Hi -
I'm sure you've all see the blocks/splotches in dim/dark scenes - it's
especially noticeable when fades from or to black occur.
No, I'm not hot on the trail of a new filter - but the visible blocks
during dark and fade scenes is very annoying and I'm not sure how to
reduce their obviousness.
What seems like a good idea is a selective median/averaging filter
that would be more aggressive in dark scenes and gradually taper off
as the frames became brighter. I'm not sure if treating regions within
a frame would be a good thing or not - might be, but might lead to
artifacts.
What's a good metric for determining the "brightness" of a frame?
Compute a histogram of the Y' values and see if there's a big cluster
below a specified threshold (such as 32)? For example, this is a
histogram of Y' for a frame that's supposedly pure black (after Y=32
it's all 0):
Y 0 0
Y 1 0
Y 2 0
Y 3 0
Y 4 0
Y 5 1
Y 6 2
Y 7 11
Y 8 14
Y 9 37
Y 10 96
Y 11 266
Y 12 734
Y 13 2223
Y 14 5806
Y 15 11904
Y 16 19854
Y 17 28770
Y 18 41353
Y 19 54259
Y 20 58347
Y 21 47760
Y 22 32121
Y 23 19341
Y 24 9487
Y 25 3593
Y 26 1285
Y 27 406
Y 28 170
Y 29 48
Y 30 16
Y 31 12
Y 32 1
Now if that same frame is run thru the medianfilter with the default
threshold of 2:
Y 0 0
Y 1 0
Y 2 0
Y 3 0
Y 4 0
Y 5 1
Y 6 2
Y 7 10
Y 8 8
Y 9 25
Y 10 43
Y 11 86
Y 12 104
Y 13 208
Y 14 542
Y 15 2709
Y 16 11383
Y 17 27625
Y 18 48690
Y 19 72625
Y 20 77236
Y 21 56771
Y 22 30253
Y 23 8061
Y 24 1219
Y 25 211
Y 26 67
Y 27 25
Y 28 12
Y 29 3
Y 30 1
Y 31 0
Y 32 0
and with a threshold (-t) of 4:
Y 0 0
Y 1 0
Y 2 0
Y 3 0
Y 4 0
Y 5 1
Y 6 2
Y 7 10
Y 8 8
Y 9 25
Y 10 43
Y 11 86
Y 12 106
Y 13 220
Y 14 561
Y 15 2662
Y 16 10036
Y 17 24510
Y 18 45282
Y 19 74879
Y 20 92223
Y 21 59140
Y 22 20823
Y 23 5774
Y 24 1207
Y 25 214
Y 26 67
Y 27 25
Y 28 12
Y 29 3
Y 30 1
Y 31 0
Y 32 0
and finally with 8:
Y 0 0
Y 1 0
Y 2 0
Y 3 0
Y 4 0
Y 5 1
Y 6 2
Y 7 10
Y 8 8
Y 9 25
Y 10 43
Y 11 84
Y 12 100
Y 13 142
Y 14 238
Y 15 573
Y 16 1691
Y 17 13763
Y 18 53222
Y 19 93559
Y 20 109834
Y 21 58018
Y 22 5464
Y 23 619
Y 24 298
Y 25 125
Y 26 60
Y 27 25
Y 28 12
Y 29 3
Y 30 1
Y 31 0
Y 32 0
It wouldn't be a good idea to use a "-t 8" over the entire movie. But
if a frame is so dark that Y' is centered around 20 perhaps a "-t 8"
would be fine.
Would it make sense to have several 'regions' or 'bands' that have
different thresholds? If the majority of the frame has a Y' around
20 +/-4 use a "-t 8", if it's centered about 28 +/- 4 then a "-t 6",
32 +/-4 "-t 4", and so on. After a certain point Y' will be high
enough (48 or so) that the scene will be bright enough to not need
median/averaging since the blocks/splotches won't be as noticeable to
the eye.
Or would that not make much difference or even worse cause visible
artifacts?
The other idea is to quantize dim scenes less accurately. For dim
frames (where the majority of the frame has Y' less than a specified
value) turning Y' values between 16...19 = 18, 18...22 = 20 and so on
up to a max (40 perhaps the default ceiling). That would reduce the
blockiness quite a bit I'd think since the encoder wouldn't see
Y' jumping up and down by small amounts.
Yet another idea (notice how easy they come when one isn't writing the
code :)) would be a filter than could be 'trained' - by that I mean
feed it a bunch of frames of what should be considered "black" - from
that it could would determine the appropriate median/quantizing
parameters to apply. What'd be great would be the ability to
"subtract" the differences between "pure black" and the "reference
frame(s)" from the movie - but I'm not sure how one subtracts in the
"YUV" space ;)
Any statistics or signal processing folks care to chime in with ideas,
suggestions, or even better working code? :) :)
Cheers,
Steven Schultz
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