Question: In RI Christmas lectures the lights in the windows of the office block seemed to move independently as the camera moved. This suggested to my cynical eye that you cheated and the lights weren't actually in the building. Is this true?

Joe Myerscough answered on 12 Jan 2015:

How cynical of you Charlie! The skyscraper hack was absolutely real as any of the volunteers who spent several hours putting lights in every window and covering each window with reflective film will be able to tell you.

Because the scene was very dark, our cameras were set to a relatively low frame rate so we could capture as much light as possible. This could have led to the slight blurring that you might have seen when the camera moved. Also each window will have had the light pointing in a slightly different direction and the reflective film would sometimes stick to the windows making each ‘pixel’ look a bit different depending on where the camera was. This could have added to the problem too.

We did it for real!

Also, if you think about it, the pictures were being fed back from the building to the theatre live (with a two second delay) so if we were going to fake it, we would have had to generate the CGI images in real time. Also the camera wasn’t just moving from side to side, it was zooming in and out, focussing, refocussing, so to make the ‘fake’ images match up with the real images would have been very difficult indeed… possibly harder than doing it for real!