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experiment to study DV conversions
It is often mentioned that transformations of DV to 10-bit uncompressed 4:2:2 result in tonal distortions — a gamma effect.
I performed the following experiment.
(1) I made a 24-bit RGB bitmap image with a grey scale. The patches were at 0, 10, 30, 55, 90, 125, 160, 195, 225, 245, 255, R=G=B.
(2) I imported this to FCP, made a 1 second clip, rendered it as DV, and then exported it as a Quicktime file. Then I reimported the Quicktime file to the timeline, so I’m pretty sure it’s now DV.
(3) I then exported the DV on the timeline to Quicktime 5 different ways:
rendering with codec None;
rendering with codec 8-bit uncompressed 4:2:2;
rendering with codec 10-bit uncompressed 4:2:2;
exporting using Quicktime conversion to 8-bit uncompressed 4:2:2;
exporting using Quicktime conversion to 10-bit uncompressed 4:2:2.
(4) I displayed all 6 Quicktimes using the Quicktime player, stopped on a frame, and grabbed the image using Grab. The grabs are .tif’s.
(5) I measured the tones in each .tif using the eyedropper in Corel Paint. (Hey, one uses the tools one has.)Surprise! All of the .tif’s EXCEPT the one from the DV show practically no tonal distortion. For example, the first 10-bit uncompressed 4:2:2 has patches 0, 9, 29, 54, 89, 125, 159, 194, 225, 244, 255. (I also viewed the original bitmap with the Quicktime viewer and grabbed a .tif and found it unchanged from the original.) It’s the .tif from the original DV that shows significantly tonal distortion. Its values are: 0, 13, 37-38, 64, 100, 135, 167, 200, 228, 245, 255. This distortion is almost exactly a gamma = 1.1 effect. 255*(DV/255)^1.1 gives almost exactly the bitmap values and those of the other codecs.
The mystery here: how can the DV be tonally distorted when the DV was the source of all the other versions which are not tonally distorted? The original bitmap tone values must be in that DV. A proper decoding of the DV must give them, while an improper decoding doesn’t.
The Quicktime player must be the culprit. It must play back DV with gamma 1.1 relative to the other codecs. In fact the DV looks good with the gamma boost and the other codecs look flat, but why is the Quicktime player playing games with this one among codecs?
I wonder how much of the rap against conversion of DV to 10-bit uncompressed 4:2:2 is due to this player artifact? I wonder what other programs, especially compression programs making mpeg2’s, will do to the 6 different Quicktimes.
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7 Replies
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[Dennis Couzin] “The Quicktime player must be the culprit. It must play back DV with gamma 1.1 relative to the other codecs. In fact the DV looks good with the gamma boost and the other codecs look flat”
Is the codec who tells QT Player the gamma to be used on play.
The DV codec was designed to be played in TVs. Haven’t changed much, while the Uncompress codec have been improved in the last few years.
I think this is what is called “Gamma Independent” codecs.
Very interesting test.
Rafael -
Thanks Rafael. DV codec: what a ball of surprises. There’s the coded image and there’s a decoder to make it a pixel-by-pixel displayable form. Apparently, when DV is decoded for display it’s with a 1.1 gamma boost. When the same DV is converted to an uncompressed format this also involves decoding the DV, but without the gamma boost. So the decoder includes a “purpose” switch. What happens when you decode the DV in the process of making an mpeg2? Tomorrow’s experiment.
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[Dennis Couzin] ” When the same DV is converted to an uncompressed format this also involves decoding the DV, but without the gamma boost”
When ever you put something in the FC time-line, is expanded to 4.4.4 whatever the sequence codec.
Only on exporting the Gamma Flag (?) will be added depending of the export codec.
AVID DV apart of the Gamma boost on play, also makes the player to execute some Chroma Filtering.
This is why the DV AVID looks better on play than the Apple DV being the YUV values the same pixel by pixel.[Dennis Couzin] “Tomorrow’s experiment.”
That’s the only way to reach founded conclusions.
Just need time and patience:-)
Cheers,
rafael -
Rafael, the next stage of the experiment gave happy results.
Compressor was used to make an .m2v from each of the 6 Quicktimes described in the first post.
MPEG Streamclip was used to play the .m2v’s.
Grab and Corel Paint were used as before.Below is a table of results.
First column is the original bitmap.
Second column is the m2v made from the DV Quicktime.
Third column is the m2v made from the None Quicktime.
Fourth column is the m2v made from either the 8-bit or 10-bit uncompressed 4:2:2 Quicktimes (made directly with rendering).
