Re: Surface detail polygons?

Hans Holten-Lund ([email protected])
Tue, 28 Nov 1995 15:02:24 +0100

Chris Marrin wrote:
> If the goal is to eliminate coincident polygon effects then you don't
> really need to specify which face you want to "attach" to. We've played
> with this in Inventor. We call it Decal. A Decal node is a group whose
> children are assumed to be coincident. Layering is according to the order
> of the children (last child on top). It's implemented by rendering the
> children in 2 passes. In the first pass you render to the frame buffer
> but not to the z-buffer. The Z test is still done so the children are
> still occluded by previously rendered objects. In the second pass you
> render the z-buffer and not the frame buffer. You're left with objects
> that do not tear if coincident but rather show the last object drawn, and
> the z-buffer has the correct values.

Yes, this would work!

like this?? :

Decal {
Cube {} # the house
IndexedFaceSet { blabla.. } # the windows
}

And the change you mentioned in the Z-buffer renderer is required.

However, when using a list-priority renderer, it is nice to attatch to
a SPECIFIC polygon.
(Oh well, everybody uses Z-buffer anyway...)

BTW. What does the word "Decal" mean? (I'm Danish...)

>
> OpenGL also has an extension called "Polygon Offset" which performs the
> same job in a single pass.

Is this something like:

if(z_framebuffer > (z - offset)) {
DrawPixel(x,y,z);
}

for the children?

What about other nearby polygons, which are NOT coincident?

>
> Adding a Decal node to VRML would be very useful with many scenes I've
> encountered. It would be especially useful for placing AsciiText on a
> wall. Today, you need to float the text or it tears.
>

Definitely!

Mvh. Hans Holten-Lund.
EMail: [email protected] WWW: http://www.id.dtu.dk/~hahl
Dept. of Computer Science Technical University of Denmark.

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