Can hair be rendered out separately

[Eric2575]

I'm working on a scene that has quite a bit of hair in it. The hair is actually leaves, ivy that is hanging on a wall. Can I render that out separately, and what about the shadows the leaves cast on the wall?

 

Eric

[robcat2075]

Describe the situation that needs this a bit more.

[Eric2575]

I have a scene with many objects in it that is taking about 7 minutes per frame to render without hair. When I enable hair to render, the time goes up to over an hour per frame. The density of the hair material isn't the problem, it's down to about 3%, it's the fact that there is a lot of hair in the scene. I created a vine running over most of one wall that is causing the slowdown. So now I'm thinking I could render the scene without hair and then render out the hair separately, then composite them together. I just don't know if this is possible, and if so, how to do it? I don't want to wait 16 passes at over an hour a piece to see that I will likely have to change something anyway. Thanks for any help on this.

 

Eric

[robcat2075]

One trick I read in Jeremy Birn's book is to not use ray traced lights on hair. Are you using ray-traced lights?

[Eric2575]

I'll have to check on that. I know I am using only three lights, all suns with only one casting shadows. It's an outdoor scene of an alley shot with IBL and the suns.

[robcat2075]

I'll have to check on that. I know I am using only three lights, all suns with only one casting shadows. It's an outdoor scene of an alley shot with IBL and the suns.

 

Sun shadows are ray traced, that's all they can be. Only Kleig lights can be z-buffered, and if you put a Kleig light far enough away it's like a sun light.

 

With light lists it's possible to have different lights for different models.

[HomeSlice]

I have a scene with many objects in it that is taking about 7 minutes per frame to render without hair. When I enable hair to render, the time goes up to over an hour per frame. The density of the hair material isn't the problem, it's down to about 3%, it's the fact that there is a lot of hair in the scene. I created a vine running over most of one wall that is causing the slowdown. So now I'm thinking I could render the scene without hair and then render out the hair separately, then composite them together. I just don't know if this is possible, and if so, how to do it? I don't want to wait 16 passes at over an hour a piece to see that I will likely have to change something anyway. Thanks for any help on this.

Eric

 

First, let me address this:

"I don't want to wait 16 passes at over an hour a piece to see that I will likely have to change something anyway."

This is not an efficient use of your time. For the first few test renders, only render 1 or 2 passes and only large enough to see what's going on. Once you think you have everything like you want it, render a half-size version with only 4 or 5 passes. If that looks good, you can be pretty sure your final render is going to look good.

 

To get render times down.

Robcat's suggestion to use Z-Buffered shadows would help.

You can render only the hair with an alpha channel, then render a separate shadow pass at only 1 pass (set the wall to "shadow only") - then blur the shadows in Photoshop and composite all the passes together.

You can render foreground/middle/background elements separately and composite them in Photoshop.

You can turn off "Cast Shadows" for all models where shadows are not really needed or seen.

You can delete all your lights except one shadow casting light and a couple of non-shadow casting lights, then use IBL to help light the scene.

You can render 30% smaller than the size you need, then scale it up in Photoshop.

[Eric2575]

Thanks guys, that will get me going again. First I'll change the suns to kliegs...

[Eric2575]

Well, I exchanged the three sun lights with kliegs and made sure that only one of them cast z-buffered shadows. A 1000x750 render still took 20 minutes with hair enabled for a single frame. Without hair it took 6:24. That's quite an improvement for the hair render. I'll try rendering the hair out separately next. Also, maybe I should rethink my hair model?

[robcat2075]

Well, I exchanged the three sun lights with kliegs and made sure that only one of them cast z-buffered shadows. A 1000x750 render still took 20 minutes with hair enabled for a single frame. Without hair it took 6:24. That's quite an improvement for the hair render.

It was 16 hours before? Yes, that's better.

 

 

I'll try rendering the hair out separately next. Also, maybe I should rethink my hair model?

You may have to actually show us what this looks like.

[Eric2575]

No, the improvement isn't that drastic - initially 16 passes at one hour per pass, now 16 passes at 25 minutes per pass.

 

There must be something wrong somewhere - a render without hair averages about 6 minutes now, but the hair only render with an alpha channel just finished rendering after 7 hours and 35 minutes at 28:30 minutes per render for 16 passes. Here is the TGA of the vines that took so long.

