Blue Screen Spill

Initially I thought this might be another "tube camera effect" but since I've already written this post here's a screenshot from tonight's Dad's Army, click for bigger:


(from this episode, 13:52 in.

It's pretty obvious the motion background in this scene is a blue screen effect, enough to make anybody sick just watching it but that's beside the point. You'll notice Godfrey seems to have a blue left arm when he puts it up, obviously the background leaking through around him.

In a previous topic here, we mentioned that the red tint/halo around the candle was due to the tubed cameras in use at the time, that from a later Dad's Army in the 1970s. Is the same true here (this episode is from late 1969), or is this just a simple case of colour spill and was that effect fixable at that time, bearing in mind the episode was recorded a week before transmission.

The cameras at the time were EMI2001s, four tube (R,G,B, and luminance) I'm pretty sure the CCUs had RGB out ( as well as the composite PAL for programme out) so hopefully they were keying using the blue output signal ( that would have had full bandwidth) It looks to me that the overspill is basically an optical effect, reflected light, and perhaps some lens flare, not much that could have been done with the primitive 'post prod' tech of 1970 ?

Another thing to remember about blue-screen, green-screen etc. and tubed cameras is that because you were saturating one of the colour tubes with high-levels of the colour they were sensitive to, you could end up with 'stick' and 'lag' where the camera tube takes a little while to 'recover' from the peak exposure. On a normal picture this is like a smeary coloured trail (not a comet tail but more a large area smear) - which on normal content doesn't look too bad. However on CSO/Chroma Key stuff it means you get a 'laggy' key.

Also - early CSO didn't have 'hue supression' (now often known as colour cancel). Hue supressors were in widespread use on cameras well into the 90s. The way they worked was that you bypassed the internal PAL coder of the camera (or CCU) and took the RGB outputs from the CCU in two directions.

One RGB (*) output fed the Chroma Key/CSO keyer input. The other RGB output fed an off-board PAL coder via a Hue-supressor. This effectively replaced saturated Green, Red, Blue, Magenta or Yellow elements of the camera output with grey (removing the saturation on the picture colour that you were CSO/Chroma keying off) This helped remove lighting spill and reduced the 'ReadyBrek' aura effects. Very odd when you saw the Hue Supressor switched on - a full colour picture suddenly goes black and white in the saturated blue or green areas. These days it is all done in the mixer...

Without Hue Supression / Colour Cancel you have to light very precisely, and with a lot of separation (and floor bounce is very tricky to handle) to reduce spill and avoid fringing.

(*) Component handling may also have been an option - but in my experience full-bandwith RGB was the usual approach.

Of course now we have 4:2:2 to cope with, we no longer have full-bandwith RGB, which is why Green-screen is a much better choice than blue-screen for a high quality video key these days (as you get much more G content in the full-bandwith Y channel than you do B content, which is mainly carried in a half-bandwith Cb channel)