Strange hdri behavior: "stains" in my rendering!

[pproestos]

Hello all arnold gurus... 🙂

I use an hdri as environment map and a skydome with physical sky as texture. The results are very good but for some reason, strange things happens. As you see inside the three pictures I attach, in the first picture I don't assign my hdri as environment map and as you see the surfaces are clear. In picture No2 and No3 where I have an hdri as environment map, illumination is better but I noticed some "stains" "spots" I don't know how to call them... Why they are appearing? Any clue?

[madsd]

It looks like you are using your old sun system together with the hdri.

In general that has to be avoided completly, so either delete the sun/sky and only use HDRI, or delete the HDRI and keep the old node rig.

Nothing good comes out of mixing 2 sun systems.

So delete the sun and sky and only use the HDRI, and check again.

[pproestos]

Mads, I am using the Physical sky shader from Arnold environment in Material editor. I assign it as texture to skydome. The hdri I use it only as map in the enviroment dialog (pressing the "8" key) I think my autocad drawing is corrupted when I insert it into max...

[pproestos]

@Mads Drøschler

You had right...

I always put my hdri images as environment map

effects dialog.

I noticed that the hdri that I used, produces shadows, so

as you mentioned before, yes I had two suns...

Instead, other hdri that I use, they illuminate my scene

in a beutiful way and without disturbing my physical sky/skydome light

with strange shadows.

Can you please explain me, why some hdri procude shadows and

other don't? I thought hdri were used only for environmental lighting,

but as I see some of them they behave like sun!!!!

[madsd · Accepted solution]

Well, if we go outside and look up we can observe a couple of things, clouds and suns position and occluding objects.
It totaly depends on under which conditions the maps were shot under. If the sun was up and no clouds, the pixels in the sun has a higher value than the surrounding blue pixels and will, with some manipulation or not ( depending on maps exposure and contrast pivot points ) give either strong or no shadows, eventhough its the same image.

An overclouded day image, has no sun, so the light is uniform ( skylight )

You need to use a map that contains the info you need, you cant extract a sun from a very cloudy image where the sun is dimmed down to virtually nothing.

If you for soem reason realy like 1 particular cloudy image but want a sun, you edit the map and paint in a sun, 32 bit image range needed, and its not enough to just use a pure white color and paint a sun, the values has to go significantly higher than the 0-1 range a normal brush can give you.

And that is all there is to know about using image based lightning basicly.

The maps themself contains ALL the lights you need to light up a scene to make it blend in the sharpest possible way and this is why you don't want 2 suns, 2 skylight emitters or anything of that nature.

A spherical environmap contains both skylight and a possible sun.

Just a recomendation, be very reluctant to add artistic suns to cloudy images, or try force a sun into a cloudy environment, it will look like crap and intergration wont be pritty. You can argue the artifical sun comes from a tiny hole in the clouds, by pure accident/luck ( pick one ) but 99% of all cases you just want to use the info the map gives.

Some architects with zero sense about realism actually DO set an environment map to "something" and direct shadows pointing in the other direction, naturally, it wont be pritty.

[madsd]

This is as bad as it gets, no filters and some clients will actually accept it and flag it "perfect"

But in general you want to avoid 3 different shadow casting directions...

[pproestos]

OMG, so many shadows is just terrifying.

I only use skydome light in combination with physical sky shader

to control exactly the sun position where I want it to be.

Then I use as environment map an hdri image that of course

produce ONLY skylight atmospheric illumination: just because

I want the entire rendering to have a tint of the surrounding

environment and also to help illuminate more naturally the darker

sides of my buildings. That's all.

I have so many hdris to use, but I figured out that only these

ones from viz people v1 are producing sun shadows!!!!

https://www.viz-people.com/portfolio/free-hdri-maps/

I am not going to find out why, I just deleted them...

I use arnold only for two months (I used mental ray) and here

it is one render with skydome light+physical sky shader and

an hdri as environment but one of those that don't emmit

sunrays...

cheers 🙂

[madsd]

You can cut out the shadow and arrange it to just 1 single master direction and give it a blue tint and fade off to the white color to match the CG camera shadows better. It will strengthen integrity significantly.

[pproestos]

Haha, I knew you would noticed that!!! 🙂 🙂

I inserted the girl into photoshop and leave her with that shadow...

Yes of course you're right... 🙂 I'll do that to my next render...