五常稻花香 I've seen many comments about HDMI 1.3 DeepColor being useless, about 8bit being enough (since even Blu-Ray is only 8bit to start with), about dithering not being worth the effort etc. Is all of that true?
It depends. If a source device (e.g. a Blu-Ray player) decodes the YCbCr source data and then passes it to the TV/projector without any further processing, HDMI 1.3 DeepColor is mostly useless. Not totally, though, because the Blu-Ray data is YCbCr 4:2:0 which HDMI cannot transport (not even HDMI 1.4). We can transport YCbCr 4:2:2 or 4:4:4 via HDMI, so the source device has to upsample the chroma information before it can send the data via HDMI. It can either upsample it in only one direction (then we get 4:2:2) or into both directions (then we get 4:4:4). Now a really good chroma upsampling algorithm outputs a higher bitdepth than what you feed it. So the 8bit source suddenly becomes more than 8bit. Do you still think passing YCbCr in 8bit is good enough? Fortunately even HDMI 1.0 supports sending YCbCr in up to 12bit, as long as you use 4:2:2 and not 4:4:4. So no problem.
But here comes the big problem: Most good video processsing algorithms produce a higher bitdepth than you feed them. So if you actually change the luma (brightness) information or if you even convert the YCbCr data to RGB, the original 8bit YCbCr 4:2:0 mutates into a higher bitdepth data stream. Of course we can still transport that via HDMI 1.0-1.2, but we will have to dumb it down to the max, HDMI 1.0-1.2 supports.
For us HTPC users it's even worse: The graphics cards do not offer any way for us developers to output untouched YCbCr data. Instead we have to use RGB. Ok, e.g. in ATI's control panel with some graphics cards and driver versions you can activate YCbCr output, *but* it's rather obvious that internally the data is converted to RGB first and then later back to YCbCr, which is a usually not a good idea if you care about max image quality. So the only true choice for us HTPC users is to go RGB. But converting YCbCr to RGB increases bitdepth. Not only from 8bit to maybe 9bit or 10bit. Actually YCbCr -> RGB conversion gives us floating point data! And not even HDMI 1.4 can transport that. So we have to convert the data down to some integer bitdepth, e.g. 16bit or 10bit or 8bit. The problem is that doing that means that our precious video data is violated in some way. It loses precision. And that is where dithering comes to rescue. Dithering allows to "simulate" a higher bitdepth than we really have. Using dithering means that we can go down to even 8bit without losing too much precision. However, dithering is not magic, it works by adding noise to the source. So the preserved precision comes at the cost of increased noise. Fortunately thanks to film grain we're not too sensitive to fine image noise. Furthermore the amount of noise added by dithering is so low that the noise itself is not really visible. But the added precision *is* visible, at least in specific test patterns (see image comparisons above).
So does dithering help in real life situations? Does it help with normal movie watching?
Well, that is a good question. I can say for sure that in most movies in most scenes dithering will not make any visible difference. However, I believe that in some scenes in some movies there will be a noticeable difference. Test patterns may exaggerate, but they rarely lie. Furthermore, preserving the maximum possible precision of the original source data is for sure a good thing, so there's not really any good reason to not use dithering.
So what purpose/benefit does HDMI DeepColor have? It will allow us to lower (or even totally eliminate) the amount of dithering noise added without losing any precision. So it's a good thing. But the benefit of DeepColor over using 8bit RGB output with proper dithering will be rather small.
It depends. If a source device (e.g. a Blu-Ray player) decodes the YCbCr source data and then passes it to the TV/projector without any further processing, HDMI 1.3 DeepColor is mostly useless. Not totally, though, because the Blu-Ray data is YCbCr 4:2:0 which HDMI cannot transport (not even HDMI 1.4). We can transport YCbCr 4:2:2 or 4:4:4 via HDMI, so the source device has to upsample the chroma information before it can send the data via HDMI. It can either upsample it in only one direction (then we get 4:2:2) or into both directions (then we get 4:4:4). Now a really good chroma upsampling algorithm outputs a higher bitdepth than what you feed it. So the 8bit source suddenly becomes more than 8bit. Do you still think passing YCbCr in 8bit is good enough? Fortunately even HDMI 1.0 supports sending YCbCr in up to 12bit, as long as you use 4:2:2 and not 4:4:4. So no problem.
But here comes the big problem: Most good video processsing algorithms produce a higher bitdepth than you feed them. So if you actually change the luma (brightness) information or if you even convert the YCbCr data to RGB, the original 8bit YCbCr 4:2:0 mutates into a higher bitdepth data stream. Of course we can still transport that via HDMI 1.0-1.2, but we will have to dumb it down to the max, HDMI 1.0-1.2 supports.
For us HTPC users it's even worse: The graphics cards do not offer any way for us developers to output untouched YCbCr data. Instead we have to use RGB. Ok, e.g. in ATI's control panel with some graphics cards and driver versions you can activate YCbCr output, *but* it's rather obvious that internally the data is converted to RGB first and then later back to YCbCr, which is a usually not a good idea if you care about max image quality. So the only true choice for us HTPC users is to go RGB. But converting YCbCr to RGB increases bitdepth. Not only from 8bit to maybe 9bit or 10bit. Actually YCbCr -> RGB conversion gives us floating point data! And not even HDMI 1.4 can transport that. So we have to convert the data down to some integer bitdepth, e.g. 16bit or 10bit or 8bit. The problem is that doing that means that our precious video data is violated in some way. It loses precision. And that is where dithering comes to rescue. Dithering allows to "simulate" a higher bitdepth than we really have. Using dithering means that we can go down to even 8bit without losing too much precision. However, dithering is not magic, it works by adding noise to the source. So the preserved precision comes at the cost of increased noise. Fortunately thanks to film grain we're not too sensitive to fine image noise. Furthermore the amount of noise added by dithering is so low that the noise itself is not really visible. But the added precision *is* visible, at least in specific test patterns (see image comparisons above).
So does dithering help in real life situations? Does it help with normal movie watching?
Well, that is a good question. I can say for sure that in most movies in most scenes dithering will not make any visible difference. However, I believe that in some scenes in some movies there will be a noticeable difference. Test patterns may exaggerate, but they rarely lie. Furthermore, preserving the maximum possible precision of the original source data is for sure a good thing, so there's not really any good reason to not use dithering.
So what purpose/benefit does HDMI DeepColor have? It will allow us to lower (or even totally eliminate) the amount of dithering noise added without losing any precision. So it's a good thing. But the benefit of DeepColor over using 8bit RGB output with proper dithering will be rather small.
