Rendering cast shadows on surfaceshader mat

[k-rex]

rendering cast shadows on (or over) objects that use surface shader material (or other self-illumination attribute, like incandescence): how can I do this in one pass? (without compositing)
I tried a Use background shader but it only works on image planes.

I achieved some interresting results using the surface luminance node, or driving the lightShadow fraction attribute of my lights (either using a stand-in object just above my self-iluminated surface to render the shadow, or blending the self-iluminated object's color with these shadow data) , but it is not very handy. I would prefer a "Use background shader" kind of solution, but I would have to render my self-illuminated surface separately first, to use it as an image plane after.

Any tip?

[empleh]

You might try using a "use background shader" as the blending value for a blendcolors node. Then one of the colors could be the shadow color and the other could be whatever shader/texture/color that you want.

I assigned a surface shader to a sphere that recieved a shadow. I assighned a blend colors to the out color of the surface shader. I assighned the red channel of a use background shader to the blender value of the blend colors. I changed my camera background to white(since my shadow color was black). It worked. I had a red sphere with a blue shadow.

I don't know if this is what you mean or not. At least it might spark an idea.

good luck
ww

[k-rex]

Very useful indeed, thanks!

Actually I was doing aprox. the same thing with either the surface luminance node or the averaged lightshadow fractions of my lights, that I was using as a blender between the color and the shadow color of my self illuminated object. But according to the kind of lights I was using in the scene, it sometimes required adjustments.
With the use background shader used as the blender (+ camera bckground set to white), I have exactly the result I need. thanks!

[k-rex]

Ooops, actually there's a problem: I was so glad with the shadows that I didn't notice that my object's color (i mean, the portion that is not in the shadow) was also affected, which is not what I want. It was supposed to remain self-illuminated.

Back to studies....

[empleh]

What happened to the portion not in shadow?

[k-rex]

Sorry, it is constant (self-illuminated) indeed, but at a brighter level than the original self illuminating color/texture/shader/whatever that I pipe into the blendColors node's Color1. (color2 being my shadow color).

[empleh]

When I was testing this I created a lambert with the blend color node attached to the incandescence. The first time I rendered the areas outside of shadow were washed out but the regions that the spotlight didn't shine on looked fine. It turned out tht I hadn't changed the color of the lambert to black so the light was illuminating the surface as well.

Just an idea.

ww

[k-rex]

Yeah this also happened to me, but this time it's sth else. use a default ramp color to compare the results between a simple self-illuminating ramp color and the same ramp color blended with the useBackground shadow as said before. Won't you notice that the color of the ramp is burned? Maybe you didn't notice that because you were testing on a sphere. Did you try on a plane?