how to do a multi - pass render

[stib]

I have a scene with some big slow -rendering hypervoxels effects in it. I've found that for my purposes (PAL Video) an anti-aliasing of low is fine for Hypervoxels, but for normal objects I'm not really happy with less than medium AA. So to save time I want to render my scene in two passes. One with the HV at low AA and one with the normal geometry at medium AA.

I've read that doing renders in multiple passes is the go, but I can't work out how to do it. There's a fair bit of stuff in the scene, and the hypervoxels move in and around it. Do i have to set up a different scene with some kind of special surface applied to all my normal objects in order to matte them out? Or is there a big red button with "matte this object out for multiple pass renders" writ large 'pon it that I'm missing (as there usually is when I ask questions like this.

Any suggestions / links?

BTW I've already tried to rtfm. I don't really know what I'm looking for, that's the problem. searching the help & these fora for 'passes' gets lots of hits, but none of them relevant.

[Mylenium]

Well, you would quite likely have to derive an extra scene from your original one. Then you can play around with your object properties (unseen by camera, unseen by alpha etc.) or your surfaces (advanced tab -> set alpha to fixed values or shadow density). It helps to turn your objects to a flat black to get a better (premultiplied) alpha for compositing. All you then need to figure out is how your layers for compositing need to be arranged.

Mylenium

[stib]

damn! You mean I've got to think? Just what I need. I was so hoping I'd missed the big rend multi-pass-render button.

Any good tips from experienced multi-pass renderers out there?

[richpr]

I know NanoGator posted a nice thread/info on Multi-pass rendering over at CGTalk... He seems to use it a lot... I remember that it usually involves different scenes and the use of black ;)

[stib]

It's not too hard. I had a think about it and came up with a solution.
I tried putting the solution up here but the free host people I use don't like images being directly linked so here's my solution as a "tutorial" (http://scriptgeek.netfirms.com/multipass.html) (www.scriptgeek.netfirms.com/multipass.html)

[richpr]

Here's the thread I was talking about... useful!

Link (http://www.cgtalk.com/showthread.php?s=&threadid=47111&highlight=multipass)

[Gui Lo]

Hi Stib,
Glad you worked out your problem.

Thanks for your tutorial. I think it can help me solve I problem I have.

Scene:
A girl in a bikini walks towards the camera.
The bikini comes into focus but the girl stays blurred.

The problem I have is that all the geometry of the bikini shows when I use Hidden from Camera on the girl.

Anyway I guess that I can 'black out' the girl and render with alpha turned on. This will give me the bra with the geometry 'as seen'. Then I can composite.

I hope I have it correct.

Thanks again

Gui Lo

[txbob]

since as you described the scene, nothing moves in front of the bra, there is a relatively easy was to multi-pass render this, and it saves a lot of time all around.

I have found that in order to get good focus effects in LW you have to jack the AA passes up really too high to make it very practical. I found that using either a blur filter, or even better, doing the blur pass in after effects or whatever does a better job, and is easier and quicker.

As far as the compositing is concened, you can render a pass with medium AA, set the Hypervoxels to unseen by camera, and they will still cast proper shadows etc. Then use this as a BKG sequence. Render the pass of just the voxels, (remove everything else from the scene) and render with low AA. The background will still be med AA and the voxels low.

Hope I understood the question, and this answer helps.

[stib]

Render the pass of just the voxels, (remove everything else from the scene) and render with low AA.

thanks for the help. My problem was that if I remove everything else from the scene I don't get shadows on my HV objects, and any objects that are in front of the HV won't matte it out, so that when I do my final composite the HV will be in front of everything. That's why I used a black texture + constant alpha set to 0 on all the objects that cast shadows on or come in front of the HVs. That constant alpha is there so that the objects matte out my HVs.

This is what i'd do for the bikini scene: Render a scene with everything except the bikini blacked out, and then one with everything blacked out except the girl- with no bikini on (ooh err), and then one with the background if you have it and no girl. You could do the girl and bg together if you want the same blurring on both.

I'd do the blurring in post, a good blur is the "squint" effect - it works the same way as an out of focus camera lens, by creating circles of confusion. Though for the background you might want depth of field happening which you'd probably want to do with LW. If the camera stays still then you could hand blur the BG in Photoshop or summat which would save a lot of render time.

I'm updating the pics in the "tutorial" (http://www.scriptgeek.netfirms.com/multipass.html) today so that they better demonstrate what's going on.

[NanoGator]

Try here:

http://www.nanogator.com/multipass.jpg

and here:

http://www.nanogator.com/multipass-ds9.jpg


:)