> My idea was to take a blue and a red light source, one without any fading and
> the other with a fade_power and fade_distance
>
> The ratio between Red and Blue intensity should give me the attenuation, from
> which I can then derive the travelled distance of each ray.
>
> This work wonderfully well in case the fade_distance is equal to the look_to
> point of the camera. However, for all shorter and longer distances the trick
> does not work and does not give good results.

You might get better results by rendering with a white light - once 
without attentuation and another render with attenuation. The reason 
being that HDR formats (I assume you are outputing to an HDR image 
format for better precision) IIRC only store a "magnitude" for each 
pixel, so if red and green are vastly different trying to calculate the 
ratio will have a large error.

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