercone. So how do you know if it's at the side?

Well... each edge is shared by two triangles. If the base of one of those triangles goes from left to right (eg. out the front), and the base of the other goes from right to left (eg. out the back) then the edge is at the side of the cylindercone. Otherwise it is not. SO we can use this to set up control points.

See the attached file. I've marked the cond-line so that it, and its control points are tan normally, but the conditional line shows up black when it's activated. Play around with rotating this and see what happens to the triangles and lines when the black line appears and disappears.

Hope that helps.


Re: Conditional lines - was Re: 30485pb01 ,,, - Steffen - 2012-03-24

you meant cone instead of cylinder, right?


Re: Conditional lines - was Re: 30485pb01 ,,, - Tim Gould - 2012-03-24

Yes. I blame the early morning Wink


Re: Conditional lines - was Re: 30485pb01 ,,, - Magnus Forsberg - 2012-03-24

I've added some pictures to my post. I hope they help in understanding.


Re: Conditional lines - was Re: 30485pb01 ,,, - Tim Gould - 2012-03-25

Cool. Actually... a wiki page on conditional lines would be a very good thing.


Re: Conditional lines - was Re: 30485pb01 ,,, - Daniel Goerner - 2012-03-25

Tim Gould Wrote:Play around with rotating this and see what happens to the triangles and lines when the black line appears and disappears.

Either my LDView is broken or absolutely nothing happens. Well, except a black line appearing and disappearing. I don't see controlpoints when I turn them on. I even deleted the condline and I don't see any difference. What should I see?

Magnus Forsberg Wrote:If there is an angle between these surfaces,
there must also be a conditional/optional line, along their common edge,

Why? what happens if it's not there?

I'm sorry, but I still don't get it. I feel like a painter who's trying to wrap his head around a crash course in architecture.


Re: Conditional lines - was Re: 30485pb01 ,,, - Tim Gould - 2012-03-25

[Image: conddemo.png]

Does that help?


Re: Conditional lines - was Re: 30485pb01 ,,, - Philippe Hurbain - 2012-03-25

Not sure it will help, but I'll try to tell it differently...
The initial purpose of conditional is to make sure to have an outline edge around the shape of the part (think "Ligne claire" cartoons characters), so that it can be seen clearly on the background. The edges of a cube are always visible so they are treated as permanent lines. But what to do for rounded shapes? if we put permanent lines, the facets forming the shape will be highlighted - something we don't want. What we need is a mechanism where the line is visible only when it appears on the border of the shape. The magics comes from control points. When projected on screen with current viewing direction, if the two control points are on each side of the conditional line, this mean that the conditional line is in the middle of two visible surfaces, so it is not on the border of the shape and must not be visible. If on the other hand the two control points are projected on the same side of the lines, this means that the conditional line is on the border between shape and background and must be visible. With this verbose explanation and Tim's image, I hope you'll get the hang of it!


Re: Conditional lines - was Re: 30485pb01 ,,, - Daniel Goerner - 2012-03-25

It dawns.

But I still don't get the control points visible in LDView. Activating "Show Conditional Control Points" does nothing. Should they look like in Tim's image?