Explain Shadow Bias to me

[tricky]

I've read the help files, but I'm still not sure what the point of Shadow Bias is. The default value is 200mm. When I render a building that has blinds or bris soleil and I am using the vraysun (for example), the shadows are inaccurate. Typically reducing the shadow bias to 0 resolves this. The help file seems to suggest that this might be dangerous in terms of light leaks.

Why do we need shadow bias? Is rendering slower set at 0?

[trixian]

I think it shows it's usefulness when used with shadow maps, as precision is a lot higher when using ray traced shadows. It also helps compensate for inaccuracies at long distances, and also glancing angled shadows, though usually one has additional parameters to help with this.
As far as VRay is concerned, I would say it should have 0 as its default value. I usually put it at 0.01 cm by habit for some reason, as 0 used to give strange results in Max back in the scan-line days.

[Morne]

Actually this is a bug in my opinion due to system units.

If you have system units in mm instead of meter, then the default for the sun is 0.2mm which works great.

I say this is a bug, because all the defaults are screwed up if your system units are meter. Another example is the stereoscopic helper which if your system units is meter, makes the default distance between eyes 6500mm

[trixian]

Well it would fit right in with the slew of other Autodesk unit issues.
Them being a US company (and the US being one of the last places on this planet to resist the metric system amongst others), several things seem to be designed with inches as the default unit.
The above mentioned, and more visibly, the sweep modifier, that gets insanely large when working in meters.
In addition, the whole procedural map size thing still has not been resolved as far as I know.