Why does fire always look so bad on TV?

H

Hywel

Well-known member
HI All,

My Google-fu is not up to finding an answer to this question.

I have an HD digital cable TV (provided by Virgin Media in the UK).

Most footage looks great, especially the HD channels.

But there are some things which the compression codec absolutely cannot seem to cope with, which always look awful. The very worst is fire, especially the Hollywood explosion/big fiery screen graphic dissolve sort of fire, but anything with caustics like swimming pools also seems to give the codec the absolute screaming heeby-jeebies.

Can anyone explain to me what feature of these images- the shifting rolling flames and the rolling caustics of swimming pools and swimming pool light reflections- that are causing such grief to the codec? Does anyone know what codec is being used, and are any of the modern codecs better or worse at rendering this sort of thing? Is it that they have lots of high spatial frequencies, or what?

Cheers, Hywel Phillips
 
why are you so interested in fire? fire is bad; and bad FOR you. are you an arsonist?

j/k

if i had to venture a guess i would say its the extremely fine, fast moving details. the GH13 can freeze easily when shooting fire or similar 'busy' scenes. i wonder if maybe the fact its mostly red could be an issue too; when watching some sports games on big HDTV's, i always notice a LOT of noise in very vivid red areas, like dancing pixels and its very distracting.
 
High end high-efficiency encoding makes use of "motion vectors" as part of the encoding process to predict frames and encode only the necessary information as objects move within the frame.

Subjects that are highly erratic (like fire, water..) have very little to no discernible motion vectors that the encoders could leverage, so they end up having to encode using much less effective means (without the data to do it well); and the results come out much worse for it.

The impressive thing is how modern encoding can make use of motion to *improve* all the other shots to make them look so good.

- Mikko
 
Ahh, that's an interesting idea.

So it might be turbulent motion which is causing it such grief, because the very unpredictable nature of the movement is absolutely stuffing up its attempts to predict motion.

As you say, probably it illustrates just how awesome a job these modern codecs are doing on everything ELSE, given the high compression... it is just that these shots are having to "run home to momma" and desperately try to build intermediate frames that resist motion analysis.

Cool!

Hywel.
 
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