The chroma aliasing certainly occurs and can be seen for example as false color fringes on high frequency details - eg closely spaced white fence posts. Chris ========================= Chris Bore BORES Signal Processing www.bores.com
Chroma Decimation in Digital Video Signals
Started by ●October 14, 2005
Reply by ●October 14, 20052005-10-14
Reply by ●October 14, 20052005-10-14
The chroma aliasing certainly occurs and can be seen for example as false color fringes on high frequency details - eg closely spaced white fence posts. Chris ========================= Chris Bore BORES Signal Processing www.bores.com
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 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?Often, the chroma data in the original content has already been band limited to below 1/4th the sampling frequency. For instance, in many digital cameras, a color mosaic in front of the sensor pixels only allows sampling color at half of the spacial sampling rate. Data converted from analog video may already have had the chroma channels band limited to meet the NTSC spec. But if the data is from, say, a non-filtered synthesized RGB source or other hires source, then there can be aliasing. IMHO. YMMV. -- rhn A.T nicholson d.O.t C-o-M
Reply by ●October 15, 20052005-10-15
Hi Everyone, Thanks very much for your replies. It is very useful to see a variety of responses - gives you more "averaging" to get a more accurate answer (much like delta-sigma processing!). Chris, it's nice to see you come out of hiding and actually make a post or two on comp.dsp! Chris (et al.), is this chroma filtering problem what causes the moire pattern you see on certain image inputs (vertical lines?). --Randy .
Reply by ●October 15, 20052005-10-15
Randy Yates wrote: ...> Chris (et al.), is this chroma filtering problem what causes the > moire pattern you see on certain image inputs (vertical lines?).What image source? With NTSC, it's the beat between luminance and the 3.58 MHz color subcarrier. A comb filter can greatly reduce that effect. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●October 15, 20052005-10-15
Randy Yates wrote:> Chris (et al.), is this chroma filtering problem what causes the > moire pattern you see on certain image inputs (vertical lines?).Uhm, I do not think that is the main reason. I saw the moire as consequence, as Jerry says, of the chroma separation filter, in case of composite signal. Or, in case picture processing, where certain bandwidth conditions were not respected. bye, -- piergiorgio
Reply by ●October 15, 20052005-10-15
>Often, the chroma data in the original content has already >been band limited to below 1/4th the sampling frequency. For >instance, in many digital cameras, a color mosaic in front >of the sensor pixels only allows sampling color at half of >the spacial sampling rate.I think a color mosaic only samples, it does not band limit. Chris
Reply by ●October 15, 20052005-10-15
It can look like a moire pattern, but is more like color fringes around the high-frequency detail. But of course a similar effect also arises from analog interference as Jerry says. The classic moire pattern is simple aliasing - the color version changes colors because the color is aliased differently from the luminance and depends on the precise sampling pattern (eg Bayer). To see that it results from sampling, take an image with lots of pixels, YUV444 or an RGB equivalent, and some high-frequncy detail (preferably white), and subsample the chroma - without filtering. Since Foveon offer sensors with coincident RGB pixles I expect they have some explanations of this (since it justifies their sensor patterns). One of the Swiss universities (I think) has a good tutorial with images on the web. Sorry, can't remember who.. :-( Chris
Reply by ●October 16, 20052005-10-16
Chris Bore wrote:>>Often, the chroma data in the original content has already >>been band limited to below 1/4th the sampling frequency. For >>instance, in many digital cameras, a color mosaic in front >>of the sensor pixels only allows sampling color at half of >>the spacial sampling rate.> I think a color mosaic only samples, it does not band limit.In all video camera systems, as far as I know, it is the optical system that does band limiting. Well, for scanned electron beam systems you might do some with beam spot size, but mostly the optical system. -- glen
Reply by ●October 16, 20052005-10-16