Shadow catcher, no background

bartovan · April 8, 2016, 9:04am

Blender noob here.
I’m trying to get this shadow catcher to work: http://blender.stackexchange.com/questions/19765/cycles-shadeless-shadow-catcher-material-for-rendering-onto-footage.

The only difference with the situation there, is that I don’t want any background: I just want the object (the cube) + its shadow, and apart from that only transparency. I want to add the background later, manually.

I can follow just fine, until the last step where the answer says

you now have a shadow to multiply over your backdrop and layer 1 to composite into backdrop

Can someone help me to achieve what I want (object + shadow only)

Here’s where I’m at so far:

which gives this render result:

which I would like without the white (where the plane was).
Thank you!!

Attachments

IkariShinji · April 8, 2016, 3:06pm

The white parts do not matter if you combine images with “multiply”.
If you read colors as numerical values, white equals 1. So whatever background color X you multiply with white, it will always stay unchanged, as X * 1 = X.

The darker colored (shadow) parts on the other hand represent values < 1, so any background color multiplied with that will become darker.

bartovan · April 9, 2016, 12:51am

Yes, exactly, the shadow is a dark value so it darkens the rest when I use multiply. So multiply doesn’t work.
This node setup (using multiply):

gives me this render result (notice how the cube got darkened at the bottom, this is undesired, and also there’s no shadow on the ground):

and switching around the multiply inputs is even worse:

How can I have just the cube and the shadow in a correct way, and all else transparent?
Is there a way to tell blender to turn white (RGB 255, 255, 255) into transparent?

Attachments

bartovan · April 9, 2016, 1:10am

Got an interesting result adding a colorramp (going from transparent black to full black)!
Node setup:

Render result:

and then, eg. in Inkscape, over a red background:

Attachments

IkariShinji · April 9, 2016, 1:32am

You’re not doing the compositing correctly in post #3. You…
a) “Multiply” the shadow layer with the background and then
b) “Alpha over” the layer with the shadow casting object on top of that…

Thereby the background receives the shadow and the shadow caster is sitting “on top”.

bartovan · April 9, 2016, 2:58am

Yes, I know that that is the original idea, but in my case I have no background. I want no background, I want the final render to be transparent. So I have nothing to multiply the shadows with… I think my post #4 solves it, but I’m a blender noob and maybe it has some drawbacks that I don’t realize yet…

wilnix · June 29, 2016, 4:01am

Hi

Take a look at this tutorial.

//Wilnix

Roken · June 29, 2016, 1:31pm

Have you tried my addon. It will do exactly what you want, Get the latest version off github (last post by me for the link)