# OpenGL Edge-on polys and Texture Stacks

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Hi all, I'm new here, please bear with me. I have a question that Clustying and Googling hasnt been able to answer so far, which is this: If a texture-mapped polygon is drawn edge-on - that is, at right-angles to the viewing direction/orthogonal to the viewplane - so that in theory it is 'infinitely thin', how does modern texturing hardware deal with this case ? ie. Does the polygon disappear (not drawn), or is the polygon drawn (presumably as a thin line) - and if so (if it is drawn), how is the clockwise/anticlockwise 'sense' of the polygon used for texturing in this case, ie. is it honoured ? I ask because I am researching (I have done a lot more graphics reading and thinking than actual programming, bear with me) real-time Volume Rendering using 2D and 3D textures. Even though some modern hardware is starting to support 3D textures, I am not yet convinced that 2D textures are as bad as they are made out to be. Specifically I am not convinced that it is necessary to store 3 copies of the texture stack - one for each of the major axes - as I think this requirement is based on the needs of the good folks trying to render giant datasets (mainly for medical imaging it would appear). If the stack is sufficiently dense (effectively making it a cell decomposition, but exploiting the hardware's ability to draw textured quads very very fast), then surely it can be rendered from any angle, provided the sorting order and anti/clock-wise texturing sense is worked out prior to drawing, - PROVIDED, that in the case where the texture stack is orthogonal (at right-angles) to the viewing direction, the quads that hit that case are actually drawn, as thin lines, and dont disappear. Can anyone help with an answer to this, eg. regarding OpenGL standards and/or NVidia or ATI or other chipsets ? Have I made myself clear or muddy ? Best Regards Jon

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I'm about 99.8% certain that gfx hardware culls 'edge on' poly's as part of the backface culling procedure. If the DotProduct of the Triangle's Normal and the vector from the camera to the triangle is <= 0, that triangle is culled. It makes no sense to draw a line, when in reality, you wouldn;t see anything, as a triangle has no depth. It's just a section of a 2D plane.

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Hey WarAmp, thanks for the reply - 99.8% is pretty sure!
This is just one of those things where, yes, I'd agree with you in general BUT...how standard is it across manufacturers is what I am wondering?
And this is one of those annoying cases where there is actually a need to do something that appears stupid on first glance: draw a textured edge-on quad as something.
Thanks again, and any further thoughts you have are most welcome!

PS, I've been looking at this guy's work:
http://www.cs.sunysb.edu/~vislab/projects/amorphous/WeiWeb/research.htm
and specifically:
http://www.cs.sunysb.edu/~vislab/projects/amorphous/WeiWeb/Software/TextureVisualizer.htm
- it works unbelievably well.

Cheers
Jon

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My guess is that you'd see at least a pixel-width line when rendering an edge-on polygon provided that back-face culling is off. Mathematically it should disappear, but a rendering algorithm isn't that clever. It will just see that the start pixel for a given scanline is the same as the end pixel and draw the one pixel. This is also the case when two lines of non-zero slope meet at a point on a regular front-facing polygon, so you can't just skip rendering when startpixel == endpixel. But in all honestly I don't know what would happen because I've never sat down and tried it.

However trying to pack many polygons close together so that they appear as a textured surface from the side sounds tricky. You may experience artifacts due to floating-point inaccuracies and integer rounding/truncation; not to mention that you'd have to dynamically change the number of stacked polygons as the world-to-screen mapping changes since depth is in pixel-space, as a function of the number of polygons. Otherwise you won't be able to maintain a constant visual depth of a cell. That alone might defeat real-time performance and storage requirements. Traditionally, polygons aren't a good way to represent volumetric data unless they're used to simulate another representation like voxels.

[Edited by - Zipster on November 12, 2005 6:31:59 PM]

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Hi Zipster and thanks.

Ah, well that is the art of the thing, the whole texture stack approach to volume rendering does use densely packed quads to do exactly that - represent a volume.

It works quite well, but as you point out there are many issues. I'm looking at a version of LOD for texture stacks using octrees (but I have a lot of catching up to do - been out of hands-on graphics programming for just a leetle too long).

Also I did a big forehead-slap the other night, one does not use backface culling at all with texture stacks, that was a basic thing that I had just totally failed to notice :)

From the side, well, thats the thing, I'd like to know if the GPU firmware is smart enough to deal with highly side-on cases, but as you both say I suspect it isn't.

Jon

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