[Paraview] Strange opacity behavior

[Moreland, Kenneth]

Yi,


I can explain exactly why this is happening.


ParaView uses a technique called depth peeling to render transparent surfaces. The basic idea is to render multiple times, peeling away the top layer every time. The more peels you do, the more time it takes to render, so ParaView has a limit to the number of peels it will do for a render.


By default, the number of peels is set to 4. That number is somewhat arbitrary but usually works OK. What is happening to you is that because you have 4 planes, you are using all of your peels on those planes. When the sphere is completely opaque, it does not participate in depth peeling and is always rendered. When you add even a little bit of transparency, it becomes part of the depth peeling, and then gets dumped after the 4 planes. When you reduce the number of planes to 3, the depth peeling makes it all the way to the sphere.


You can control the depth of the depth peeling by going to ParaView settings under Rendering -> General. You will see an option named Number of Peels that is set to 4. Increase it to 5 to see the sphere. Increase it to 6 to see the back of the sphere.


-Ken


Kenneth Moreland
Sandia National Laboratories
email: kmorel at sandia.gov
phone: (505) 844-8919
web:   http://www.sandia.gov/~kmorel
________________________________
From: ParaView <paraview-bounces at paraview.org> on behalf of Yi Chen <chenyiaero at gmail.com>
Sent: Monday, August 25, 2014 3:30 PM
To: paraview at paraview.org
Subject: [EXTERNAL] [Paraview] Strange opacity behavior

Howdy,

I am using paraview 4.1.0 64bit on Ubuntu and when I play with the opacity of overlaid objects I found the behavior is interesting.

Basically I would like to see the sphere through several overlaid planes (along the normal direction of the planes) , as shown below. I first uses 4 planes.

[Inline image 1]

When the sphere has an opacity of 1.0, the effect looks correct, as shown below

[Inline image 2]

But if I lower the sphere opacity for a tiny bit ( like opacity = 0.999 ), the sphere is not visible at all, as shown below

[Inline image 3]

More strangely, when I use 3 planes instead of 4. The behavior looks normal. I mean when I adjust the sphere opacity I can always see the sphere unless the opacity is too low.

[Inline image 4]

Anyone has similar experience and happens to know why? Thanks!

Yi
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