Alpha problems with particle systems in Vray

[pxm-vray]

Hi Chaos group

I am in production at the moment and I am having a Vray issue.

My scene consists of a TP particle setup. When I rendered certain particles would disappear and then reappear down the line (reminds me of quantum physics). This creates a flickering in the render, and can be most easily seen in the alpha channel. The alpha ranges from white to grey and some objects have no alpha at all but show up in the rgb - super weirdness. The same setup rendered with scanline produces no problems.

I have tried everything to fix this, and finally settled on exporting my particles to animated meshes. This effectively eliminated TP as the cause. However the problem persists.

At the moment I have merged my animated meshes into a completely new max scene along with the camera - so there are no lights, no hidden geo or anything else that could be causing problems.

As I mentioned I am in the middle of a production with a deadline looming so any help on this matter would be saving lives....

Software being used:
Max 2010
Vray 1.50sp3a
Windows XP 64 bit sp 2

Hardware:
Dell precision T3400
Intel core duo 2 quad
8GB ram
Nvidia quadro FX 1700

Thanks in advance...

Conrad Murrey
Contracted to Pixomondo

[stevesideas]

Are you using DR ? Do all the DR machines have the TP plugin installed ? Are you using maps or instances for the particles ? Are all the paths to textures etc all networked paths so all machines can see them ?

Do you have dynamic memory set to auto...sometimes setting it to something other than Auto can lead to funny outcomes with particle flow...maybe it affects TP too....

Thats all I can think of...it may be more techincal that that...

[joconnell]

Any chance the particles are tiny and you're using adaptive subdivision aa? If you've got tiny particles and use undersampling (like aa settings of -1,2) then some of your particles might disappear.

[pxm-vray]

Any chance the particles are tiny and you're using adaptive subdivision aa? If you've got tiny particles and use undersampling (like aa settings of -1,2) then some of your particles might disappear.

You are correct that the disappearing objects are small. I have been using adaptive DMC for AA. My initial setting were min subdivs = 1, max subdives = 8, clr thresh = 0.001. However I have pushed these settings up considerably (min = 8, max = 512) with no great improvement.

We are working with cm as our scale and it is a large scale scene, however scaling the scene down or up makes no difference. Would changing my units form cm to meters make any difference?

Thx for the feedback

[pxm-vray]

Are you using DR ? Do all the DR machines have the TP plugin installed ? Are you using maps or instances for the particles ? Are all the paths to textures etc all networked paths so all machines can see them ?

Do you have dynamic memory set to auto...sometimes setting it to something other than Auto can lead to funny outcomes with particle flow...maybe it affects TP too....

Thats all I can think of...it may be more techincal that that...

Thanks for the feedback. We are not using DR for the renders. There are also no problems with missing textures etc. I have done tests with Auto, dynamic memory and static. Same problems occur.

[pxm-vray]

Another weird thing that we have discovered is that the alpha channel is different depending on whether you use the vray frame buffer or the default max buffer....

Vrays frame buffer is missing objects and is also way darker. Not using vrays frame buffer still misses some objects but the alpha is a lot stronger. Even stranger is that if you duplicate the Vray frame buffer render to max buffer then the alpha appears stronger.

Is there a reason for this? Initially I thought it could be a gamma issue but gamma should have no effect on an alpha channel should it?

[pxm-vray]

Just to let people know - we did find a solution to this problem sort of.....

The issue is definitely in the AA sampling. Pushing the sampling up to ridiculous amounts helped a little bit but not enough to fix the problem. Also increases rendering times astronomically.

Solution is simple - render at double the size! sure renders take 4x as long but the problem is completely solved. Luckily my original renders were super quick so I can live with this, but if I had really long renders this solution would not work.

I believe that Vray has problems with sampling any sub pixel objects and I hope that the Vray guys get this fixed.

Having said that, I am a big fan of the software and will continue to rave about it.