What color chroma key to use when actors are lit by deep blue light?

[umbrella]

Using lights with full CTBs on actors for a stylized shot and need to pull a few keys, I’m curious what the best chroma key color would be to use?

My natural instinct went to green but I’m curious if I wouldn’t get a better key with a yellow/orange/red screen?

Thank you!

Topic

[gabj3]

Technically, yellow for a blue light.

However, when lighting your key, it’s important to be careful when using LED light as an RGB LED doesn’t emit any spectral power in the Y region. Then you have an even RED and GREEN channel.

It might be worthwhile using gel’d tungsten fixtures for a clean key (I would light your key with yellow light as therefore you have the most spectral content in the region you desire.)

 

 

 

Infinityvision.tv

[umbrella]

This brings up another question which maybe you can answer.

I know many digital cameras have problems with blue light clipping too easily. My plan was to use daylight balanced LEDs and gel them blue. Would that avoid the issue?

[gabj3]

I’m unsure exactly what you mean by blue clipping –

 

By that, I assume you mean your B channel has become saturated?

OR potentially, you’re highlighting errors in some cameras OETFs (opto electric transfer functions).

 

The prior –

If your blue channel becomes saturated, this means that your B channel can’t perceive any more information as some part along the signal process, there is a saturation point, whether it’s the photodiode, photosite, or ADC.

This means that if you have a light with a fuller spectrum at great intensity, the other R and G channels will read while your blue channel remains saturated, reducing the saturation of your blue channel. This is akin to RGB clipping.

 

The latter –

A camera doesn’t perceive colour as you do – in that its RAW spectral response produces an image that looks quite different to the eye. It looks significantly more magenta than our eye and the final output of the camera.

All cameras have OETFs to transfer from their initial RGB space to XYZ to match our LMS cone functions.

This is done with a simple 3×3 matrix and simple matrix multiplication to adjust the gain of the RGB triplets for each spectral primary.

However, this equation is over-determined and error-prone. Hence, some colours are prone to errors and can cause pretty gnarly artefacts – this is especially common in more saturated far-gamut colours.

Each spectral response and OETF matrix is dependent on the camera and manufacturer, so – test test test!

Thanks

G

 

 

Infinityvision.tv

[Fedor Dokuchaev – Colorist]

umbrella wrote:

This brings up another question which maybe you can answer. I know many digital cameras have problems with blue light clipping too easily. My plan was to use daylight balanced LEDs and gel them blue. Would that avoid the issue?

gabj3 is right, usually it is a problem of the transform, not the blue channel itself. ACES historically has that issue. But if your colorist is tech-savy enough, he will deal with it with no problem.

But I, as a colorist, would advise against using harsh blue light (gel or not) as a single source of lighting in the scene. It is quite hard to make this image colorful and natural in post. But it is much easier for a colorist to make daylight-lit scene naturally blue.

[Author]

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