[LaMereHic]

i have character moving on a scene, the render is done in Global illumination
i have the shadows of the character on the floor, and i want to composite it without the floor , but only with the shadows.
i did put the alpha channel property of the floor to shadow density in the surface editor
but i got nothing in the alpha channel in the render, (maybe the shadow is too diffuse)
if anyone has a clue on how to get the shadows of a render in GI....

[CrazyMerlin]

are you planning to use the animation frames within another image that has a backdrop, like a street say?

if so, choose a color for the floor that is in neither image, some oblique color, and then using a video application remove this color completely.

kinda like the blue screen idea.

you could remove it by setting the image frames transpency color to the color of the floor.
this way the floor would just appear as transparent in the new backdrop.

you may need to touch up the shadow but this shouldn't be a problem, and if you have a large frame count, maybe a macro/script could sort this out.

hope this helps

~~CM!

[colto]

dont use a differnt color like a blue screen. That will degrade your shadows quality. just make the floor as white as possible and apply that image with the shadow ontop of whatever you want it on with a Multiply filter. A multiply filter removes all white from a scene based on its grayscale value. So perfect white is completely transparent, 50% gray is half transparent, and pure black has no trasparency.

[colto]

Here is what I got using your image with a multiply filter ontop of a picture of concrete.

[CrazyMerlin]

nice post!

~~CM!