Help me to get the gist of GISD ;)

N-drju · June 19, 2015, 07:45:06 AM

Hello,

Could you perhaps tell me what GISD does? Terragen wiki does not offer any definition or suggestions on that and I'd like to know what it does. I used it several times but did not see much difference in my renders. Except perhaps for much darker shadows on some parts of my models.
While we're at it - shadows on my newest render turned up horribly dark, almost black. This does not happen too often for me and I'm wondering what the cause might be? I have a "strength on surfaces" set to 1.35 and GI at 2/3. I also have GISD turned on and I wonder if that might cause black shadows... And yes, I DO have Global illumination on.

I did a GI cache file as well to use it during rendering but I precalculated illumination on a very small portion of an HD image. Do you think this may influence the shadow brightness too? Should I precalculate the whole 1920x1080 area or maybe that makes no difference?

Tangled-Universe · June 19, 2015, 07:57:12 AM

GISD is a different way of doing "GI surface details" from earlier TG2 versions. It's much faster with similar, if not more pleasing, results.
I'm not at liberty to tell how it works.

Crop renders do affect GI, but usually not so much that a crop = complete black shadows, while a full render = brightly lit shadows.
So I would not worry too much about that, but keep in mind that differences *do* occur and that it can vary for each situation.
So I guess it wouldn't hurt if you would increase your crop area and set GI to 4/4/8.
You will notice that the effect of GISD is already greatly reduced with these settings, because you're already sampling GI more accurately now!

So if this doesn't help, then let's think...
How large are those shadow areas, few pixels? Larger areas? TG's GI engine is...well...not the best. Tiny shadows often stay entirely black.

But if your shadows are of reasonable size then there are likely other issues:

1) dark surface shading = dark bounced light = darker shadows
2) GISD is set to values far from the default? If you over-do the AO in conjunction with a large radius then you may introduce very dark shadows.
Turn off GISD or set AO to 0.5 or 0.

How does that work?

N-drju · June 19, 2015, 08:24:21 AM

Okay, it's good to know that crops does not influence the precalc lighting that much. But I'll try to increase the precalc part of the image either way.

The shadows I'm talking about are indeed rather small... I'm mostly referring to the human and tree shadows seen from a distance. In fact, shadows they cast do not occupy large area of the final render. $:-\$ Shadows from bigger trees that are far away turn out fine though, while cloud shadows I got work wonders! Confusing at least... Oh, and my surfaces are bright all the way so this is probably not a problem.

What do you mean by "setting the values of GISD"? The GI and AO in the "Enviro light", right? I didn't even touch the AO slider... yet.

I'll check out your suggestions today and perhaps post the problematic part of the image if there's still a problem.

Tangled-Universe · June 19, 2015, 09:10:04 AM

I meant the settings in the GI dialogue of the renderer, like bounce to the ounce etc.

Can you post your crop render results?

Oshyan · June 19, 2015, 06:30:51 PM

Yes, it's really pretty difficult to say anything with specific value without looking at an example of the problem being reported.

Also, if you're saying you're using a GI cache generated by a crop render *to render a non-crop full rendering*, that's definitely a bad idea. In that case I would think it *would* totally screw up the GI of the scene, possibly resulting in dark/black shadows, because there are only GI samples for the cropped area.

If instead you are simply saying that you're doing a crop render using a GI cache generated from the same crop area, and wondering if areas *outside* the crop are contributing to GI, then the effect is much more limited and of less concern. Just don't ever use a crop render GI cache for a non-crop render.

- Oshyan

N-drju · June 20, 2015, 04:21:14 PM

Okay, so here I am again. I checked my settings and did the following; I turned off GISD feature and made another GI precalculation of the whole image. This way I got natural, nice looking shadows!

Thanks to you, I now know for sure that the size of the image being used as a GI precalculation does matter. I've suspected this for a long time but wasn't really sure until now...

TU, all the more reason for me to believe that GISD feature is not really a render enhancement but rather a tool for whimsical experimentation.

As soon as my render is finished, I'll post a good and a crappy version of the part of the image I was having trouble with. Perhaps someone else can learn from my mistakes that way.

Oshyan · June 20, 2015, 05:34:40 PM

GISD is intended absolutely as a fundamental, valuable part of realistic rendering in Terragen. Doing a GI prepass with GISD disabled will result in a different result than one with it enabled; GISD affects the prepass as well as the final render.

- Oshyan

Dune · June 21, 2015, 02:57:27 AM

I noticed that GISD particularly deepens shadows between separate parts of an object, more so than if the corner in a house, e.g. is made up of two linked parts.