a question about light:
I have a scence with a Sphere inside a open Box and a Point light backside ,show below:
When Light Shadows is on
if Umbra parameter Equals 0,then everything is OK
if Umbra parameter Greater than 0,then Light illuminate the Sphere
if the Box is obturate,then everything is OK
WHY?
the light do not direct irradiation the Sphere,but Why the Point Light can illuminate the Sphere ???????
WHY?
Bonjour Curry,
Very simple. The "umbra" parameter defines the transparency of the projected shadow. It means that if the value is 1, then the transparency value is 100%. If the value is 0, then the transparency valye is none.
In other words, the lower the value, the stronger the shadow. Thus, a value of 0 means that the casted shadow is totally black. The only way to make this shadow not black is to have a light that illuminates that shadow.
If you want to check just make several tests with different values and you'll see.
Hope this helps
Salutations - Cheers
Bernard Lebel
THX Atyss
I understand The "umbra" parameter defines the transparency of the projected shadow
But I think it is do not accord with nature physical attribute 
Maybe I should to accept this
THX again
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But I think it is do not accord with nature physical attribute
@@@@
What do you mean? Every 3D package I saw has this type of control!
What do you define a physically accurate attribute?
Salutations - Cheers
Bernard Lebel
there are architectureal lighting packages that enable you to define and test environments accurately.
agi32
radiance
lightscape
etc....
lights can be defined with *.les files (provided by industry light manufacturers) which describe 3d photometric falloff webs...
im not entirely convinced by what ive seen from them...
the first thing they do is remove most of the surface finesse (eg just diffuse map, all other properties are global) im sure surface qualities play a significant role in light accuracy.
thats all
Te Anau
Just a few corrections here:
1) umbra is not a shadow transparency factor - it's a percentage of the source illumination (there's a difference). If umbra is 0, then no illumination is present in the shadow. If umbra is 0.75, then 75% of the illumination cast by the light will be present in the shadow.
If the surface is not opaque, then umbra is the percentage of source illumination plus the amount of illumination that is transmitted and filtered as it passes through the surface.
2) Shadows can be colored with shadow shaders applied on the occluding surfaces. However, light shaders can also cast colored shadows if a shader (such as one I've written for myself 
) is programmed to do so.
The only way to solve your problem is to keep umbra at zero. Any other value will let illumination from the light pass through to the sphere. If you need the sphere to be visible at a level other than what you've shown, then you'll need to use another light, or shading trick to make the surface visible.
Matt
Matt Lind
Animator / Technical Director
Softimage certified instructor:
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speye_21@hotmail.com
speye_21
THX so much 
I quite understand