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On NTSC Video, Blue Blurring, Chroma Subsampling
NEOGEOman writes "Something I've been fascinated with for a long time is video signals. On my website I've spent over six years collecting video and other hacks for game consoles. I've recently put together the fourth revision of my video signal primer and it's expanded to six pages now, including strange subjects like chroma subsampling, horizontal colour resolution and rather interesting revelation: your eyes suck at blue."
Can anyone possibly tell me why the hell this site is listed in my company's web filter as pornography?
My eyes suck especially badly at blue. a pure-blue image is something my eyes completely refuse to focus on. I can see the image is there, see that it's blurry, but whatever makes my eyes focus just doesn't work on blue. Light a room with pure blue light and I'm almost blind. gah!.
:)
Add some other colours and I'm fine. Curiously, given a red line of text, a green line of text and an off blue line of text, I have to focus differently for all three. (Fully blue is, of course, a complete waste of time
You may want to read a report on misbehaving censorware. Blocking the the article as "pornography" is misbehavior. If your company's business has anything remotely to do with video production or video games, ask your IT department to review gamesx.com (the site on which the article is hosted) and consider whitelisting it.
who's the girl? She's hot ;)
It's well known; as our eyes drift to the blue and red end of the spectrum, we lose our sensitivity, off by many orders of magnitude from say, yellow. This is why you see blue, and more commonly, red, lights as "night" light sources.
The general reasoning: our eyes evolved with a single primary light source: the Sun. Which has quite the yellow tinge to it. Our eyes adapted to this, and as such, gave yellow the highest sensitivity and drifted off in a rough bell curve from there.
It was an interesting article, and certainly put the RGB sensitivity into perspective, but ... it's not entirely new or surprising, either. Nor does the human eye really respond at RGB -- its response curves (beta, gamma, and rho) more closely correspond to blue, green/yellow, and yellow/orange.
That all being said, thanks for letting us meet Traci. ;)
"All your base are belong to this file I send in order to have your advice."
I realise this competely off topic, but is it ever a small world... I went to high school with the lady in the picture laying on the car (Traci). A year after high school I moved 400km's away to a large city and about 2.5 years after that I ran to her working at the Red Robin a block from my house. Now I see her picture on Slashdot of all places, whats the chance of that happening? :)
To top it off, the guy who apparently owns the website (gamesx.com) runs (or ran?) a console game rental store in my home town, and used to date my sister!
Open Source Time and Attendance, Job Costing a
I'm red-green colorblind, and the pictures on the LEFT, with the low-resolution red images, look as good to me as the original or the one with the low-res blue. Does anyone else notice this?
IIRC, the irises are actually opaque; the only light that gets through to the retina passes only through the cornea and pupil. Colored contacts, likewise, don't color light because they have a transparent "dot" in the middle of the lens.
Beyond that, I also remember reading that it's actually the brain that does all of the color (and gamma) correction; nothing in the eye's machinery--it's all done in the (pre?) processing.
Our eyes suck at seeing blue for an even better reason - there are very few blue things that:
From an evolutionary perspective, that's the most important thing. We're get good at seeing green, because many green things are edible, and some things that want to kill us are good at hiding in green areas. So people who were especially good at seeing movement in green areas, and finding edible green things tended to survive, while those who didn't died out.
"How perfectly Goddamn delightful it all is, to be sure" Charles Crumb
Not necessarily.
SCART connectors are huge chunky things that can handle a number of video formats, including RGB, S-Video and Composite (maybe others too). But that's not the same as saying that a given SCART cable or socket will support all those formats. Many cheaper cables only support Composite (fewer wires means cheaper cost). And on some high-end TVs with multiple SCART inputs, only some of those will support RGB.
So if you're playing your PS2 or whatever through a SCART cable, the TV might be using the SVideo or Composite signal rather than RGB.
The lesson is, be sure to check your TV inputs, and always buy good quality cables!
It's only RGB after decoding.
e rt/
NTSC video uses the YIQ color space, very similar to YUV (used in PAL, JPEG, DVD, & stuff). Y is the brightness, which gets the highest resolution, and I & Q (or U & V) are the chroma values, which can be greatly subsampled because they have no effect on brightness (when everything's working correctly).
Most lossy image compression formats involve first transforming the image to the yuv color space. The RGB->YUV transform is also used by many paint programs for things like estimating differences between colors for color reduction & such.
First match on google for "YIQ YUV":
http://astronomy.swin.edu.au/~pbourke/colour/conv
After fooling around with video for quite some time now, I have came to the same conclusion that NEOGEOman gets: Macrovision and the entire industry blows. Sure, we all know that the MPAA sucks, but the drop so low that to mess with the video to the point of almost unwatchable is absurd. Here's a small list of things they do to mess up composite video (NTSC):
- variation of the black level (confuses AGCs)
- phase modulation of the color burst (later macrovision versions, like DVD players)
- removal of lines from one field and putting them on the other field.