Fifth column is the m2v made from either the 8-bit or 10-bit uncompressed 4:2:2 Quicktimes (made with Quicktime conversion).BMP DV None 8/10 8/10 conv
0….. 0…… 0…… 0…… 0
10… 9…… 9…… 9….. 11
30… 30… 29… 30… 30
55… 55… 54… 55… 55
90… 90… 89… 90… 90
125 125 124 125 125
160 160 158 160 160
195 193 192 193 193
225 226 224 226 224
245 243 242 243 245
255 255 255 255 255The DV gamma boost does not occur. The m2v made from the DV is exactly the same as the m2v made from the 8- or 10-bit uncompressed Quicktimes (made directly with rendering). All six m2v’s are very like the original bitmap on the greyscale. Likewise for color tinted scales.
Thus FCP conversion of DV to uncompressed formats does not appear to introduce color or tonal compromise. Relatively washed out appearing conversions are artifacts of players. It’s funny that Avid diddles the DV for display even more than Quicktime does. What’s the point? To see how good the DV can look in ideal display?
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[Dennis Couzin] “It’s funny that Avid diddles the DV for display even more than Quicktime does. What’s the point? To see how good the DV can look in ideal display? “
It makes the DV look less blocky and it really looks more pleasant.
If you want the Apple DV looks similar you need to use Nattress Chroma Smooth/Sharpen.
Works great when you set a 422 codec in your sequence.
rafael(and here some clips for the friends: https://www.vimeo.com/2694745 )
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Oh, so it’s more than display. Avid has a better DV-to-uncompressed 4:2:2 converter than FCP. DV compression being lossy, it makes sense that there are various ways to decompress it, some making less visual loss than others. Converting the 4:2:0 of DV-PAL to 4:2:2 only partially undoes the color subsampling, however. Something else, maybe the display, has to produce 4:4:4 ultimately.
A moral of my little experiment might be that digital display is where the funniest tricks get played. For example identical signals going into two displays can produce very different images, with different motion qualities. What displays (and projectors) do to video isn’t normally specified or even named.
It’s a pity if Apple offers inferior conversion out of DV. My experiment showed small (tonal) differences between the Quicktime made by FCP straight from the rendering and that using Quicktime conversion. I don’t know if there are other differences, and I didn’t try Compressor’s conversion which could be different again. (Is software amateurism Apple’s charm?) Are we sure that all three Apple routes from DV-PAL to 8- or 10-bit uncompressed 4:2:2 need Nattress help?
Do you recommend the Nattress Chroma Smooth/Sharpen filters just for FCS Color or also for FCP? (I’m still using FCP 5.1.4 and don’t have Color.)
I can’t grasp how the Nattress filter can be applied before the FCP/Quicktime/Compressor conversion or after it. If the filter is used first, then the DV isn’t DV anymore for the FCP/Quicktime/Compressor conversion. If the filter is used second, then the 4:2:2 is already done and the Nattress filter has nothing to do. Do Nattress filters work in conjunction with FCP/Quicktime/Compressor. That would be nifty. Mr. Nattress would have to know FCP inside out.
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[Dennis Couzin] “Converting the 4:2:0 of DV-PAL to 4:2:2 only partially undoes the color subsampling, however. Something else, maybe the display, has to produce 4:4:4 ultimately.”
When you are working with 411/420 and is played/processed at full quality, the file is expanded to 444. Each of the for pixels in the block get their own Y’Cb’Cr” value. The Cb’ and Cr’ values are common for the four, but each of them may have a different Y’ value. You end up with four different RGB values.
Plugins like the Nattress doesn’t apply the Cb’/Cr’ lineally but using a kind of algorithm that try to fake how was the Chroma before the in camera 444 to 411/420 downsampling.[Dennis Couzin] “It’s a pity if Apple offers inferior conversion out of DV. My experiment showed small (tonal) differences between the Quicktime made by FCP straight from the rendering and that using Quicktime conversion”
The problem is that there is no much information available about how QT works internally.
All the controls in the Visual Setting window, I have no idea how thy work. They can really change the look of your picture.
I deeply recommend it. I started working with Betacam more than 25 years ago. Working in DV was really frustrating for the blockyness of the picture. Betacam is much more “organic”. The Nattress filter changed everything for me.
It supposedly have to re-build the original Chroma so should treat the original footage.The ideal is to apply it over the raw footage and the first in the stack if you put few filters.
Cheers,
rafael(and here some clips for the friends: https://www.vimeo.com/2694745 )
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