## Thursday, October 6, 2011

### Matlab: SURF, creating a surface.

I have no problems admitting that i am not a smart person at all.  When it comes to programming/mathematics/image processing/medical imaging analysis, every day, i make sure i tell myself i am a  real novice and there are boundless rooms for me to improve.

Just like one of Matlab's function: SURF/SURFACE.  The function creates a surface based on a matrix or columns(x,y, and/or z).  Reading Matlab's help on surf bewildered me further.   My intention was simple.  I wanted to create a surface, but i didn't have a z function and my z was not anyhow defined by x and y.  If my z is well defined, i might as well have a pretty good model for what i am trying to do.

Anyway, imagine that you just want to create a square plane.  How do you do it with surf ?

After fiddling with this and that, reading this and that, i finally realized what needs to be understood.

Say, i want to create a 2 x 2 plane flattened on z = 2 using coordinates.
As unintelligent as this might look, i could actually go by doing the followings:

> x = [-1  1; -1 1];
> y = [-1 -1;  1 1];
> z = [ 2  2;  2 2];
> surf(x,y,z);

For a smart person like you, you should already recognize the that there are 4 x-y-z points (-1 -1 2), (1 -1 2), (-1 1 2), and (1 1 2). Note that even you know z is 2 across the whole plane. You cannot just input 2. You need a MxN matrix (which happens to be the M and N of X or Y), which makes sense other wise, Matlab won't know how to reconstruct that plane. In the above case, it will be a 2 x 2 matrix. This might seem introductory to you. But it gives me a better understanding of making a surface in matlab.