Hi Bob,
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>Date: Wed, 2 Jan 2002 18:11:31 -0500
>From: RWGRAY <[EMAIL PROTECTED]>
>Subject: [JAVA3D] Transparency Bug Report????
>Comments: To: Kelvin Chung <[EMAIL PROTECTED]>
>To: [EMAIL PROTECTED]
>
>Hi Kelvin,
>
>Has a bug report been issued for any of the transparency problems I
>demonstrate at
>http://www.rwgrayprojects.com/Java3D/Transparency/trans01.html??
>
>Toward the end of this web page shows the object as 3 different quadArrays,
>and a very bad transparency problem. If you don't think this is a bug,
>could you explain what the cause of the effect is and possibly suggest
>another way to code it?
>
>Thanks,
>Bob Gray
>
Seems to me the color of three toruses are
green, blue, red from inside to outside.
As you may know, the way Java3D doing depth sort is to
compute the centroid of each GeometryArray and use the
distance from the viewpoint. In the torus like
shape there is a vertical part which connect the top and
bottom. When view from the top, the
distance from for all three (green/blue/red) of this
vertical parts after you subdivide the torus are about the
same to the viewer so the sorting order is not correct
and green circle is seen. This is expected.
Breaking the geometry into 3,168 quadarrays
and depth sort this one EVERY frame is going to be
very slow if not memory expensive. For 30 of these
object you are taking about sorting ~100K
geometry EVERY frame.
Using only one GeometryArray for a Shape3D under SharedGroup
should have better memory usage. However it will not solve
the transparency problem that you want. In general
this is a hard problem without hardware support for
depth sort per pixel or using stencil buffer
(which Java3D do not support) technique. You
can also try to simulate the effect if hardware
support 3D texture as in Volume visualization.
One thing you can try to do is instead of using
BLEND_SRC_ALPHA for source blend function
BLEND_ONE_MINUS_SRC_ALPHA for destination
blend function, use BLEND_SRC_ALPHA for source
blend function and BLEND_ONE for destination
blend function instead. In this way there is no need to
subdivide the torus and doing depth sorting since the effect
after rendering is order independent. The drawback of this
approach is that the effect is additive.
i.e. the color of second layer will multiple by alpha1
and add to inner layer. The final color is the
color of third layer multiple by alpha2 and add to
the previous result. So you can only achieve the effect
from dim to bright from inner to outer layer.
The weired banding effect that you get when
(1) Turn off depth sort transparency
(2) Disable setDepthBufferFreezeTransparent
(which is true by default)
(3) Set PolygonAttributes.CULL_NONE
is expected. Since the backface from the far side of
the sphere may drawn in front of the frontface as
Z buffer turn off. The color of backface drawn
is darker than the color of frontface. It is perfectly
legal for the graphics card to tesselate the QuadArray
and drawn in whatever order it prefer when the geometry
array is pass down to the native library.
Hope this helps to explain the transparency issues.
- Kelvin
-------------
Java 3D Team
Sun Microsystems Inc.
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