Levels(0,1,128,0,255) - why does it work like that with U and V?

[rgr]

This is the original YUV range without modification (histogram="levels"):



I wanted to cut out only the left half (ranges 0-127 in each planes) and I did it like this:

levels(0,1,128,0,255,coring=false)

And the graph looks like this:


And while the luma graph is as expected, U and V have only expanded proportionally to both sides.
These graphs don't represent values ​​from 0 to 255? (like luma)

When I do:

levels(128,1,255,0,255,coring=false)

the U and V graphs look the same.


What don't I understand?

And why am I doing this -- a captured miniDV cassette has a Y range 16-255. I wanted to check if the chroma also exceeds 16-240 range.

[rgr]

At least I found out that chroma does exceed the 0-240 range.

[Selur]

iirc. Levels in Avisynth is meant or 'brightness, contrast, and gamma', so it does not change the chroma.
(different story in Vapoursynth, which is why at the beginning folks were confused that is didn't work like Avisynth unless you only process the luma or rgb.)
hmm, on the wiki it states:

For adjusting brightness or contrast in YUV mode, it may be better (depending on the effect you are looking for) to use Tweak or ColorYUV, because Levels changes the chroma of the clip.

[rgr]

"because Levels changes the chroma of the clip"

And that's exactly what I was looking for. But this change of chrome is not known by what rules.

[Emulgator]

At least I found out that chroma does exceed the 0-240 range.

I would be interested to see the script that outputs such histogram.
With DV and HDV cam chipsets Y going 16..255 is encountered often, but U,V ?
Canopus ADVC-300: Analog Device's Video DAC's ADV7172, ADV7173 implemented modulation range for U,V values is 128±112

[lollo2]

Originally Posted by Selur

iirc. Levels in Avisynth is meant or 'brightness, contrast, and gamma', so it does not change the chroma.

Levels in AviSynth affects the chroma. The simple functions LevelsLumaOnly does not: https://forum.doom9.org/showthread.p...606#post536606

AFAIK Chroma manipulations with Levels is undetermined and not suggested. Use ColorYUV to act on U and V channels.

[DTL]

For YUV if you want to tweak U and V - try to separate planes of planar YUV and apply Levels() to each plane as Y-only (single plane) clip. And merge YUV back after.

[Sharc]

Avisynth's Levels() was designed to emulate VirtualDub's Levels filter working in RGB mode. The transform that calculated from the values given to Levels() is applied equally to each RGB channel. The result is that the U and V channels change along with the Y channel. It increases saturation when one increases the contrast.
If you want Y to be affected only use YLevels() which preserves color saturation better.
http://avisynth.nl/index.php/Ylevels
https://forum.doom9.org/showpost.php...2&postcount=41
Or use Levels() with chroma copyback from the original chroma.

Edit:
@rgr. The screenshot with histogram of post#2 doesn't make much sense to me. (How) did you manipulate the source?

[rgr]

Originally Posted by Emulgator

I would be interested to see the script that outputs such histogram.

The whole script is just the above mentioned "levels(0,1,128,0,255,coring=false)".

With DV and HDV cam chipsets Y going 16..255 is encountered often, but U,V ?

I was also surprised, but that is clear from the histogeram.

[rgr]

Originally Posted by Sharc

ses saturation when one increases the contrast.
If you want Y to be affected only use YLevels() which preserves color saturation better.
http://avisynth.nl/index.php/Ylevels
https://forum.doom9.org/showpost.php...2&postcount=41
Or use Levels() with chroma copyback from the original chroma.

No, I also want to change the value of all planes.
Y - from 16-255 to 0-255 (or 16-235)
U,V - from 16-255 to 0-255 (or 16-235)
I expected a simple "Levels (16,1,255,0,255)" to do just that.

Edit:
@rgr. The screenshot with histogram of post#2 doesn't make much sense to me. (How) did you manipulate the source?

