Hi, I've seen several cases where the chroma decimation, e.g., when going from YCrCb 4:4:4 to 4:2:2, is done by simply throwing away every other sample. I have two questions regarding this operation: 1. If there were a 1-D signal, simply throwing away every other sample without first filtering would cause (potentially) aliasing. Why doesn't this happen in a chroma signal, or why doesn't it matter if it does happen? 2. Since this is a 2-D signal, why isn't a 2-D filter required? --Randy
Chroma Decimation in Digital Video Signals
Started by ●October 14, 2005
Reply by ●October 14, 20052005-10-14
Hello Randy, This may be only a little related to your question but here goes. In still photography the common ways to represent images are in RGB and CMYK color spaces where the color and luminance info are entangled. However from what I understand YCrCb is a lot like L*A*B in that the color info and the brightness info are separated from each other. And my experience with processing images in L*A*B colorspace shows most of the high spatial frequency info is in the L channel (Luminance) and the two color channels A and B have very little high spatial frequency content. Since in this viewpoint, the color channels are sort of already lowpassed in the sense that they don't contain much high frequency info so aliasing is not much of an issue. This is not to say the data has been filtered, but rather this lack of high color spatial frequency content seems to be a common property of many images. There are cases where the color channels can have high frequency data and aliasing may prove to be a problem here. But it may not be strongly perceived. Clay "Randy Yates" <yates@ieee.org> wrote in message news:1129298509.776847.49730@z14g2000cwz.googlegroups.com...> Hi, > > I've seen several cases where the chroma decimation, e.g., > when going from YCrCb 4:4:4 to 4:2:2, is done by simply > throwing away every other sample. I have two questions > regarding this operation: > > 1. If there were a 1-D signal, simply throwing away > every other sample without first filtering would cause (potentially) > aliasing. Why doesn't this happen in a chroma signal, or > why doesn't it matter if it does happen? > > 2. Since this is a 2-D signal, why isn't a 2-D filter > required? > > --Randy >
Reply by ●October 14, 20052005-10-14
Clay S. Turner wrote:> Hello Randy, > > This may be only a little related to your question but here goes. In still > photography the common ways to represent images are in RGB and CMYK color > spaces where the color and luminance info are entangled. However from what > I understand YCrCb is a lot like L*A*B in that the color info and the > brightness info are separated from each other. And my experience with > processing images in L*A*B colorspace shows most of the high spatial > frequency info is in the L channel (Luminance) and the two color channels A > and B have very little high spatial frequency content. Since in this > viewpoint, the color channels are sort of already lowpassed in the sense > that they don't contain much high frequency info so aliasing is not much of > an issue. This is not to say the data has been filtered, but rather this > lack of high color spatial frequency content seems to be a common property > of many images. There are cases where the color channels can have high > frequency data and aliasing may prove to be a problem here. But it may not > be strongly perceived.I was under the impression that much of the low-frequency nature of the color signal arose in the eye, rather than the image. I can imagine that that doesn't matter. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 14, 20052005-10-14
Randy Yates wrote:> Hi, > > I've seen several cases where the chroma decimation, e.g., > when going from YCrCb 4:4:4 to 4:2:2, is done by simply > throwing away every other sample. I have two questions > regarding this operation: > > 1. If there were a 1-D signal, simply throwing away > every other sample without first filtering would cause (potentially) > aliasing. Why doesn't this happen in a chroma signal, or > why doesn't it matter if it does happen? > > 2. Since this is a 2-D signal, why isn't a 2-D filter > required? > > --RandyAliasing could occur, yes, but see the other response. As for #2, in 4:2:2 the chroma is subsampled only in the horizontal dimension, so a 1-D filter along that horizontal would be all that is required to prevent aliasing. In 4:1:1, you would need a 2-D filter since chroma is subsampled horizontally and vertically. Cheers! --M
Reply by ●October 14, 20052005-10-14
"Jerry Avins" <jya@ieee.org> wrote in message news:jqmdndlO187BedLeRVn-sw@rcn.net...> > I was under the impression that much of the low-frequency nature of the > color signal arose in the eye, rather than the image. I can imagine that > that doesn't matter. > > Jerry > --Hello Jerry, I'm just speaking from my experience in working in the L*A*B colorspace. Many graphic artists subscribe to the maxim of blur early and sharpen late. Often the color channels can take a little bit of gausian blur without affecting (much) the detail in the image. This follows from there not being a lot of high frequency color info. This blurring really helps reduce color noise due to over processing an image. The L channel on the other hand gets sharpened at the end of the processing. And sharpening this way avoids colored halos. Certainly the human visual system has its own lowpass filtering as I'm sure you know how this was implemented in NTSC. Maybe this natural adaptation is in response to most scenes lacking a lot of high color frequency detail. Clay
Reply by ●October 14, 20052005-10-14
