Hi, back again.

I've tried to my best to recreate the setup as described in all the above posts:

• One infinite light, one skydome with no ambient (passive IDL deflector), IDL set ON with high quality settings
• Head with only Scatter (in Alt_Diffuse) and specular
• Cap like Head, and transparency
• Hair with regular hairnode setup, and varying densities (1, 10 and 100 hairs/cm2)
• Head, Cap and Hair have all properties ticked ON

Findings:

• Light Emitter on/off on hair does not make any difference in result or rendertime. That's a new one!
• Scatter is not of influence in all this, it's an independant (diffuse) effect that disappears with all other diffuse effects at 100% transparency. Good.
• No difference between 1.0 transparency and 1.0 transparency with black color attached, so there is just one kind of "100% transparent", as expected
• Main difference between 100% transparent and (total) Visibility OFF is in specularity (in transparent, not in invisible), as expected
• The overlap between 100% transparency and Visible in Camera OFF is in shadowing and IDL emission, both do that.  The ViC check only removes the source of the shadows / IDL emission, not the effects on other objects. As expected.
• 100% transparency does leave traces in the render, on darkening/shadows and visibility, when Transparency_Falloff is not 0.0. In combi with the previous: 100% transparency, Falloff > 0.0 and ViC unchecked does leave the darkening/shadows on other objects. IDL emission is minimal.
• So for invisible SkullCaps shadowing on the Head, the Falloff dial is your friend (or enemy).  And since the object is invisible in camera anyway, transparency can be reduced a bit too. That will increase shadows and IDL. Then Scatter will have some effect, softening the shadow pattern and reducing brightness somewhat too.

So:

• Put the SkullCap slightly over the head, such that the hair fits the head while the cap warps shadows onto the head
• For the cap, set Visible in camera OFF, give it a dull black material and assign a hair-shadow map to transparency such that the cap is opaque where you want shadows on the head.
• For the cap, set Transparency_Falloff to 0.0 or raise it, put scatter on the cap at will and so on affect the shadowing on the head somewhat. Or leave it all and let the hair-shadow map do the work.

Now the scatter on the head itself might produce some red or cyan artefacts, but that's something entirely different. Custom_Scatter offers uncolored scattering instead.

happy Posering.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though