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Colm Whelan

fast movement and green screen

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I've been doing some testing at varying frame rates and shutter speeds for a martial arts film. The speed of the kicks etc make for some very challenging images as they blur and blend with the green screen a lot. the options for this project were Red One or Sony F3 recorded to 4444 pro res at 360mbs.

 

Just wondering if anyone has found an optimum frame rate/shutter speed for keeping sharp line details on such fast movement. Don't think a higher end camera is an option with budget.

 

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Obviously the shorter shutter time will reduce the amount of blur, which is fine for action to give it more of a crisp staccato feeling, but at some point, this is mainly about how good your compositor is -- because some transparency with fast motion is inevitable, even necessary. ProRes 4444 should be fine for chroma key work.

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Hi David

 

thanks so much for your response. Yeah I thought the pro res would be more than enough. its crazy how fast these guys hit and kick and for performance they feel they have to maintain that speed and energy. so the frames are showing transparency but its keying ok. in your opinion is shooting twice the frame rate worth it or is it just the higher shutter speed that will help. we want to keep the action as natural looking as possible. also shutter speed or angle? normallly I use angle but the smoother motion is actually not helping the process. hence I was on here wondering what people prefer to go with for best results.

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I think everything is a factor when it comes to keying

The lighting
The framing of the shot (wide, medium, close)
Wardrobe and skin tone
Resolution of camera
The optics
The shutter angle
Frames per second
Uncompressed color (4,4,4)

Most things are both asthetic and technical considerations at the same time.

In my opinion resolution and color compression from the camera are a major factor.
Wider full body shots are harder to key. Minimize any green spill on the subjects for clean motion blur key.

Frames per second is really determined by the project frame rate and if its over or under cranked. We did a 50fps PFR a few months back with a red epic. It was decent to key but in my opinion motion looks better at 25fps or 24fps PFR (It was a european project projected at 50p)
Shutter is also both a technical and aesthetic call. Technical consideration of lights beings used, exposure time, and the aesthetic is the desired amount of motion blur. You can still accomplish great keys with heavy motion blur if it was shot with a decent camera and proper set up. I’ve had great results keying 4k red and 3k alexa footage.

I would say along with a great compositor you need a great DP. Lighting ratios are important for a good key.

On some films where I have done only vfx, post production ends up cutting down the image size due to the amount of data before we receive dpx or whatever. Unfortunately that takes away some of the ease of keying. On my own projects I always key with the full resolution sequences and uncompressed color, its a much cleaner key. Having equal color to luminace per pixel is the key.

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