3D mapping on back projection screen

[headfull]

Hi, in Station 8.5 I want to use 4 projectors to project onto a large curved back projection screen. See setup:

The problem with the 3D virtual projection method is that it's designed to work for front projection onto objects. In my situation I'm projecting from behind, and doing the mapping from the other side of the screen, so all of the mapping controls will be in reverse from my viewpoint. Is there a workflow for doing this? Previously I've done this in Resolume with the physical projectors setup for rear projection, which is the same as doing a 2D mapping workflow in Smode. But I'd like to try doing it with the 3D workflow method if possible.

Thanks
Jamie

[francis]

Hello Jamie,

Here is the trick for rear projection in Smode Station:

• On your Surface element, add an "Invert Normals" modifier (it's inside the right click menu 'Modifiers' in Smode 8.5, otherwise you can find it with the instantiator after having selected the Surface in previous versions)
• In the parameters of the Surface renderer, set "Renderer - Side" to "Both"

All the best,
Francis.

[headfull]

Hi Francis, Thanks for those tips, but the reverse normals on a stage surface element didn't seem to do anything. Anyway I've already flipped the normals by doing that in the FBX model of my surface.
I'm not sure flipping normals is the solution I'm looking for. Since I'm projecting from the back, but doing the mapping from the front, the UI handles and view of the surface are reversed (from my viewpoint), hence making it pretty hard to do mapping. This is not an issue when doing the 2D workflow (since flipping is done in the real-world projector). But using the 3D workflow, we are working with what the projector "sees" from it's position. I think this is just a limitation of using the 3D workflow (in any app). But I was hoping you guys had some tricks to help. This time the solution for me is to just use the 2D workflow for mapping.

[francis]

Hello Jamie,

I guess what you are missing here is an option to horizontal flip the 2D view of the video projector processor?
Another related problem is when projecting at 90°: things would be easier if we could apply a 90° rotation to the 2D view of the VP processor.
Did I correctly understood?

[headfull]

Hi Francis,
um, yes, no, maybe? It's a hard thing to explain unless you see it, then it's obvious. Maybe a better way to explain it:
if you were doing a dome 3D mapping onto a transparent dome, but the projectors are outside the dome, and you are mapping from inside the dome. I guess the same problem arises, where all the content in the smode viewport are flipped in relation to your viewpoint, making it hard to do mapping (move something right in smode viewport, and real world thing moves left). compare these 2 images:

img1: 3D mapping method

img2: 2D mapping method

See how in img1 the 3D method correctly flipped the content for back projection (if the real-world projector is set to normal front projection mode), but if you try to adjust that mapping from the audience side of the screen, things get confusing. If you add any 2D transforms in a processor you either break the 3D mapping, or make the content appear backwards from the audience perspective.
in img2 with the 2D method, the content is not flipped in smode (because the real-world projector is set for back projection), but this means that what you see on the audience side of the screen is the same as what you see in the smode viewport, so mapping is easier.

Possibly there needs to be an option in the smode stage projector to tell it you are doing back projection? But I'm not sure what the resulting effect should be. I think there is no solution. If you want to do 3D mapping on a back projection screen, you have to adjust the mapping from the same side of the screen as the projector. that is not an option for this project unfortunately, so I guess I have to do a 2D mapping workflow.
I'll email you my project, maybe that will help you see the issue.
thanks!