What's the secret to RTS?

reck · October 15, 2009, 03:53:18 AM

I thought i'd try out RTS (Ray Traced Shadows) on one of my renders, not something I normally use because of the render time increase. To compare what difference it made to an image I rendered a scene without RTS on and then rendered again but this time turned on RTS without changing any other settings.

The image with RTS took a lot longer to render of course but it also doesn't look as good as the image without RTS. I was hoping to see better shadow and lighting within the cloud and maybe some rays, instead it just comes out a lot darker and flatter and the non RTS image looks a lot better.

Here are the two images. The top one has RTS turned off, while the second one has RTS turned on in both the atmosphere and the cloud layer.

Render settings for both images.

Detail = 0.5
AA= 5 (cubic)
Bloom = on
GI=2,4
Soft shadows=20,9

Render time for non RTS timage = 3 hours, 49 minutes
Render time for RTS image = 17 hours, 7 minutes

Henry Blewer · October 15, 2009, 06:42:50 AM

I increase the Global Illumination or Ambient Occlusion on the surfaces. It softens the shadows, but allows the objects/terrain to be more visible.

reck · October 15, 2009, 08:15:23 AM

But don't you think a GI of 2,4 is already pretty high? Yeah I can brighten it up a bit, it's just I didn't expect the RTS option would make it look this bad.

jo · October 15, 2009, 08:19:37 AM

Hi,

Try changing the exposure. You have a very bright background so of course the foreground is going to be darker. It's like you were using a camera.

You can adjust this after the render if you save in EXR format and have something which can work with HDR files. It's a bit liking using RAW in your camera, if you have a camera which supports RAW.

Regards,

Jo

domdib · October 15, 2009, 08:33:08 AM

What I think njeneb means when he says he increases GI on surfaces has got nothing to do with the GI settings in the render - he means in the Lighting tab, in Enviro Light, adjust the Strength on surfaces higher. It's a trick I've used in certain unusual situations - e.g. to enhance the reflection of the sky colour in snow. In your render, it would make the darker areas a bit brighter.

Also, please re-read the render setting recommendations tab stickied in this Forum. Although it doesn't mention the RTS, it does point out that for enhancing rays, you should increase atmo samples. The general consensus on RTS, which you have basically confirmed, is, turn it on only in the Render settings (and if GI isn't a major feature in your scene, you can sometimes get away without it); but elsewhere, don't turn it on unless you have to - in the atmosphere, this is if the Sun is behind the landscape (which isn't the case in your render - and there's also a hack to avoid this, using metaclouds - see http://forums.planetside.co.uk/index.php?topic=7369.0); and in the clouds, I think it is only if the landscape is casting shadows onto clouds, e.g. opposite the sunward side of a mountain.

Hope this helps.

P.S. Why do you have bloom on? As far as I understand, this is intended to smear out very sharp highlights, like the ones from reflections in water. And of course, soft shadows is also going to up your render time.

Tangled-Universe · October 15, 2009, 12:45:39 PM

Quote from: domdib on October 15, 2009, 08:33:08 AM
What I think njeneb means when he says he increases GI on surfaces has got nothing to do with the GI settings in the render - he means in the Lighting tab, in Enviro Light, adjust the Strength on surfaces higher. It's a trick I've used in certain unusual situations - e.g. to enhance the reflection of the sky colour in snow. In your render, it would make the darker areas a bit brighter.

Also, please re-read the render setting recommendations tab stickied in this Forum. Although it doesn't mention the RTS, it does point out that for enhancing rays, you should increase atmo samples. The general consensus on RTS, which you have basically confirmed, is, turn it on only in the Render settings (and if GI isn't a major feature in your scene, you can sometimes get away without it); but elsewhere, don't turn it on unless you have to - in the atmosphere, this is if the Sun is behind the landscape (which isn't the case in your render - and there's also a hack to avoid this, using metaclouds - see http://forums.planetside.co.uk/index.php?topic=7369.0); and in the clouds, I think it is only if the landscape is casting shadows onto clouds, e.g. opposite the sunward side of a mountain.

Hope this helps.

P.S. Why do you have bloom on? As far as I understand, this is intended to smear out very sharp highlights, like the ones from reflections in water. And of course, soft shadows is also going to up your render time.

No.

