From: teunis <teunis@computersupportcentre.com>
  To  : ggi-develop@eskimo.com
  Date: Thu, 20 Aug 1998 18:29:02 -0700 (MST)

Re: LibGGI3D RFC

On Thu, 20 Aug 1998, Jon M. Taylor wrote:

> On Thu, 20 Aug 1998, teunis wrote:
> 
> > On Thu, 20 Aug 1998, Jon M. Taylor wrote:
> > 
> > > On Wed, 19 Aug 1998, teunis wrote:
> > > 
> > > > On Wed, 19 Aug 1998, Jon M. Taylor wrote:
> > > > 
> > > > > On Wed, 19 Aug 1998, teunis wrote:
> > > > 
> > > > I'm curious, as moving any of the X,Y[,Z] coordinates will affect the
> > > > remainder of the coordinate list...
> > > 
> > > 	For one triangle this shouldn't be a problem.  For a set, you do
> > > what you can. 
> > 
> > so what happens if you clip a triangle and you lose the corner [u,v]
> > coordinates?  All of a sudden your texture is moving around when it gets
> > clipped.  The same happens for colourblending when you have discrete
> > colour components per-corner - all of a sudden your blends get messed
> > up...
> > 
> > How do you mean to handle this?
> 
> 	Clip and retesselate the triangle before you apply a texture map. 
> Won't that take care of the problem?  I have to admit that my texture
> mapping knowledge is not a strong as it could be, but I'm not seeing why,
> if you clip and retesselate the triangle(s) before you do any shading, you
> won't be OK.


Finally!  -THIS- is the issue behind clipping.

According to the person who wrote the GGI3D FAQ, tesselation is -not-
something to be handled by libGGI3D.

[and applying a texture is alot easier before tesellation imho]

> > > > > > >  - I want clipping in libggi3d also.
> > > > > > 
> > > > > > See above...  and this requires also the camera info (view frustrum,etc)
> > > > > > to be available....
> > > > > 
> > > > > 	Right. 
> > > > 
> > > > So how does this work?
> > > 
> > > 	Attach it as a struct to the visual. 
> > 
> > not what i meant *sigh*.  I mean how do you expect clipping to work?
> 
> 	Oh.  Sorry |->.  Well, I guess it would really depend on the 
> rendering target.  If rendering directly to triangle hardware, maybe no 
> clipping at all (the HW would do it).  If rendering to a depth buffer, 
> you could render eatch triange or triangle set into a secondary buffer 
> sized to not need clipping and then copy that into the master buffer, or 
> directly into the master buffer if no shading/texturing issues are present.
> 
> 	The point is that the clipping is target-specific.  there won't be
> one (or more) "standard" LibGGI3D clipping techniques, because there are
> so many different ways to render 3D.  In LibGGI2D, things are a lot
> simpler.  In 3D, you can render to a 2D buffer and clip there, or you can
> render to 2D hardware with hardware clipping, or you can do a
> sutherland-hodgman algorithm in 3D, or you can retesselate, or you can
> retesselate with varios kinds of interpolation, or you can render into a
> depth buffer and clip there, or you can render into a hardware depth
> buffer and the hardware will clip, or you can render triangles directly to
> the hardware, or... see my point?

Ah - but available clipping methods may vary per renderer as well...
texturemapping - none, tesellation, interpolation, depth-buffers only
	(you -don't- want to lose where the U,V should be when you clip!)

> > [clip]
> 
> 	Heheh.  At last that clipper is simple |->. 

yep!

> > > > Though there should be some kind of internal mechanism for handling
> > > > display memory!  (so it doesn't get all fragmented and can be used
> > > > effectively!)
> > > 
> > > 	I guess the buffer system doesn't *have* to be in a target.  AAIK 
> > > is isn't(?) in LibGGI2D.
> > 
> > Supposedly DirectBuffer handles this.  Will someone -please- tell me the
> > status on non-primary-display DirectBuffers?!?!?!?!??!
> > WHAT NEEDS DOING?
> 
> 	Don't ask me, I'm just learning this stuff.  If it hasn't been
> done, it will probably have to be implemented as part of writing LibGGI3D. 
> Not initially, but eventually.

It's missing in libGGI2D as well.  the libGGI powers-that-be said "not in
GGI" and noone has finished an extension to handle it.

That's why I keep asking!  (and getting the runaround)

I'm wondering what -it's- API / modus operandi should be!

G'day, eh? :)
	- Teunis