**comp.graphics.algorithms**

## Subject: **Re: Basic perspective/normal q**

"Hans-Bernhard Broeker"

news:4bkqq4F11au1iU1@news.dfncis.de...

> [jongware]

> > 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]

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