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Re: IDLgrClipboard and IDLgrPrinter: wrong vector output order ?



Hi.

kschultz@rsinc.com wrote:
> 
> I think what we have here is a Z-buffer "tie" issue.  The primitives
> are being drawn in the same order.
> 
> In your example, the red poly is drawn first, then the green one.  On
> the Window device, the red one overlaps the green one, so the Z buffer
> rule is that the pixels are not modified when there is a tie.  This
> corresponds to the default OpenGL rule of GL_LESS, which means draw
> only if the fragment's Z is less (closer) than the Z currently in the
> frame buffer.
> 
> The IDL vector-mode support essentially does not have any notion of a
> Z-buffer.  However, it makes a reasonable effort to sort primitives by
> their Z values.  You can get into problems with this, particularly
> when polygons overlap each other, or intersect, etc..  In other words,
> we just do a real simple Z-buffer sort to try to approximate things.
> I think that the 5.3 manuals discuss the fact that you won't get
> really good Z-ordering in complex scenes.
> 
> Anyway, apparently, the Z-sort is implemented so that if there is a
> tie, the order that the primitives are drawn is retained.  So, in this
> case, since both polys have the same Z value, the relative drawing
> order of these two polys is maintained and the green one is drawn
> second.  Since there is no Z-buffer in vector mode, the green one ends
> up overlapping the red one.
> 
> So, what is the RIGHT way to do this?  I'm not sure it is clear.  The
> argument to try to be consistent with raster Z buffer rules is a good
> one. But with vector output, people are used to thinking in terms of
> the "painter's algorithm" of drawing the thing you want on top last,
> so the intuitive notion of maintaining the order of prims that are at
> the same Z is also useful.

In fact, for postscript output it is more than "people are used to",
postscript _do_ draw the last object on top. So, if I understand well
the Z-buffer thing, I think that to be as much as possible near this
Z-buffer rendering, you must do the following in vector mode for
postscript (and the like) output: if there are several objects at a same
Z value you output them in the postscript in the reverse order of the
graphic object tree. This way I think it will work for my very simple
square example. For more general situation, I am not enough experienced
with IDL.

Any comments ?

Anyway, I think that in a near future I will use the Mark Hadfield's
suggestion: "code-control" of the order. While (for the moment) I only
work with 2D graphics, I will use the Z value of my objects to implement
a layered organisation of them.

Of course a fix into 5.4 will be nice too.

Later.

Nicolas Decoster.

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