TP8 : Uniform B-splines as Subdivision Surfaces

1 April 2016 in

Code

Do

git pull

or, if you don’t have the local repo,

git clone https://github.com/bbrrck/geo-num-2016.git

As usual, test by

cd TP8/
mkdir build
cd build
cmake ..
make
./geonum_TP8 torus

If everything goes well, you should see a cube. The viewer can be controlled with the mouse: click and drag to rotate, scroll to zoom (also works with pageup/pagedown keys), press W to toggle the wireframe mode (or Z on AZERTY keyboard).

In case the SimpleViewer does not work, you can still export and visualise the surfaces via the python script, included in plots/.

Algorithm

Much like in the curve case (recall Chaikin in TP4 or Lane-Riesenfeld in TP5), a uniform bi-quadratic B-spline surface ($k=2,l=2$) can be evaluated by iterative subdivision of the initial control net $V^0_{i,\;j}$. Here’s the algorithm to perform one step of the subdivision.

At the $k$-th iteration, for each cell of the grid composed of the vertices $V^k_{i,\;j}$ $V^k_{i+1,\;j}$ $V^k_{i,\;j+1}$ $V^k_{i+1,\;j+1}$ we define a new grid as follows

Important: If the surface is closed (cyclic) in the direction $u$, the subscripts $\scriptstyle i+1$ of $V^k$ are taken modulo the number of vertices in the direction $u$. The same applies for the direction $v$ and the subscripts $\scriptstyle j+1$.

open 1 open 2 open 3

A subdivision step, open surface.

closed 1 closed 2 closed 3

closed 4 closed 5

A subdivision step, surface closed in the $u$ direction.

Implementation

As before, the surfaces are represented via three coordinate matrices X, Y, Z. Apply the algorithm to each matrix individually. The openness/closedness of a surface in a particular direction is controled via the parameters u_closed, v_closed (these are read from the input file).

Start by implementing the algorithm for a surface closed in both directions as this is easier to do; test on [torus]. Don’t forget to use the modulo arithmetic where needed. Then, think about what needs to be changed if the surface is open in one or both directions.

ToDo

  1. Implement one step of the above subdivision algorithm for closed uniform B-spline surfaces. [torus]
  2. Modify you implementation for surfaces which are open: either in one direction only [cylinder] or in both directions [grid].