Displacement problems

[adamsb]

Due to the "broken mirror" problem that seems to be created when using the SPD setting on the displacement, I've been trying to render a stone wall without this setting. However, when I do, I don't get any displacement at all, especially at the wall corners.

Is this always the case? Can you only affect the corners of a wall with displacement if you have SPD turned on?

Thanks much for anyone who knows their way around this!