No, this is what the histogram looks like after applying "levels(0,1,128,0,255,coring=false)" -- theoretically converting the range 0-128 to 0-255.

[StvG]

Originally Posted by rgr

And why am I doing this -- a captured miniDV cassette has a Y range 16-255. I wanted to check if the chroma also exceeds 16-240 range.

There is Limiter() function to check/display if planes have outside range pixels.

Originally Posted by rgr

No, I also want to change the value of all planes.
Y - from 16-255 to 0-255 (or 16-235)
U,V - from 16-255 to 0-255 (or 16-235)

Separate planes, apply Ylevels for every plane, combine planes.

[Sharc]

Originally Posted by rgr

And why am I doing this -- a captured miniDV cassette has a Y range 16-255. I wanted to check if the chroma also exceeds 16-240 range.

You don't have to tamper the levels for this test.
The standard histogram("levels") provides this information. See your very first screenshot. U and V are well within the limits (as usual). It is only Y which exceeds 235 and clips at 255.

or use

ColorYUV(analyze=true)

http://avisynth.nl/index.php/ColorYUV
Scroll down to the bottom of that page.

or use to see the Y,U,V waveforms and Vectorscope:

VideoScope(DrawMode="both",HistoTypeBottom="UV",HistoTypeSide="Y",FrameType="UV")

http://avisynth.nl/index.php/VideoScope

Edit:
And if you want to modify YUV data use

ColorYUV(...........)

http://avisynth.nl/index.php/ColorYUV

[DTL]

Originally Posted by rgr

No, I also want to change the value of all planes.
Y - from 16-255 to 0-255 (or 16-235)
U,V - from 16-255 to 0-255 (or 16-235)
I expected a simple "Levels (16,1,255,0,255)" to do just that.

UVs are completely different from Y. They are zeroed at 128 code level and bi-polar. If you really execute U,V - from 16-255 to 0-255 (or 16-235) it mostly probably will shift coded zero level from 128 to lower and it will cause some constant colour tone and saturation shift.
So Levels for YUV in good case simply do not change anything with UVs (copy).

[wonkey_monkey]

They are zeroed at 127

Isn't it 128/0x80?

[StainlessS]

"Isn't it 128/0x80?"

+1.

[wonkey_monkey]

Originally Posted by StainlessS

"Isn't it 128/0x80?"

+1.

No, it's definitely not 129

[DTL]

Yes - Cb Cr are (colour difference)+128 in ITU BT.601 equations. So its zeroes are at 128 in 8 bit. I fixed that post. I typically make +-1 errors and it cause bugs in software too.

"it's definitely not 129"

But 128 is also 129th code value counted as integer from 0. The digital 8bit range of 0..255 (counted/mapped as 0..255 not 1..256) is centered at 255/2=127.5 . And ITU select to round it to 128. So UVs are also +0.5 offsetted from real 255/2 mid value of 8 bit.

[StainlessS]

Originally Posted by wonkey_monkey

No, it's definitely not 129

I was agreeing with you, and not providing an alternative answer [and I think you know that ]

and, for 8 bit TV Y levels mid point is,
(16 + 235) / 2 = 125.5 # Rounded to 126.

and for 8 bit U/V,
(16 + 239) / 2 = 127.5 # Rounded to 128.

[rgr]

Originally Posted by Sharc

You don't have to tamper the levels for this test.
The standard histogram("levels") provides this information. See your very first screenshot. U and V are well within the limits (as usual). It is only Y which exceeds 235 and clips at 255.

Look post #2.

[rgr]

Originally Posted by DTL

UVs are completely different from Y. They are zeroed at 128 code level and bi-polar. If you really execute U,V - from 16-255 to 0-255 (or 16-235) it mostly probably will shift coded zero level from 128 to lower and it will cause some constant colour tone and saturation shift.
So Levels for YUV in good case simply do not change anything with UVs (copy).

Yes, I noticed that it doesn't work. White (YUV 255,127,127) stops being white (UV -> ~120). The image becomes greenish overall.