What's wrong with my shadow plane?

I’m compositing a 3D element into video, so I created a shadow plane to catch the object’s shadow. I’ve got “only shadows” selected, but I can’t get the plane to completely disappear. I get sort of a ghosted image of the plane, like it’s not completely transparent:


What gives? I’ve tried ZTransp and a dozen other things, but I can’t get the plane to go away.

Try these settings
And turn premul on instead of sky and use png as your file type,
premul is in the render buttons


Looks like a task for render-layers to me.

It’s possible to render an image, in which the shadows are the only bit that gets kept from the image. These shadows are then added onto the video, and the 3d objects added to this.

I’ve been up all night, and it’s been a while, so I can’t give you a better explanation right here, though I did do a write-up on how to do just this a while back. You can find the PDF and the blend files here if you’re interested.


I’ve never looked into shadow planes etc., but if you’re trying to generate transparent images for overlay, make sure you’re saving in a format that includes an alpha channel.


I tried those exact settings. No dice. This is driving me crazy.


I downloaded your tutorial and tried to duplicate the effect with my scene, but I can’t get the shadow layer to work.

And you had Premul set?
and saved as a png?
i know what it is,
set the it RGBA


You will not see the shadow till you combine the 2 images back together,

Done, done and done. No dice. The plane still shows up in some shadowy ghost form (very different from when I render it as a regular, solid object). The shadow is there, but it’s combined with the plane ghost.

that is wierd,
I just had this problem and I made it work,
here is a link to the thread I started on monday

Yeah, I had read that thread when I did a search (prior to posting), but I didn’t see anything that I thought would help. I’ll take another look.

In the meantime, I just opened a new file and started with a simple sphere and a plane. Made the plane “OnlyShad” and rendered. Perfect. There was a shadow of the sphere on the plane, but the plane itself was invisible.

So why can’t I get that to happen in my scene? No matter what I do, I get this sort of shadowy image of the plane. I’m seriously losing my mind here. I’ve wasted half my day on this so far.

Okay, I figured it out. This is weird.

That 3D object is a sort of robotic device, and inside of it is a 3D lamp to illuminate the inside (which is not visible from this angle).


Well, the “Ray Shadow” button was active in the Shadow and Spot panel for that lamp, and that’s what’s casting a shadow (in 360 degrees around the robot, because the light is inside) on the plane, which is otherwise invisible.

If I unclick the “Ray Shadows” button and re-render, the plane is perfectly invisible, but the shadow of the robot (from the sun) shows up.

I only figured this out when I replaced the robot with a simple sphere and saw that that took care of the problem. So, then I knew it was something about the robot model that was causing this.

Sigh. What a waste of time. Thanks for the help everybody.