# comp.graphics.algorithms

## Subject: Re: Basic perspective/normal q

"Hans-Bernhard Broeker" wrote in message
news:4bkqq4F11au1iU1@news.dfncis.de...
> [jongware] wrote:
> > I'm still struggling with this basic 3d stuff!
> > I have a 3d rotation matrix, a set of planes, and a couple of normals
> > associated with these planes. To draw a plane I rotate the associated
normal
> > and test if its z is positive (right-handed AFAIK). If so, the plane is
> > visible and should be drawn.
>
> That test is wrong. You have to test not only the normal vector, but
> also the position of the plane along that normal, to know if it's
> visible. A plane can very well be completely behind the camera, and
> still have it'z normal.z be positive.

I know it's no good -- that's why I get substandard results. Can you
substantiate your comment with the required math? All inputs are as
mentioned: a number of x,y,z coordinates for a plane; its normal; a camera
position. I guess that's all what it takes for input. I assume I'm correct
in stating that if the (transformed!) z coordinates are in front of the
camera position the plane should be visible (apart from its normal vector).

TIA -- [Jongware]