bump map shadow bug #41930

System Information
windows7 64
ASUS ATI Radeon HD6870

Blender Version
Broken: (All)
Worked: (None)
Short description of error
from this #41929 use attached file{F111931}
to this scene add a cube, select one of this lamp and set ray shadow mode

and now we have object with bump map, cube(which will cast a shadow),
and we must have 2 lamps in scene:
first - sun, randomly rotated, ray shadow mode
second - select first, shift+D and R,Z,180,set no shadow mode
(it is very important) only in this combination bug is active

go to rendered shading mode

Exact steps for others to reproduce the error

open this shadow_bug.blend
set rendered shading mode, switch on norm influence - and we will see incorrect shadow
switch off norm influence - working normal

Changed status to: 'Open'

Added subscriber: @Inwader77

Will have a look later.

Added subscribers: @Sergey, @lichtwerk

Hm, I would almost suggest this is to be expected?

If you tweak the geometry normal to such an exteme extend with same kind of noise bump, then the shading will depend more and more on this quasi-random normal (meaning the original geometry normal will have less and less of an effect). Thus what you call "real shading" will get less and less while what you call "virtual shading" will get more and more.
The virtual 'bumped' normal is all over the place and thus doesnt really have a noticable 'connection' to the original geometry anymore. (same explanation as in #41929)

So in this case the lamp with shadows activated will act 'normally' but we will see a difference in the shading related to the lamp without shadows.
A lamp without shadows will still result in a dark surface when the geometry normal is pointing away from the lamp. Once you start tweaking that normal so extreme with a noise-bump, the surface could 'pick up light' because now it is virtually not pointing away from the light anymore. Resulting in what you call 'incorrect shadow' [and I would call 'expected' :)]

Might be wrong on this. @Sergey: what do you think?

Changed status from 'Open' to: 'Archived'

@lichtwerk, you're sound about right.Using such an extreme noise texture on normals could easily flip "microfacet" to totally different direction, making it to receive the light. This is like rougness woks in cycles, just in really extreme way.

Thanks for the report, but i don't really see a bug in here.

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bump map shadow bug #41930