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  1. Member
    Join Date: Aug 2010
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    So, I was reading some of the docs on the avisynth website and a few of them mention luma leaks and chroma subsampling and colorspace. I was wondering if I could get a simple explanation on how those 3 video properties work and if possible, what kind of consideration they should be given when doing DVD backups.
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  2. Member
    Join Date: Sep 2007
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    Not sure about "luma leak" . Do you have a link ?

    But for chroma subsampling and colorspace, you will be using YV12 colorspace and the sampling is 4:2:0 . All MPEG2 from DVD's will use this. If all your filters are in the same colorspace, you don't have to worry about anything. So if you're doing a simple DVD backup, no worries, because input colorspace and chroma sampling = output colorspace and chroma sampling

    If you convert to RGB (maybe do some filtering in vdub for example) , or use RGB filters in avisynth , then there is special consideration. Any time you do YUV<=>RGB conversions you have to specify how the converision is done (what matrix) . There are also considerations for YV12 interlaced material <=> RGB ; when you convert, you have to specify that it's interlaced otherwise chroma samples can get associated with wrong plane (wierd artifacts)

    There are considerations for HD content as well which are supposed to use Rec.709 , and you may want to use Rec.601 which is typically used for SD (if you were downscaling from a blu-ray source for example, but you only mentioned SD=>SD for a DVD backup)
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  3. Member edDV's Avatar
    Join Date: Mar 2004
    Location: Northern California, USA
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    Re: Luma, Chroma and Colorimetry

    Big subject but in the context of DVD backups poisondeathray hit the major points.

    Y = Luma
    C= Chroma, or CbCr with YV12 pixel placement (saturation and hue)
    Colorimetry relates to the Rec.601 (SD DVD) and Rec.709 (HD ATSC/DVB/Blu-Ray) standards

    You won't get into trouble unless you use filters and/or convert to RGB space. Or, unless you start importing TV captures or camcorder files.

    Probably 90% of "color" problems have nothing to do with CbCr. It has to do with proper scaling of Y. An improper RGB conversion will change 16-235 Y into 0-255. Not only does this cause "colors" to go off, it also crushes blacks and clips white overshoots.
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  4. Member
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    @ poisondeathray :

    Sorry, I saw that 'luma leak' somewhere and I can't find it again in my web history. What I was looking for at the time is guides on how to reduce the "ghosting" that sneaks through on some scene transitions when using SRestore(). The two are probably not related...

    What I usually have in my DVD9 -> DVD5 is MPEG2Source("xxx.d2v", info=3), followed by ColorMatrix(hints=True), but now that you mention it, I'm wondering if ColorMatrix is even needed in this context? I only use CCE SP2 for MPEG-2 encoding, but after going through the options, I see it has a "Try decoding in YUY2 format" checkbox for input. Should I leave that checked?

    I don't even have a Blu-Ray player or drive yet and on the island where I live, we only get 3 HD channels from a company called Canal Satellite. Those are broadcast in 1080i50. Great for watching soccer games. But original commercial pressed discs are a rarity over here and nearly double their original price if you manage to find them. All the shops sell pirated / crappy-quality stuff. I choose to buy all my DVDs online just because of that.


    @ Jagabo : thanks for the links. I don't even know what chroma subsampling means or how it works. But the test reminds me of the advantages of working in the LAB colorspace in Photoshop. Well, not for everything, but sometimes it can improve things a bit there.


    @ edDV : sorry, I still don't get it. Are Rec.601 and Rec.709 like subsets of the RGB colorspace? I guess I should have asked, what is Luma and Chroma in the first place? Also, what do the numbers like 4:2:0, 4:1:1, etc mean?
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  5. Originally Posted by Island_Dweller View Post
    ...but now that you mention it, I'm wondering if ColorMatrix is even needed in this context?
    It's not. You remain Rec.601 all the way through. Nothing has to be changed.
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  6. Member edDV's Avatar
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    Originally Posted by Island_Dweller View Post
    @ edDV : sorry, I still don't get it. Are Rec.601 and Rec.709 like subsets of the RGB colorspace? I guess I should have asked, what is Luma and Chroma in the first place? Also, what do the numbers like 4:2:0, 4:1:1, etc mean?
    Back to basics.

    You are dealing with end product MPeg2 video in DVD spec. That is by definition...

    Y = 8bit luma scaled 16 for black and 235 for white. Overshoot transitions 236-255 are allowed.

    Color components CbCr are sampled as 4:2:0 with YV12 pixel arrangement.
    http://www.fourcc.org/yuv.php
    http://en.wikipedia.org/wiki/Chroma_subsampling

    Colorimetry = Rec601 (refers to YCbCr color space)

    Since you are converting DVD Mpeg2 to DVD MPeg2 you wouldn't change anything unless you think you can decode it and remaster better than the original program producer. Most people just want to recompress into smaller files (e.g. xvid, h.264). In that case avoid any conversion to RGB. Just convert YV12 MPeg2 to YV12 AVC.

    Next if you want to start with a camcorder and go through the whole production process to DVD we can talk about that. But I sense you just want to shrink some DVD files.
    Last edited by edDV; 8th Sep 2010 at 12:39.
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  7. DVD can use rec709 too.
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  8. Member edDV's Avatar
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    Originally Posted by jagabo View Post
    DVD can use rec709 too.
    How does a DVD player "know" to use the Rec709 matrix and do they?
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  9. Originally Posted by edDV View Post
    Originally Posted by jagabo View Post
    DVD can use rec709 too.
    How does a DVD player "know" to use the Rec709 matrix and do they?
    It's flagged in the MPEG 2 data. It can be flagged rec601 or rec709, or it can be unflagged. If unflagged rec601 should be assumed. DgIndex can tell you what's flagged.
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  10. Member edDV's Avatar
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    Originally Posted by jagabo View Post
    Originally Posted by edDV View Post
    Originally Posted by jagabo View Post
    DVD can use rec709 too.
    How does a DVD player "know" to use the Rec709 matrix and do they?
    It's flagged in the MPEG 2 data. It can be flagged rec601 or rec709, or it can be unflagged. If unflagged rec601 should be assumed. DgIndex can tell you what's flagged.
    How many or what type DVD players will respond to this flag?

    I claim no expert knowledge of DVD authoring.
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  11. Originally Posted by edDV View Post
    How many or what type DVD players will respond to this flag?
    I don't know. Most people probably wouldn't notice if the player used the wrong colorimetry. As you know, the difference is pretty subtle.
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  12. Member edDV's Avatar
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    Originally Posted by jagabo View Post
    Originally Posted by edDV View Post
    How many or what type DVD players will respond to this flag?
    I don't know. Most people probably wouldn't notice if the player used the wrong colorimetry. As you know, the difference is pretty subtle.
    I agree, I can't see much difference for HD broadcasts vs commercial DVD for color shift. I haven't compared Blu-Ray.
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  13. Video Restorer lordsmurf's Avatar
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    Differences in MPEG decoders, DACs, wires, TV types (LCD, CRT, etc), proper/improper calibration ... etc..
    Nobody will really notice. Most other things are off anyway.

    Broadcasters, satellite co's and cableco's all screw these up too in their workflow paths.
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