Global illumination issues with Pathtracer and standard default render.

Started by Nala1977, November 05, 2021, 12:54:40 pm

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Nala1977

Hey guys so im having lots of issues with the global illumination render.
Basically it looks like Terragen doesnt take into consideration the bounces of light on the surfaces.
I have rendered this image where the sun is high in the sky, i have these palms model with just a plain diffuse texture both on trunk and foliage.
The problem is that the face not lit by the sun are super dark. Now in a normal environment this shouldnt happen, because the Light from the sun bounces around and hits the surface from the back, bouncing off the ground.

In every 3d software the global illumination does a good job and produce a more or less accurate result, in Terragen im having this issue where everything is really black as if there is no secondary bounces of light.
My settings are also high but i noticed no difference.
here they are:

Path tracer: 25 Landscape
GI settings: Gi cache detail 12 | Gi sample quality 12 | Gi blur radius 8 | Supersample prepass on.

anyone have any clue how to increase the overall global illumination bounce?



if you take a look at the docs, here https://planetside.co.uk/wiki/index.php?title=Terragen_4_Global_Illumination
these 3 renders, it looks like my render is just lit by Direct illumination. while if you check the "direct + indirect" the foliage and the overall shadow blackyness is way softer, look at the ground the shadows arent that hard black. Something is wrong



in the zoomed image you can see the shadow projected into the ground from the tree trunk its way too dark. It shouldnt be like that considering how strong is the sunlight

Hannes

Well, to my eyes it looks more or less correct.
If you use the legacy renderer, you can increase the environment light's strength to make dark areas brighter (which looks way better than using second suns, or something like that...).
But unfortunately this doesn't work with the path tracer. I was asking a similar question some time ago, and Matt answered, that the PT renderer would be more or less physically correct (as far as I remember). So, you could increase exposure for example.
Something else: what base color does your trunk shader for example have? If it's grey (0.5 or so...), make it white. The brighter the base color, the brighter your texture.

WAS

PT is more photorealistic, so you'll have more real-world anomalies, such as the bright ground, foreground, and sky, creating contrast in darkness, making it darker. Additionally, nothing IRL has "zero" reflectiveness, so you need some Fresnel. I think even the most absorptive material we have produced still technically has some Fresnel.


Here are some things to keep note of:

  • Surface diffuse emission plays a role. If the colour is neutral, and too much under 0.5, it may not have a huge impact on highlights in shadows.
  • Surface displacement direction plays a role in the direction light is bounced. A flat surface is mainly bouncing up and away from the trees.
  • The shadowed surface will not produce much light, it will be coming from the sides, and if the surface geometry is right, cast on the sides of the shadowed trunk area.
  • Objects themselves need some Fresnel to help pickup GI lighting.
  • Low AA quality can muddle the effect (though yours seems to be relatively high).
  • GI Caching has no effect on PT renders (that I am aware of)

In this following example image, the sand high colour is 0.5, and it's low colour (a duplicate of the high) set at 0.25. This creates a lot of ambient light that bathes the undersides of the tree foliage and the shadow boundaries of the trunks lower on the trees.

WAS

PS when you think of shadow highlights you may be thinking of something like rocks in sand or something. The reason this is so strong is because the shadow isn't cascading well beyond the object and ends closer to the object allowing lit surface on the shadow side to light it.

Matt

It appears to me that the tree's surface colours or textures are too dark. Notice how they are pretty dark on the sunlit side too.

The default scene has a ground texture that is fairly dark (with albedos ranging between 0% and 30% due to the fractal outputting values between 0.0 and 0.3). The ground doesn't look dark in the render because of the default sunlight intensity and camera exposure. I feel that if the sunlit sides of the trees look darker than the ground then their textures might be too dark. The "Base colour" slider in the Default Shader is an easy way to adjust the overall brightness.

If you're imagining the kind of bounce light that you'd get from a light-coloured sandy beach, the default scene's ground texture won't give you that. You'll need to use lighter ground colours to bounce more light. You might need to brighten it up to a level where the image appears over-exposed, and that's fine because you can adjust the camera exposure down just as you would with a real camera.
Just because milk is white doesn't mean that clouds are made of milk.