I’m not sure if that option exists, but one way to do it would be to give the backdrop an emission shader with a value of 1 (you can also use other values to make it lighter or darker). The emission shader does not receive shadows or reflections etc. Use your background image in the colour input.
DNorman, I have a follow up question if you don’t mind
Back in 2.79 - if I had a character walking in front of a backdrop, I could add a “ground” plane that was set to be “Shadow Only” . (To give the appearance that there IS a shadow on the backdrop.)
Is there a “Shadow Only” shader? I tried a Transparent BSDF shader, but the ground plane is completely black. I tried hooking up a Principled BSDF just for for fun, but that makes the plane purple - but I do see the shadow that I want.
Ha, ha, that’s a bit like having your cake and eating it!
I take it you want a shadow plane in a defined part of the image, being transparent and only “receiving” shadows.
Again I am not sure if there is an easier way of doing this for Eeveey, but you could make a shadow catcher, explained here:
As explained in the video depending on the light source you may find that you see unwanted parts of the shadow catcher. If you do not want to change the light intensity you can change the position of the white colour slider to contrast the effect. The black/grey slider value changes the opacity of the shadow.
I had to look this up so credit for this answer is for Jeannot Landry the author of the video.
in this image I made a circular “shadow catcher” and placed it on top of an emission plane.
You can see the shadow is only showing on the circle. I can not find any reference to the “express” way mentioned by Giri for Eveey.
It is not that it does not work in viewport, «material preview» mode by default uses an hdri to light the scene, the hdri does not give any shadows. You can choose to use the hdri in this mode or turn it off in the drop down menu at the end of the viewport icons. To get shadows in Eeveey you have to use lamps. If you want the viewport to resemble the render results you have to check the two boxes in the drop down menu as shown.
Here the default hdri (no shadows):