Use RTS when:

Shadows are being cast into the atmosphere (from terrain for example...hence your "sun behind terrain" example) -> enable RTS in the atmosphere node
Shadows are being cast on clouds from terrain/atmosphere -> enable RTS in the cloud node

That's "it" basically.

Tangled-Universe · October 15, 2009, 12:53:02 PM

A GI setting of 2/4 is really enough for this scene. I'd increase the sun's strength to 4-4.5 and along increase the exposure ~1.4. This also makes the bloom work better.

Like Jo suggested then save as EXR and postprocess. Success guaranteed.

Your softshadow-settings are unrealistic: in "real life" the softshadows diameter is 0.5 degrees (default setting) and with ~9 samples you'll get smooth results.
You have 20 degrees as setting and just 9 samples which is really really too low and possibly also a source for the grain in your atmo (besides the too low atmo sample number).
Also, these settings increase rendertime considerably.
If you want slightly exaggerated soft shadows then go for 1-1.5 diameter and use 10-12 samples. That would give nice results without too long rendertimes.

So my suggestion for rendering it next time:

Detail 0.75
AA 6
GI 2/4
128 atmo samples
AA Bloom enabled

RTS enabled in atmo --> the dark result can be "solved" by tweaking the density of your atmosphere.
You could also add a low cloud and then enable RTS there as well, but I warn you for insane rendertimes then.

This image has no clouds, just atmo:
http://forums.planetside.co.uk/index.php?topic=7718.0

Cheers,
Martin

reck · October 15, 2009, 04:35:06 PM

Thanks a lot guys.

domdib I use bloom whenever I see the sun in the image, it softens the edges of the sun and I think it looks a lot better.

Regarding the darkness of the image, well I like Jo's example and suggestion, tweak the exposure setting. I'd rather do this than mess around with the GI on surfaces setting.

Martin, thanks for you post, it helps me A LOT.

You mention to use RTS in the atmosphere node when shadows are cast into the atmosphere from, for example, terrain. As you can see from the image the sun is behind some tress and facing the camera, therefore shadow would be cast into the atmosphere from the trees right? Is that why you're suggesting turning the RTS in atmos setting on? Also there is actually already a low level cloud (trying for fog) in this image but its not very clear at the moment. So for the same reason as above (tree's casting shadows onto fog) would you suggest I turn RTS for the cloud layer as well? I'm just not clear what effect I would have from just having RTS on for atmos, or just having RTS on for the cloud layer or having it on for both. As you mention it takes a long time to render when you start using RTS so i've not been able to test each of these settings out.

Also i've been using 20,9 for soft shadows for a long time, I thought this was the default, that's why i've been using it. It was only after you mentioned that 0.5 is the default and more realistic that I went back and noticed that this setting is being loaded from a default tgd I created so I must have changed this setting in the past and accidentally saved it to my default scene.

One last question for you Martin, if I use the settings you suggest will it look as good as your image here:

http://forums.planetside.co.uk/index.php?topic=7718.0

Henry Blewer · October 15, 2009, 06:35:15 PM

I have two questions. What is EXR and how do you save EXR?

Kadri · October 15, 2009, 07:15:01 PM

http://www.openexr.com/

As you are saving the render look in the menu at the bottom.

Kadri.

Henry Blewer · October 15, 2009, 10:37:25 PM

Thanks. I will have to try this out.

Edit: Did a quick render and saved it. Corel Paint X2 does not support this.

domdib · October 16, 2009, 03:43:42 AM

Just to clarify Martin - so what's the use case for RTS in atmosphere (other than if the sun is behind terrain)? Does it help give rays more definition?

Oshyan · October 16, 2009, 03:51:23 AM

It will give you shadows from terrain into the atmosphere (which gives you rays, where the shadows *aren't*), which is relatively rare. More often what you get is rays through clouds. You do *not* need RTS to get those. If you have objects that would be casting shadows into the atmosphere and potentially creating rays, then you'd need RTS, but the reason it didn't work in the original example above is probably that there were objects actually entirely covering the sun, so your camera is actually in shadow, but without RTS you don't see that.

Anyway the point is that "correct" results aren't always prettiest.

Bottom line, leave RTS off unless you're really sure you need it. If you like the results of your scene and you don't have it on, you very well may not like the changes that come with enabling it, not to mention the dramatically longer render time. Also keep in mind that you will rarely ever need to have RTS on in both atmosphere and clouds.

- Oshyan