8
votes
Accepted
How do people come up with subdivision schemes?
The subdivision schemes are not arbitrary. Catmull-Clark, arguably the most used subdivision scheme, generalizes bicubic B-splines to meshes of arbitrary topology.
Most, other subdivision schemes ...
- 1,188
7
votes
Accepted
How to calculate matching roundness of two offset rectangles?
What you (probably) want to achieve is something like this:
When having a closer look at one of the corners and add a few lines, we see this:
The black lines indicate that the center points of the ...
- 1,310
7
votes
Ordering a set of unorganized points along a curve
You have an instance of a problem called curve reconstruction from unorganized points. Now that you know what to search for you'll find several methods, such as the crust, NN-crust, etc. Here are a ...
- 724
6
votes
Accepted
Determining Rational Quadratic Bezier Curve Weights for Circle
Check out the section on Circular Arcs and Circles, from Ching-Kuang Shene's excellent computational geometry course notes:
[G]iven three control points P0, P1 and P2 such that P0P1 = P1P2 holds, if ...
- 881
5
votes
Move position in smooth gradient
What you want is linear interpolation like @Tim points out. However he uses a formula that is not so easily understandable form, we can refactor it differently. Basically linear interpolation is a ...
- 8,397
5
votes
Accepted
Move position in smooth gradient
The math you need is linear interpolation. I think of a player sprite which moves from position X to position Y. Pseudocode:
...
- 538
5
votes
Accepted
Converting raster shape/blob into displacement map
One way I can think of is to make a "signed distance transform" of the image where there is information for each pixel about how far the pixel is to the closest surface of the shape. Since it's ...
- 7,751
4
votes
Accepted
If you can use subdivision surfaces for 2D curves
Subdivision can be used for curves in 2D just as easily as for surfaces in 3D. Usually the subdivision algorithms applied to 2D are called subdivision curves. Subdivision curves do not suffer from the ...
- 1,188
4
votes
Conversion from cubic catmull-rom spline to cubic b-spline
Quoting the comments above for context:
Just to confirm, are you asking, given a set of $N$ CatRom control points,
$$\{CR_0, CR_1, CR_2, CR_3 ... CR_{n-1}\}$$
forming a piecewise curve, what is the ...
- 4,181
4
votes
How to use Monotone cubic interpolation in 3D?
Ok, monotonic interpolation depends on what you are monotonic about. For a simple 1D function interpolation monotonicity is easy to define. But for a 2D and 3D dataset its not so self evident what the ...
- 8,397
4
votes
Ordering a set of unorganized points along a curve
After some clarifications, there is probably a much better approach that doesn't even require knowing the parametric form of the curve, and also avoids the potentially problematic numeric minimisation ...
- 2,720
3
votes
How to take the derivative of a Bézier curve?
A simple example of taking a the derivative of a B'ezier curve can be shown using a cubic curve.
$$C_3(u) = \sum_{i=0}^3 B_{3,i}(u) P_i,$$ where $u \in [0,1]$ and $B_{n,i} = {n \choose i} u^i (1-u)^{n-...
- 1,188
3
votes
Accepted
Polygons versus curve primitives in software rendering
Most software rendering engines dice the parametric primitives to micropolygons, usually on the fly as needed. In essence this reduces the needed complexity to determine intersections. The surface ...
- 8,397
3
votes
Ordering a set of unorganized points along a curve
Since you've only got floating-point representations of the points, there is no guarantee that these still lie exactly on the curve, due to rounding errors. So I think the only generic approach is to ...
- 2,720
2
votes
Does the blending matrix change between calculating various curve segments in a uniform cubic B-splines approximation?
No, $B$ is constant for given type of cubic spline, e.g. B-spline, Bezier, Hermite or Catmull-Rom cubic splines have different $B$ matrix. To make B-spline continuous, you need to copy 3 control ...
- 3,616
2
votes
Accepted
Does the blending matrix change between calculating various curve segments in a uniform cubic B-splines approximation?
No, the $B$ matrix (basis coefficient matrix) does not change from one segment to the next. It is a property of the type of spline you're using, in this case cubic B-splines. If you used Bézier ...
- 24.8k
2
votes
Accepted
How to deform some mesh so that it fits along a spline curve?
I have no knowledge of the literature on the topic, but I did something very similar to what you're asking some time ago: I wanted to generate lathe meshes and bend them according to a spline. I think ...
- 4,380
2
votes
Accepted
Why cubic curves provide the minimum curvature interpolants?
For a function $y = f(x)$ the (signed) curvature at $x$ is given by:
$$
\kappa(x) = \frac{f''(x)}{(1+f'^2(x))^{\frac{3}{2}}}
$$
If you assume that the slope is very small compared to $1$: $ f'^2<\!...
- 2,083
2
votes
Why cubic curves provide the minimum curvature interpolants?
Splines confuse me which is one reason I asked somebody else to write that chapter.
But I like this explanation:
https://www.johndcook.com/blog/2009/02/06/the-smoothest-curve-through-a-set-of-points/
2
votes
Accepted
B spline curve generation in Python
In line 15 use the half-open interval, i.e.,
if u>=t[0][i] and u<t[0][i+1]:
Otherwise, at knot values, you evaluate two basis functions at the k=1 basis ...
- 166
2
votes
Continuity of parametric and geometric continuity
I would rather draw the $G_1$ example something like this:
This makes it clear that $t_2$ and $t_3$ are parallel, but have different lengths in general. (They both start at the same point, but $t_3$ ...
- 24.8k
1
vote
Continuity of parametric and geometric continuity
Yes, your understanding of $C1$ and $G1$, as shown in your drawings, is roughly correct: $C1$ means equal derivative vectors, and $G1$ means parallel derivative vectors.
I say “roughly” because there ...
- 338
1
vote
Fake cubic Hermite spline interpolation with smoothstep
A Hermite cubic polynomial interpolates 2 points and the derivatives at those points. To be exact, let us have the points $p_0, p_1$ and the derivatives (tangents) at those $d_0, d_1$. Then we want a ...
- 2,083
1
vote
Fake cubic Hermite spline interpolation with smoothstep
However, Hermite interpolation it links to is different from Cubic Hermite spline interpolation; they're two different articles.
These are nested subjects. That is, "Cubic Hermit interpolation&...
- 9,697
1
vote
Spline interpolation library in cpp
So you have a series of points and, at each point, a supplied derivative?
Is a piecewise cubic sufficient or does it need higher derivative continuity? If the former is ok, then Cubic Hermite Splines ...
- 4,181
1
vote
How do you compute the winding number of a closed poly curve?
Winding number as in https://en.wikipedia.org/wiki/Winding_number?
Unless I'm mistaken, one way is to fire, say, a horizontal 'ray' out from your point towards, say, $+\infty$. Each time you cross an ...
- 4,181
1
vote
Non least squares formulation to fit catmull rom spline
Disclaimer: I've done some work in the past with wavelet decomposition of image data but it occurred to me that it may be applicable to your problem. Admittedly, I don't actually know if the ...
- 4,181
1
vote
Accepted
How does the Lane Riesenfeld algorithm work?
What you're looking for is called de Boor's algorithm. It lets you compute a point on a b-spline curve by doing a series of linear interpolation (LERP) calculations. So, it works very much like the de ...
- 338
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