Re: [Visualpython-users] surfaces and gradients

[David S. <dsc...@cm...>]

> To kick off, below is my amateur attempt to create a potential surface and
> its gradient (field). I was wondering whether there's a better way of
> handling this. This isn't very smooth but it's fast and gives the
> flavor of
> what I want. Would there be a better way to handle exceptions, like the
> singularites that can result at the origin?

It would be nice to have a "surface" primitive in Visual, but I think your
basic approach is the best way to approximate one given what's in there now.

I would suggest moving the arrows completely above the surface:
   arrow(pos=(i,j,z+0.2), axis=gradf((i,j,z)), ...

As for singularities: One approach is to change the gradf() function to
return the zero vector at the origin.  A terse way of writing this is:

def gradf(v):
    return vector(v) and norm(v)

Alternatively, you could allow norm(v) to raise an exception, and catch it:

def gradf(v):
    try:
        return norm(v)
    except ZeroDivisionError:
        return vector(0,0,0)

> As a suggestion, when I want to clarify for my students the order of some
> vectors (like the unit vectors forming the coordinate basis), I always use
> the order RGB (red green blue) since that is the canonical ordering of the
> colors. Further ordering uses CMY (cyan magenta yellow) since that's the
> common abbreviation for printer inks. This way I don't have to
> use labels to
> slow things down (but the real reason is that I haven't figured those out
> yet -- help?).

The .label attribute does nothing for most primitives and will eventually
vanish.

If you have a very recent build of visual, you can use the new
(experimental) label primitive.  For example:

xhat = vector(1,0,0)
xaxis=arrow(pos=origin,axis=xhat*10,shaftwidth=vsize,color=color.red)
label( pos = xaxis.pos + xhat*8, text = 'X' )

> Finally, I can't seem to define "up" for the screen or view or camera
> objects. What is the format?

The direction the camera points is controlled by the "forward" attribute,
not by "up".  Up only controls the camera's rotation around that axis.

Try this:

scene.forward = (-1,-1,-1)
scene.up = (0,0,1)

Dave