Only shadows ?

[Ronny Bang]

Hi
I have car i want to render with a background.
I Have a plane i want only to recieve shadows, how do i do that ?

[Ronny Bang]

No one can answer me this ?
I want to intergrate my car into a background image, therefor i need a transparent plane to show the shadows

[Stephen Thomas]

You need to apply a Matte/Shadow material to the plane. I assume Maxwell can deal with that.

[Matthew Spencer]

It's done in Maxwell a little differently than most renderers
The quick way:
1. Make a ground plane big enough to catch the whole shadow of your object
2. Make a new white material with roughness 100. In Material Properties, check the Shadow box. Apply this material to your ground plane.
3. Render. Output will be 1 render image and 1 shadow image.
4. Go back to your setup and hide your ground plane. Re-render, this time only an Alpha channel.
5. In Photoshop, add your render/shadow/alpha images to one document as separate layers. Clip the render layer with the alpha layer.
6. Put your backplate or environment image as the bottom layer.
7. Put the shadow layer above the backplate layer and set its blending mode to Multiply.
If you want subtle coloration, reflections, or caustics in your floor shadow:
1. Render just the main object, with alpha channel as well.
2. Go back to your setup. This time apply your floor material to the ground plane but do not check "shadow." Make the material as white as possible.
3. Select your main object and in its Object Properties select "Hide from camera"
4. In render options, make another render, with only the standard "reflections/diffuse" channel as your output. This will output a color image of just your floor, with a big fat shadow where your object used to be.
5. Again composite these layers together and alpha mask where necessary. Your floor layer won't be a "shadow" channel this time, but is instead a standard color render, so that things like caustics and reflections show up.

-It may be necessary to select your render layer in Photoshop and do Layer > Matting > Remove Black Matte to get rid of a thin black outline
-It may also be necessary to tweak the Levels on your various shadow layers