I notice in many 'making of' features on movies, blue chromakey sets use red markers as tracking markers for 2D and 3D tracking. In Lord of the Rings, I notice they used green markers on a bluescreen in some shots. And in some cases, both green and red markers were used on a blue set.
Any particular reason why red is used? I'm guessing it's another primary RGB color that's easy to replace?
>Any particular reason why red is used? I'm guessing it's another
>primary RGB color that's easy to replace?
For the most part yes - you can remove it by simple channel operations, which also works for YUV colorspace plus it already provides a high contrast basis for any tracking algorithm. In addition, red tends to not require too much additional lighting and yet each marker will look "flat" enough to register as a single consistent tracking point. It is also worth noting that red will also register in infrared, which has relevance for some motion capture and motion control systems that use infrared cameras and reference markers. Finally, red LEDs are the cheapest ones but have the highest radiant output, which also makes them perfectly suited as markers...