Is this normal? Btw, my computer is an Intel Core 2 Duo 6600 @ 2.40 GHz running Win XP SP2 with 2 GB of Ram running AM 15.0i.

 

Had to convert the TGA to a jpg for uploading.

[robcat2075]

Why do you need multi pass for this hair?

[Eric2575]

To be honest, I forgot to turn multipass off this morning when I set the render. In any case, isn't it taking a bit too long for the hair to render?

[robcat2075]

It does seem long. I haven't done much with decaled hair, but Nancy has and I don't recall her mentioning 7 hour frames.

 

How big is the decal for the leaf shape?

[NancyGormezano]

It does seem long. I haven't done much with decaled hair, but Nancy has and I don't recall her mentioning 7 hour frames.

 

How big is the decal for the leaf shape?

 

You betcha - I wouldn't be doing hair if it took 7 hours/frame. I have to wonder Do you also have AO on? - and Roberts question about the size of the image for the leaf shape is a good one. You don't need a high resolution image. And I would also wonder about the alpha channel for the leaf shape as well - transparency (anywhere) will also increase render times -

 

In my testing - that I just ran (15j plus) - I took Orkarella - rendered at 1280x960, 5 pass, soften ON, default lighting, alpha on - took 33 secs (or 13 secs at 640 x 480). When I switched the image for all the hair systems, to a leaf shape (approx 400 x 400) with a more complicated outline - render took 1:10 minutes.

 

Have you tried rendering with 15j plus? 15i might have a problem?

 

EDIT: - 3rd example just tried with a more complicated emitter image - ie more transparency, increased thickness of emitter to make more overlapping of transparency areas - and still only took 5 mins

Edited April 15, 2010 by NancyGormezano

[Eric2575]

Thank you both for looking into this with me. Off to work I go, but when I get back, I'll check out all of your suggestions.

[Eric2575]

Wanted to add the tga.

Ivy_trio_86.zip

[NancyGormezano]

I don't think it's the emitter (just tried - 1:25) - it's some other setting - either in your render settings (AO) - or some property of the hair system ... density?, dynamics?, collision?, roughness? etc - can you upload the hair material ?

 

EDIT: uploaded another emitter example in previous post in case you missed it

Edited April 15, 2010 by NancyGormezano

[robcat2075]

I got this in 9 seconds in the default lighting. Alpha channel doesn't add significantly.

 

 

 

Does AO even do anything with image hair? Does image hair really shadow itself?

[robcat2075]

Here's an AO only render. It does look different than straight lights.

 

 

This took 9 minutes at 10% AO so that's 60x longer. I think that's the culprit.

 

How do these vines need to animate?

[robcat2075]

Here it is under one of my fake AO domes.

 

49 passes in 3 min 01 sec

 

 

 

that's not perfect AO but it's plausible AO in a very fairly brief time.

 

Perhaps one might use fake AO for the vines and perfect AO for the more architectural elements and combine the two.

[robcat2075]

I changed the decal to white so we could see the AO effect on the leaves better.

 

 

 

that was 121 passes in 7 min 42 sec

[Eric2575]

Fake AO dome? I'd like to give that a try.

[robcat2075]

This is just experimental but you can try it. It's several very wide Kleig lights arranged in a dome over a central point. They are very wide so they overlap. When you use multipass they randomly jitter themselves in their width. With enough passes they start to approximate a continuous dome of light emitting light from every direction. Because they are all Z-buffered each pass is fairly short. It's like a spinning light rig but there's no spinning and and you don't need to mess with motion blur.

 

If you adjust the properties of "Light1" in the model (Objects>OneLight>Bone1>Light1) you adjust all the lights in the array simultaneously.

 

Generally, the smoother the surface of an object, the more passes needed to hide the stepping of the lights. The vine needs not many, the ground plane needs very many.

 

The greater the "softness" setting of the light, the fewer passes needed for smooth shadows, but the less that small details will get good occlusion shadowing.

 

This technique doesn't seem to work well with creating occlusion shadows around very small details, which is what the scene in the chor was testing.

 

Dome_8_lights05_vine_small_details.prj

[jason1025]

Thanks Robert

[robcat2075]

Here's another attempt. The self jittering lights seem to have too many clumps in their jitters so i went back to a spiral path rig that spins from the top of a hemisphere to the ground. that isn't really random either but gets me a fairly even distribution of lights.

 

This has somewhat better occlusion on the small details than the above version. I level adjusted this to deepen the AO look.