- bursts in the VBI
And then the industry refuses to move on until they can get some other "protection" on the video feed. Who do they worry about? The "Casual copier," "hobbyist," "hacker," "small scale pirate," and the "professional pirate" (DDWG powerpoint presentation [http://www.ddwg.org]). The cost? Remotely decent video and your right to fair use.
Arg!
</rant>
As a side note, if you're interested in chroma sampling and how it can go wrong, check out this page: http://www.hometheaterhifi.com/volume_8_2/dvd-ben
It's an interesting read.
The author knows he knows nothing, but knows more than the hundreds of info-starved fanboys on forums the world over, and knows enough to put together a primer (And I'm repeating myself here) that is an introduction to a subject, not a comprehensive guide.
;)
Also, as far as I know no one pumps "NTSC DV" through an svideo cable, unlress they're way off spec. Svideo is analogue, not digital. Or are you using a non-spec definition of "DV"?
Alright, I'll tell you exactly that. I used a digital camera, and reduced the image to 25% using an interpolated scaling to create an image much closer to 'balanced' than a raw digital image would be. By scaling it this way the image was resampled to the new size by combining and averaging the pixels, negating any effect the camera's CCD had.
According to the histogram the colours are very nearly the same but for some variation in the highs.
What you say is irrelevant for the discussion, really, because a digital camera also uses JPG compression which, as we've discussed, creates a high-res green and low-res red + blue image. That's the other reason I resampled it, it negates the JPG effects and creates a (very lovely) new image without effects from the CCD or JPG compression.
Does that satisfy you?
Actually, it's Never Twice the Same Color. A little less of proper English, but the initialism works cleaner.
You have to focus on different colors differently because red, green, and blue light are all different wavelengths and therefore are refracted slightly differently by your eye (shorter wavelengths, i.e. blue, refract more, longer wavelengths - red - refract less). You have to focus closer in on a red light than a blue light for it to be sharp, if you can even do so.
Something I don't understand is that blue light seems to exaggerate my mild astigmatism; I have a Logitech mouse that drives me crazy if I try to focus on the blue taillight. Red lights I can focus on quite clearly and from further away (I am also nearsighted) than any other color lights.
Fun related trivia bit (and forgive me if this is common knowledge):
If you have a decent old 35mm SLR camera with a normal lens (other lenses may have this too) look at the focus ring. There is a marker for where to line up the focus ring in normal conditions, and then there should also be a little red dot a fraction of an inch to the side of it to show where to line up the ring when shooting on infrared film. You have to focus the lens closer for infrared than for visible light, because longer wavelengths refract less.
This is all related the prism rainbow effect, too.
Oh, I think we've got decades of Y'CbCr to look forward to. it's really an advantageous format in a lot of ways.
First, a 4:2:0 Y'CbCr is half the bandwidth of 4:4:4 RGB. We're a long way away from having half the processing power required, bandwidth, storage, etcetera simply not mattering. My RAID is 2 TB formatted, but I regularly have projects that take up over 50% of the space.
Second, Y'CbCr is a better native space for video processing, since the channels align better with what we want to filter. Luma filters like gamma or contrast are more than 3x faster in Y'CbCr than in RGB, since only one channel needs to be processed. Saturation is more than 6x faster in 4:2:0, since only two channels, each at 25% bandwidth, need to be processed. Plus a lot of filters have to convert from RGB to another color space to run. Y'CbCr is closer to those other spaces, and often doesn't require any conversion. You can say whatever you will about Moore's law, the difference between 4 and 8 real-time layers will matter for a while. Even the audio guys, who have it a lot easier, still run into performance limits with enough simultaneous tracks and such.
Lastly, our entire video infrastructure is build around subsampled Y'CbCr. Never underestimate the lock-in of standards like this. If computer people couldn't kill interlaced video in HDTV, they're never going to kill subsampling for lots of applications. Color video has always been Y'CbCr, and that's how everyone works and thinks for decades now.
That said, Hollywood is likely to pick a >8-bit RGB solution for digital projection. For digital projection, bandwidth is a non-issue, and quality, and quality like that of film. Film guys live in RGB. Plus, it's a win for that industry to have digital cinema be as INCOMPATIBLE with consumer digital video tech as possible, in order to reduce the ease of piracy, and to maintain an advantage of the theatrical experience over home theater.
FWIW, I'm a member of the SMPTE groups working on both video compression and digital cinema.
My video compression blog