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."

Suck at blue something horrid.

[danamania]

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 :)

Wow, what a small world...

[IpSo_]

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!

Wow.

[TheSHAD0W]

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?

Even better reason...

[K8Fan]

Our eyes suck at seeing blue for an even better reason - there are very few blue things that:

1. You can eat.
   
2. Can eat you.

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.

Re:Obvious Physics

[Anonymous Coward]

You have been misled.

It is in fact BLUE at 445nm that the eye is most sensitive to. Blue receptors are the most sensitive.
This "sucking at blue" thing has nothing to do with sensitivity of the receptors, but with the fact that only 2% of the cone receptors are the blue sensitive ones, so you have no resolving capability in the blue part of the visible spectrum.

This is an issue of resolution NOT sensitivity.

Furthermore, I have been researching vision for about 10 years now, and I can tell you that the curve you linked to is totally fucked up. The leftmost curve is not far enough to the left at all. 445nm, which is what your blue cones are sensitive to is far more purple than that stupid graph would have you believe.

You need more reputable sources.

Re:Wow.

[NEOGEOman]

I'm not colourblind at all, and I consider my vision to be pretty good. The difference on the red-reduced image is less noticable than the blue. It varies though depending on the source image and reduction method, where green is always noticable and blue almost always invisible.

Fun and depressing

[pc486]

<rant>
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-benc hmark-special-report-chroma-bug-4-2001.html

It's an interesting read.

Re:Obvious Physics

[kaphka]

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.

No. Red lights (certainly not blue) are used in low-light situations, e.g. a ship's bridge at night, because the photoreceptors that are used in scotopic conditions are most sensitive to short wavelength ("blue") light. If you stepped out of the starlight outside and into a room with a blue (or white) light source, the rods in your eyes would immediately be saturated, and it would take up to a half hour of darkness before your night vision was fully restored. However, red light is way over at the other end of the spectrum, so it has no affect on your night vision.

Re:Suck at blue something horrid.

[Xzzy]

What I find especially straining is the default colors for ls --color, at least under fairly recent linux distros.

Compressed files get a bold red, directories get a slightly dimmer blue. I use a black background on my xterms, and I've found that when I try to read a directory that has a lot of both I'm constantly having to refocus when I go between blue and red areas.

It's annoying enough that on any new machine the first thing I do is change my alias for ls to no colors. :p

I wonder if this is something that could be considered a genetic defect of some sort, or does everyone react that way to blue/red?

Re:Suck at blue something horrid.

[robwmc]

The color blue exists at the low end of the spectrum of visible light. The bluer the color, the harder it is to focus on. The wavelengths are so short that they won't focus on the retna properly, hence blue is blurry.

It is a common thing to see a deep blue color and not be able to distinguish edges but notice a "glow" around the colors.

Oh great, now I can't find my keys.

Re:Good explanation but...

[NEOGEOman]

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"? ;)

Re:Suck at blue something horrid.

[zakezuke]

Wavelenth issue? As in better manual focus lenses have an infrared mark on them that basicly you focus normal, and step it back a notch for IR to be in focus. Whether or not the human eye much focus diffrently for objects closer up showing diffrent wavelenghs is beyond me.

I do know that in order to detect red from green I see the effect of the surrounding, as in green reflects more light then red does... as in green leds are annoying cause they make the whole room bright... but the red ones do not.

Being color blind, i've studdied this quite a bit. I find that i'm fond of purple text rather then blue dispite the total lack of contrast between the two. I'm the one who made purple british flags in gradeschool didn't understand what I was doing wrong. Stupid unlabled markers. Red text on black is my worst color combo, can't see the contrast usually.

I find i'm better with blues then others, but never the less, i'm familar with this lack of focus esp when the blue is on contrasting background, yellow/green/red. I know on my old amiga... my text colors were hard for others to see, as I picked what I considered to be high contrast in my eyes... blue / black / green. No other bugger could read the blue I picked.

It's due to wavelengths

[SirNarfsALot]

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.

Words

[cirby]

I have one odd little hobby... I collect foreign language dictionaries. One of the funny things you notice when you browse through languages is that the less "sopisticated" ones have fewer color words. Some of the lesser-known tribal languages have one word that stands for both blue and green, because the difference is really not very important to the average guy living way out in the middle of nowhere.

The more urban/technical a culture is, the more words for color the average person knows.

Re:Chroma off by one frame?

[NEOGEOman]

Sounds like your Tivo works like my capture card, which only captures at 30fps. This makes it very difficult to get clean results for anything but normal live-action video, because every second frame is blurred together with the first. What you describe could be the next frame blurring into the previous one, ahd perhaps a de-interlacing controller is eliminating sharper details and leaving only the blended colour?