On 14 Oct 2005 07:01:49 -0700, "Randy Yates" <yates@ieee.org> wrote:>I've seen several cases where the chroma decimation, e.g., >when going from YCrCb 4:4:4 to 4:2:2, is done by simply >throwing away every other sample. I have two questions >regarding this operation: > > 1. If there were a 1-D signal, simply throwing away > every other sample without first filtering would cause (potentially) > aliasing. Why doesn't this happen in a chroma signal, or > why doesn't it matter if it does happen? > > 2. Since this is a 2-D signal, why isn't a 2-D filter > required? > >--RandyAliasing will only happen if there's energy above fs/4, which Clay suggests there likely won't be in the color info. So throwing away every other sample in that case won't affect much other than sample spacing. The perception issue is key. A while back when somebody was asking about maintaining registration between the compressed audio and compressed video during packet dropouts or rate mismatches, I learned from our internal people working on that that a common way to handle it is either drop or repeat video frames. Dropping or repeating a video frame is apparently (and I can easily see this) much more tolerable to a user than dropping or repeating an audio packet. The impression I get from that is that such things that we'd suspect to be travesties from a signal processing perspective aren't really so bad to the eye. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by ●October 14, 20052005-10-14
Clay S. Turner wrote:> "Jerry Avins" <jya@ieee.org> wrote in message > news:jqmdndlO187BedLeRVn-sw@rcn.net... > >>I was under the impression that much of the low-frequency nature of the >>color signal arose in the eye, rather than the image. I can imagine that >>that doesn't matter. >> >>Jerry >>-- > > > > Hello Jerry, > > I'm just speaking from my experience in working in the L*A*B colorspace. > Many graphic artists subscribe to the maxim of blur early and sharpen late. > Often the color channels can take a little bit of gausian blur without > affecting (much) the detail in the image. This follows from there not being > a lot of high frequency color info. This blurring really helps reduce > color noise due to over processing an image. The L channel on the other hand > gets sharpened at the end of the processing. And sharpening this way avoids > colored halos.I think that's because of the way that the visual system processes images. Most sharp color transitions coincide with luminance transitions and can be inferred from them. High color frequencies without corresponding luminance transitions would seem to be rare.> Certainly the human visual system has its own lowpass filtering as I'm sure > you know how this was implemented in NTSC. Maybe this natural adaptation is > in response to most scenes lacking a lot of high color frequency detail.If the high-frequency color information can be recreated from intensity, maybe it's a form of data compression. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 14, 20052005-10-14
Randy Yates wrote:> I've seen several cases where the chroma decimation, e.g., > when going from YCrCb 4:4:4 to 4:2:2, is done by simply > throwing away every other sample. I have two questionsWhich is not really a good thing... Throwing away samples, I mean, not the two questions :-)> regarding this operation: > > 1. If there were a 1-D signal, simply throwing away > every other sample without first filtering would cause (potentially) > aliasing. Why doesn't this happen in a chroma signal, or > why doesn't it matter if it does happen?Yes, it could cause aliasing, but it depends also on the source. For example color information in PAL/NTSC modulated composite signals is quite low passed, so subsampling can be done without further filtering. Different story if the UV signals come from direct RGB444 conversion. In this case low pass filtering is required, also because the 422 result could be, in case 420 is the final target, co-sited or interspersed (whatever is written), then filtering is a must.> 2. Since this is a 2-D signal, why isn't a 2-D filter > required?422 sumsamples the UV signals only in the horizontal direction, so 1-D filter is enough. In case of 420, 2-D filtering is required, of course. bye, -- piergiorgio
Reply by ●October 14, 20052005-10-14
Throwing away every other sample is often done, but is not the best idea. The media processors I know best (Philips TriMedia/Nexperia) apply a 5-tap filter when changing from eg YUV444 to YUV422 (and from YUV422 to YUV420 etc). The shape of filter is important, too, and may be changed for different types of image. In some cases this os done by digitizing or reconstruction hardware, in some by dedicated co-processors. Chroma decimation by throwing away is often acceptable, but it depends what you mean by 'acceptable' - it does not give you such a good picture. In fact, the new Nexperia processors enhance the chroma by some clever processing that tries to put back what was left out, and this is beginning to become perceived by viewers. I thik the reason for the less-demanding processing being so common is, that viewers have been trained to accept fairly poor picture quality. Horizontal filtering is really a quite simple 1-D case, albeit one that requires polyphase filter, and so can be done 'on the fly' as a signal is captured or rconstructed - to do a 2D filter requires line or frame buffering, but this is also often done. One problem in this area is, lots of people 'know' how to process video - but fewer seem able to produce pictures that look good - that requires a lot of domain knowledge. When I was involved with demonstrating video to prospective customners (ie TV manufacturers) I noticed that engineers would often be proud of their video processing but the intended customers would write it off very quickly because he picture quality was not good enough. To be able to look for video quality is not all that easy, as we get distracted by the content and by being so used to watching awful quality. Chris =================== Chris Bore BORES Signal Processing chris@bores.com
Reply by ●October 14, 20052005-10-14
