Shadow Catcher Surface "Light Bulb" Went Off

[treddie]

Hi.

Maybe this is old news. Maybe not.

I discovered something interesting. When you set a shadow catcher's surface color to pure white, naturally, when it catches shadows, it also catches an overall tone based on the lighting in the scene. Therefore, it is not just catching shadows, it is also getting contaminated by the scene lighting and exposure.
That's my take on it anyway.

But that is a conundrum, because then you have to spend a lot of post time in PShop, carefully erasing out the majority of the shadow catcher surface.

So then I thought that maybe Octane needed something analogous to a green screen, where a certain narrow wavelength of color is given to the shadow catcher surface that Octane understands is meant to be perfectly transparent. Then it dawned on me that green screens in movie effects work are actually EMITTERS putting out that green light. So it then dawned on me why couldn't you just give the shadow catcher material a low power emission node, so that it EMITTED WHITE? Hot damn if it didn't work! The shadows are still there, and the shadow catcher surface is now gone.

[treddie]

The shadows ARE washed out from the emission node, even at extremely low power. But that is far easier to fix in PShop than not having an emission node. For instance, you can duplicate the shadows layer any number of times, setting them all to multiply mode to get the desired darkness. If there is any residual tone to the shadow catcher surface, a slight Brightness & Contrast adjustment should nix that tone easily.

UPDATE: One caveat...If you give the shadow catcher surface material an emission node as discussed, it will cast light up underneath surfaces. However, if you set it to a reasonably low level, it may not be that bad. In PShop, I would rather come in with a weakened shadow from the emission surface and a lighter toning of the overall shadow catcher surface, than a properly dark shadow area with a very dark toned shadow catcher surface...When doing a Brightness & Contrast adjustment in PShop, the integrity of the shadows will be better maintained if you don't have to adjust too much. You can always get the shadow density back by duplicating the shadow layer one or more times, setting each layer's mixing mode to Multiply to get the shadow density back.

[xxdanbrowne]

My 2c is that I noticed the color of the shadow depended on the light source.

[gordonrobb]

I thought that the colour of the shadow was affected by the diffuse colour of the shadow catcher object. No?

[treddie]

xxdanbrowne > That makes sense. From a Color Theory point of view, shadows are not really black or gray, but the surface color mixed with the compliment of the light source color.

gordonrobb > That too. That's why I choose pure white for my shadow catcher, but it still gets toned to a darker color when lit by the environment, which is a drag.

After playing with the shadow catcher, I think a "green screen" approach would be best. You would not have to deal with a toned shadow catcher plane anymore, and the shadows would be pristine on a perfectly transparent background.

[abstrax]

Sorry, I don't get what you are rambling about. The matte material is very simple: It creates black pixels with an alpha that corresponds to the amount of shadow (or better to the amount of occluded light). So when you compose it, 100% shadow will be 100% opaque black and 50% shadow would be 50% black, i.e. 50% of the background would "shine through". You can reduce the overall opacity by controlling the opacity of the matte material.

[treddie]

For me anyway, I'm rambling about how a shadow catcher surface is not 100% transparent where there are no cast shadows. I'm just guessing that the surface picks up a value of gray due to the exposure of the scene.

[abstrax]

For me anyway, I'm rambling about how a shadow catcher surface is not 100% transparent where there are no cast shadows. I'm just guessing that the surface picks up a value of gray due to the exposure of the scene.

No, it's because there is some shadow. Even a small light still produces some shadow. In the past we only took emitters into account, but now we also take environment lighting into account, which should reduce this effect. Maybe the problem is, because the light is behind some fully transparent but specular surface and with that counts as fully occluded. One way to solve the problem is to enable the fake shadows option of the specular material.

[treddie]

I'm not sure about specular being the problem. Here is an image I am rendering as we speak. The car is the only geometry in the scene, so there are no specular materials behind the camera. You can see that the shadow catcher plane is not 100% transparent, even way back along the upper edge, which is way in the distance away from the car. Also note the hard edge below the front of the car. Someone remarked that they had this hard edge problem too and it only happened when they had a small light on inside a car. For me, I happen to have the turn signal and rear taillights backed up by a very weak emitter so that they would not be so dark. Not sure if that is causing it, but it was interesting that someone else had a problem with emitters and hard edges in the shadow catcher.

[xxdanbrowne]

I thought that the colour of the shadow was affected by the diffuse colour of the shadow catcher object. No?

I could be talking crap here but it seems to me that both could be true.
i.e. the color of the shadow is affected by the color of the light source
AND/OR
the color of the shadow is affected by the diffuse color of the shadow catcher.

But again, I could be talking gibberish. No way to test it till I get home.