Shadow Bias

Megarig

I'm not quite sure where in the forum to post this question, so I'll just put it here.

I have made some images, and I still am experimenting with light and shadows. I get shadow softness, but what is the purpose of shadow bias? I've tried spot-renders with it set at 0, and with it set at full. I'm not sure I see a difference.

Richard Haseltine

One problem with rendered shadows is that they can get confused - a surface can end up casting a shadow on itself, usually showing up as dark and light bands. Shadow Bias can help to fix this by offsetting the shadow, but since it leaves a lit gap it can cause problems itself (glowing nostrils are usually down to Shadow Bias being too high, and it can also lead to things like straps casting not or incomplete shadows). So you need to look at your render - if there are weird tonal variations shadow bias may be too low, if the shadows don't meet the object that cast them or are incomplete it may be too high. In general, since the setting is in cm, it needs to be lower for close ups than for distant shots.

Megarig

Thanks for offering your insight on this. So far, in a lot of the things I've experimented with, I've tended to look at the shadows and go, "Wow! That looks pretty cool!" But from your explanation, I can see there is a little more to it. In most of my experiments, I'll use more than one light source, positioned differently, and usually at least two kinds (spotlight, and distant light, or maybe one of those with the New Linear Point Light). Maybe it's how I have the NLP Light falloff set, but I will get an effect where say, a spotlight is casting a main shadow, and the other seems to cast a slight, nearly imperceptible shadow. I guess I'll need to play with it some more.

Jaderail

For the extra shadow just select your second light and in Pramaters turn Shadows to NONE. You can set each light to have shadows or not. SB is what causes stuff like a shadow from a bra strap to only go most of the way down to the bra. It will seem to just stop short. If you lower it the shadow will go all the way down to the Bra.

adamr001

Every light source (generally) should cast shadows... at least if they're intended to represent real lights. You "layer" shadows by varying intensity and varying shadow softness.

So let's build a test scene. Create a 1m sphere, and a 20 m plane. Create a Distant Light. Name it Stage Left Distant. Rotate it -50 on the Y-axis and -25 on the X-Axis. Now create another Distant Light and name it Stage Right Distant. Rotate it +50 on the Y-axis and -25 on the X-axis. Set both lights to Raytraced Shadows. The default Shadow Bias is fine.

When you render the scene you should get something like this image: (note I've straightened the camera to head on instead of the default angle).
Sample ONE
Note the very dark, very sharp edged shadows. You have two clear sets, one cast by each light.

Now, let's lower the intensity of the Stage Right Distant to 50%.
Sample TWO
See how the LEFT shadow is far less "dark" than the right... This is because the intensity of the stage-left light is now over-whelming the shadow cast by the stage-right light due to LIGHT intensity. So that's closer to what we're after, but it still looks pretty odd or light a "photoshoot studio" type setup.

So let's try adding some shadow softness...

Let's give the Stage-right light 50% shadow softness on top of the 50% light intensity it already has...
Sample THREE
Okay, so wow! It's not quite gone, but it's almost invisible. The end result is a very natural looking set of shadows that gives the render that natural feel while still retaining the bright-all-around-the-scene light we've built.

Hope that helps!