Green Screen and Motion Blur

Bershka ad campaing

Last week I did a short video for a Bershka online campaign for their new collection. I recorded the model over a green screen to duplicate her and add some mist. I only had a low-end camera available, which meant the chroma subsampling was 4:2:2. If you are using a consumer camera chances are you also suffer from this, so keep reading.

Camera manufacturers rely on chroma subsampling to allow for higher resolutions without sacrificing processing power. When the image is encoded, less resolution is allocated for chroma information than for luma information.

At plain sight, this optimization is almost unnoticeable. But when editing and especially when extracting a matte, artifacts start to show up. Semi-transparent areas (e.g. motion blur) where color accuracy is essential are the hardest to solve.

THANKS TO BRYCE BAYER

This encoding process is not a software optimization, it is inherent of the sensor filter array used. The most common filter array used in digital sensors is the Bayern. You can see in the image below how this really clever distribution of filters gives full luma information but reduced chroma.

Some photographers scratch the filters of their sensor to obtain a full resolution sensor but losing the possibility to capture color.

Bayern filter

Bayern pattern

 

 

 

 

 

 

Illustrations from wikipedia

TYPES OF SUBSAMPLING

Even with cameras that shoot 4:4:4 video, which contains a full resolution image of all the channels, this is usually only sampled information from a debayering process, not from actually having three filters of light per pixel. Subsampling images fit in these categories:

 

chroma subsampling

 

 

 

 

 

Here you can see a screenshot of the green screen from the Bershka project where motion blur caused issues:

Motionblur shoot

GREEN SCREEN TIP

When I tried to pull out the green screen, getting the motion blurred areas were a challenge and artifacts were definitely visible. Fortunately, there is a cool trick that helps minimize this issue. Shoot at a high enough shutter speed to avoid motion blur! And then add it back in post after pulling out the green screen matte.

 

Check the final video here, and check out the profile of the model on her IG Nuria Oliu

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