What I don’t get now?!
Shadow catcher enabled, but with Transparent activated there is only checkboard-texture, not really transparent:
Without transparency:
That is correct because when you set “transparent” you are supposed to use the shadow catcher in the compositor, so it’s transparent now. If you look closely you will see that the shadows are there.
Forget the transparent part if you want to use the background image directly from the render, without compositing. That second render has something wrong, as the shadow catcher is transparent by definition.
Thanks, so I need use compositor then!
This background scene is made with Meshlab and it is the scene where we filmed video footages. I only need that for set lights and camera in same place and getting right kind of shadows when adding 3d-objects to video footqage. I will add video footage there with “images as planes”.
So first I tried to get this background mesh as shadow catcher, but tested also with this plane (and it is easier to show here).
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Still no shadows (or mist)! Why? Is there some setting wrong?
I made this Composition with CG Geek’s tutorial:
background-layer:
Any clue?
When Ray is visible, shadows are there,
but when adding shadow catcher shadows are there first 0,1 seconds when activating shadow catcher, BUT IN DIFFERENT PLACE 
,
but disappear very soon. Only very small amount is there after rendering.
Sorry, I’m a bit lost here. I can’t understand what you are doing with your nodes.
First, why aren’t you using the shadow catcher pass?
Second, I don’t know what you are doing with the mist pass. What are you trying to achieve?
I copied nodes and settings from this tutorial: https://youtu.be/-BYH5pDYakk
because those worked there. First picture in last message there is shadow that I would want to come only with object. I have shadow catcher enabled (if that means shadow catcher pass?) this two lasts pictures, but shadow is not working. And I’m asking why not?
Mist is not must-have, decided to include it if I would need it or give spooky color for the scene.
I trying to achieve this:
This robot (with glossy of green bed on it) and it’s shadows…
…to pasted over with video footage (motion tracked):
Ok, then I will tell you later what to do cause now I need to work. Where are you from, by the way? Maybe I should explain that by message in Portuguese or Spanish. I deal better with those languages.
Ok, thanks a lot if You can help! 
Sorry, I’m from 
, but allways can use G translator (often using when answers in English too), if You want to write in Spanish.
Lol. I would never guess that judging by your name.
well, basically you can be having problems with your shadow catcher cause as I can see you have the catcher over another mesh there. Why isn’t that mesh your shadow catcher object? What do you need that for if you are compositing everything with video footage?
Do You mean “texturedMesh”-object? It is whole scene “scanned” by camera and Meshroom-software. Face count number is 160 975, so thats why I cut pieces of it for this mesh “texturedMesh.002”, which is now only one mesh in collection “background”. (I tried first decimate this “texturedMesh”, but it was going so bad, that I think this was better idea).
And I’m needing this “texturedMesh” for set up Blender’s “virtual” cameras close the position of camera in real footage.
If I would use this texturedMesh for Shadow Catcher same problem appear:
Then, maybe the problem is that your objects are disabled for the render.
Well, that’s weird. Can you make a copy of the file only with the lights and the shadow catcher so I could see what could be the problem here?
Nobody have a clue?
The problem in your scene is the lights. Too many light sources and they are too strong, You are washing away the shadows.
I reduced the lights intensity from the emission materials to 5 instead of 50 and from the point light from 1000 to 500 w. You don’t need to activate the shadow catcher pass cause it becomes already transparent and ready for compositing. I also reduced the amount of bounces as too many bounces tend to wash the shadows as well.
The result I got is:
I’m also not sure if your shadow catcher geometry is the best for you, but you will decide that.
One advise: when you want to reduce the file size in Blender you can compress them. You could use that to all your files, as I do to mine, but that is your decision. I prefer them smaller and I never had problems with compressed files.
When you save a file in Blender you click on this gear and select “compress”.
You can also set that behavior to be your standard behavior in Blender, under preferences > save and load > compress file.
Your file after the compression takes only 19.5 mb instead of the 53 you had before.
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Thanks again, but I did not get same result. 
But what I noticed accidentally was that when I activated shadow catcher for light (=plane with emission 50) shadows are coming like it should be?
And thanks for this tip to compress! Never heard about it, this is huge! 
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Yes, it is, and as I can see, most people don’t know about that either. 
Sorry for that I’m asking same question all the time!!
BUT THIS IS ENDLESS (two weeks is not endless, but it start to frustrate) ! 
Shadows are right in viewport rendered view, BUT NOT IN RENDERED FRAMES! 
I tried to do totally new compositing with help of this tutorial Youtube
But NO shadows, no transparent background:
Rendered frame:
