| Re: Depth visibility with Object Graphics !!! [message #39611 is a reply to message #39477] |
Wed, 26 May 2004 07:30  |
Karl Schultz
Messages: 341
Registered: October 1999
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Senior Member |
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"Antonio Santiago" <d6522117@est.fib.upc.es> wrote in message
news:40B239EE.10308@est.fib.upc.es...
> Hi,
>
> i am working with Object Grpahics and i hace a problem :)
>
> I have put on a model two objects. Firts, i must put an IDLgrPolyline
> betwen two points (a line) that i can modify to select one trajectory.
> Second i put an IDLgrImage.
> My problem is that image is drawing over the polyline. I play with
> DEPTH_TEST_DISABLE and DEPT_TEST_FUNCTION, but always the image is
> drawing over the polyline.
>
> Any idea?
The IDLgrImage does not interact with the depth buffer as it is drawn. So,
if you draw the image after the polylines, the image will overwrite the
polylines where they overlap, independent of the depth of the polylines. If
you draw the image first, you will always see the lines. In general, it is
usually better to arrange scenes containing images so that the images get
drawn first.
The reason behind this is that IDLgrImage objects are drawn with an OpenGL
pixel primitive (glDrawPixels), which is a special 2D image drawing
primitive. The intent of IDLgrImage is to provide a "billboarding" style
image drawing primitive, so the usage of this primitive makes sense.
The discussion in this thread about using a texture-mapped polygon gives you
the flexibility of drawing the image in space whereever you want with the
full interaction with the depth buffer if that is what you would expect.
So, you have both methods available to you in IDL.
The IDL (6.1) docs say:
Image objects do not take into account the Z locations of other objects that
may be included in the view object. This means that objects that are drawn
to the destination object (window or printer) after the image is drawn will
appear to be in front of the image, even if they are located at a negative Z
value (behind the image object). Objects are drawn to a destination device
in the order that they are added (via the Add method) to the model, view, or
scene that contains them. To rotate or position image objects in
three-dimensional space, use the IDLgrPolygon object with texture mapping
enabled.
Karl
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