Strange shadow


Can anybody explain to me what’s going on here: I have a simple scene containing a kind of “flower pot”, a white plane underneath it, a directional light source and a camera. I guess I must have overlooked something in the texture or rendering settings, because, strangely, the shadow that the pot throws onto the white plane is darker, where the light has to “pass” through more layers of the pot…

Clearly, I want the pot to block all the light: alpha is 1, translucency is 0, Frsnel is 0, ZTransp is off, RayTransp is off, but still…

Can anybody help me? Please?


Maybe you could post your .blend file so we can take a look at it. Because i cannot regenerate your problem.

gr, Twan

Thanks for the quick reply. I will post the .blend-file as soon as I am home (about 3 hours). Can you tell me how to attach a file to a posting in this forum (without ftp’ing it somewhere and linking to it, as I did with the image)?
Or should I just copy-paste it here?

You need to adjust your Clip Start and End on the spot as close to the rendered part of the scene as possible. You also need more samples to get rid of the jagged edges.


also bias being lower would help, but considering how bad your clip start/end are it will quickly result in artifacts

[adjust the clip start/end first, then bias]
[bias controls how far behind a shadow casting surface a shadow is shown, too low and you get artifacts on the shadow casting surfaces]

Thank you all for your help. Adjusting clip start and end really did the trick. Although I’m still at a loss :expressionless: as to the exact effect of bias, I played around with some parameters and managed to set up the scene as I had in mind.

Thanx :slight_smile:

Thanx for the illustrations… that at least made the effect clear to me.
But can you give me a more theoretical explanation? I mean, why exactly does a low bias produce such artifacts? Why is it necessary to have such a parameter at all? Wouldn’t it be possible to design a lighting algorithm that implicitely uses the optimal bias setting? Or are there really situations in which I would want to manually adjust that value?

the bias value is only for buffer shadows. A buffer shadow stores the distance away from the light for every pixel in the shadow buffer. Because this distance isn’t entirely precise the bias value specifies how far away from the shadow casting object the shadow starts. If the bias is too low surfaces cast shadows on themselves, causing artifacts.

If you use a ray shadow instead [much slower] there is no bias value. However getting the same amount of blurring is much slower, but can be of higher quality [the blurring of the shadow near the wall would be minimal, but away from it there would be a large amount of blur]

Thank you very much for the explanations. I’m beginning to get the hang of it.