Circular Polarizer Effect

[gregtee]

Oftentimes in landscape photography, the photographer will us a circular polarizer to darken down the sky, making clouds pop out in the process.  Photos always look more dramatic when this is done.  While it's possible to mimic this effect in TG by rendering alpha mattes of the clouds and atmo layers and adjusting them a program like Nuke, is there anyway to do this natively in TG? 

-Greg

[Matt]

One of the effects of a polarizing filter is to darken the atmosphere. You can do this simply by darkening the colours of the atmosphere in TG (the "bluesky horizon colour" and "haze horizon colour"). The problem is that this also affects GI, which you don't want to do.

There is an easy solution to this. Create two Planet Atmospheres. On one, turn on "Enable primary" and turn off "Enable Secondary". This is your visible atmosphere, the one that the camera sees directly. On the other atmosphere, use the opposite settings. "Enable secondary" is responsible for GI, reflections and shadows (including the reddening of light through the atmosphere), so you probably want to use realistic atmosphere settings for this one. But once you've done this, you can tweak the visible atmosphere as much as you like without affecting the lighting of the scene.

In the node network, chain the two atmospheres together just like you would with cloud layers.

Matt

[gregtee]

Great, I'll try that. 

Thanks Matt

Greg

[Tangled-Universe]

Circular Polarizers do quite a lot more actually.
In fact, they do affect GI if you translate how a polarizer works to a renderer.

In essence they do just one thing and that's transmitting light of a certain polarization (the orientation of the electromagnetic wave) to the camera sensor.
When light bounces on particles in the atmosphere it changes polarity and this polarization effect is the greatest when the sun is behind the observer.
So if you choose to do it in TG and want to do it realistically then keep this in the back of your mind Greg.

Besides that also reflected light changes in polarization.
When using a circular polarization filter you rotate this filter in such way that you can reduce reflections in water, allowing you to emphasize the transparency on the water.
This is something you can adjust easily in TG.

However, what you can't do in TG is adjusting for the effects of a polarizer on the surfaces.
Surface colours, contrast and shadows can change quite dramatically.
If you would think about this in a GI way then it also affects indirect lighting, since indirect lighting is bounced light it is likely also different in polarization and thus will be affected by using a polarization filter.

I have enclosed 2 photos I have shot about 2 months ago in New Zealand, demonstrating the effect of a polarizer on atmosphere and surfaces.
The sun is not even in the ideal position to demonstrate it perfectly, but the effect is already very dramatic.

There's not an easy tweak in TG to get a polarizing effect on surfaces, so you would have to design them on purpose for emulating a polarizer.

Cheers,
Martin

[Matt]

Circular Polarizers do quite a lot more actually.
In fact, they do affect GI if you translate how a polarizer works to a renderer.

That's true (sort of, see below). I only provided a solution for the one thing that I thought Greg was asking about, which is darkening the sky in an artistic sense.

In essence they do just one thing and that's transmitting light of a certain polarization (the orientation of the electromagnetic wave) to the camera sensor.
When light bounces on particles in the atmosphere it changes polarity and this polarization effect is the greatest when the sun is behind the observer.
So if you choose to do it in TG and want to do it realistically then keep this in the back of your mind Greg.

Yeah, the darkening effect is certainly dependent on direction. Polarization affects the Rayleigh component (e.g. bluesky in Terragen) more than the Mie component, and the amount of polarization of Rayleigh scattering is strongest at 90 degrees away from the sun (not opposite the sun), and opposite the sun the sky gets brighter again.

Besides that also reflected light changes in polarization.
When using a circular polarization filter you rotate this filter in such way that you can reduce reflections in water, allowing you to emphasize the transparency on the water.
This is something you can adjust easily in TG.

However, what you can't do in TG is adjusting for the effects of a polarizer on the surfaces.
Surface colours, contrast and shadows can change quite dramatically.
If you would think about this in a GI way then it also affects indirect lighting, since indirect lighting is bounced light it is likely also different in polarization and thus will be affected by using a polarization filter.

I have enclosed 2 photos I have shot about 2 months ago in New Zealand, demonstrating the effect of a polarizer on atmosphere and surfaces.
The sun is not even in the ideal position to demonstrate it perfectly, but the effect is already very dramatic.

There's not an easy tweak in TG to get a polarizing effect on surfaces, so you would have to design them on purpose for emulating a polarizer.

Fully diffuse GI is not polarized, AFAIK. Only Fresnel reflections are. If all of your TG surfaces are purely diffuse, I would say that a polarizer should have no effect on them. Now, if you have added some reflectivity to your surfaces, then this is the thing that you'd want a polarizer to affect. But if you want the image to appear polarized, it seems to me that you should simply not add any reflectivity. What happens in nature is that most surfaces have a mixture of diffuse and Fresnel reflectivity, whereas TG's default surfaces are all purely diffuse until you specifically add some reflectivity.

Matt