garageCube

I am preparing a building facade projection with two projectors double stacked to increase brightness via Matrox.

I have about 20 surfaces mapped to the building, which is fine for one beamer. Is there a way I can copy across all the surfaces to the second beamer, but also make sure it still is exactly aligned to the first beamer?

Of course I can try and align them pre-mapping with test patterns, but I would like to compensate for subtle lens variations via madmapper...

I mean, something like a group function for the surfaces would probably fix this problem to be able to slightly stretch all the surfaces at once.

Hope it makes sense.

its best avoided is the simple answer.
you will get parallax issues,which are a function of the lens focal length and the distance the two lenses are apart.
if you have long throw lenses you may well be fine, here is a video of two pairs of triple stacks onto a building. we only had a problem on the dome at the top

https://vimeo.com/17138044

some projectors have independent warp controls - christies for example.
you might try pointing the projectors at each other then onto 90° mirrors. this may reduce the parallax by reducing the effective distance between the two beams. but lineup will be very fiddly.
but the smart idea is to trigger two computers via timecode or MIDI and have individual maps for each output.
Qlab can now patch into madmapper..

usually we use christies projectors with a warping card.
For two projectors, (proj. 0 and proj.1),
projector 0 is unwarped, and projector 1 is warped onto proj.0, using a built in warping card.
Then only one output is sent to the projectors.
This is the easiest/cleanest way of stacking 2 projectors.
If a warping card is not an option (too expensive sometimes), then use a dual head to map two times your setup, one for each projector.

for this project, we used 4 projectors stacked using warping cards, then only one madmapper output:
https://vimeo.com/18888136