[3D Graphic]Normals of a Face

[KeepingRealBusy]

Steve,

That makes sense. I am familiar with both methods.

My dad taught me electronics as he learned it in the 20's 30,s and 40,s left hand rule, current flow from positive to negative, with the diode arrow in schematics. Nowadays, it is more correctly the electron flow from negative to positive, but the diode (and transistor) arrows remain as a relic of the past. With the old left hand rule, current flow (+ to -) through a coil in the direction of your left hand fingers wrapped around the coil, meant that the north pole of the magnetic field (center of the coil) was in the direction of your thumb or up, the same as if you wrapped your left hand fingers around one of the wires in the coil with the thumb in the direction of the current flow then the finger tips (in  the center of the coil) would point up (north). The right hand rule, however covered the mathematics and geometry.

As far as the cross product goes, use determinants for the value, use the right hand rule (rotate one planer vector counter-clockwise to the other planer vector and the thumb points out (is the normal).

Kind of makes sense to allow either, but on some severely convoluted surfaces, where only polygon vertex coordinates  were known, it might be difficult to visualize just what was IN and what was OUT.

Dave.

[dedndave]

20's ? - he must be a real OT   :P
spark-gap and all that

in physics class, they taught us the right-hand rule using i, j, and k
i never did get a good explanation as to why x, y, and z weren't good enough - lol



of course, x, y, and z are axis'
i, j, and k are vectors   ::)

the arrow you are talking about...
the arrow shows the direction of "conventional current flow", as opposed to "electron current flow"
which really reflects the way solid state devices work
a "hole" or "depletion" leaves an empty spot for an electron to fill
so - the arrow points in the direction of travel for holes   :P

when i was a teenager, i struggled a little with transistors
because i was raised on vacuum tubes - lol
later on, FET's came along and made the world right again - sorta
by that time, i was comfortable with transistors and had to re-learn tubes   :lol:

[KeepingRealBusy]

Dave,

He started out with Crystal radios, the old germanium crystals and cat whiskers (I think). He never got into spark gap transmitters(per-se). He served in the Signal Corps during the war (in Detroit) in research and suppression of radio interference, especially the noise suppression of ignition systems for tanks and jeeps.

Dave.

[dedndave]

very cool
i think we all wound our wire around a toilet paper roll at one time or another - lol
fortunately, we had 1N34 and 1N60 diodes
no farting around with a cat whisker

i was raised not too far from Detroit
that's where we used to go to take our FCC exams

[dedndave]

i hope your last name isn't Wilson   



what ??? - that's not MY dad - i don't know the guy   :lol:

[KeepingRealBusy]

Dave,

Interesting find! No, not my dear ol' dad.

Dave.

[dedndave]

i bet that guy had all the gals, in his day - lol

[Farabi]

i bet that guy had all the gals, in his day - lol

Sounds like had a lamborghini  :lol:

[dedndave]

he had a crystal set that was only good for 10 miles   :lol:

when i was a kid in Michigan, i could get stations as far as Oklahoma City (at night) on my crystal set
of course, it was one of those 250 KW transmitters   :P

[xanatose]

A Point (vertex) is in the Form
T(x,y,z,1)

A normal is not a point or vertex, but a direction. So the form for a normal is:
T(x,y,z,0)

When you multiply a vertex by a 4x4 matrix. It will move its position. When you multiply a normal by a 4x4 matrix, it will just cause a rotation. IF is in the form (x,y,z,0).

[Farabi]

I tested it, it is rotated. So when you rotate an object, the object normals is rotated, or should be rotated.