Colourspaces (JPEG Pt0)- Computerphile 

Yeah, because the terminology refers to the light. The dyes subtract light. He got confused because you're adding pigment to make it darker, and more pigment means more light subtracted

point6000x You're right. An easy way to remember is to ask: How do you get the colour white? Add all colours of light, subtract all ink from a page.

point6000x Wow, you're so smart! Nevermind the fact that he was actually talking about the ADDITION of dye not subtracting of light...

Bob Chap en.wikipedia.org/wiki/CMYK_color_model read first sentence. If that's not what he meant he should have specified better. But it's just a minor mishap, nothing major - just wanted to point it out, no harm intended.

Bob Chap It's clear what he meant but the way he expressed it is contrary to convention. While in the grand scheme it makes little difference, it is probably better to conform to conventions for the sake of educational content imo.

Jimmy De'Souza Colorspaces that behave like light does, are additive, like paint does, are subtractive. This is what the word means in color spaces. He is adding confusion by using the term inappropriately for its intended use.

***** Yes please, do this!

***** I was also hoping for a little more along those lines, plus Adobe RGB, and increasing bit depths, and how RAW images work with all that.

***** YCbCr doesn't necessarily have a smaller gamut than RGB. It is just a way of encoding RGB. Barring any rounding errors or range-limiting, RGB to YCbCr and back is a lossless transform.

***** RGB by itself is inaccurate, because it simply tells the monitor how much red, green and blue light to mix. What it doesn't tell is the exact color of those three light sources. The tone is also an important factor. That's where sRGB and Adobe RGB come in, they precisely describe the base components and the tone. CMYK is even more critical. Every ink is completely different, and it also depends on the type and color of the paper, and the color of the light under which you view the paper. XYZ and Lab are physical color spaces, which means they're precisely defined in absolute terms. Lab is just a non-linear transformation of the XYZ space. When you transform between different calibrated color spaces, you either go through XYZ or Lab as an intermediate space. For example, when you go from sRGB to CMYK, you really go from sRGB to XYZ to Lab to CMYK. Gamut means a collection of all colors that a device can display, or a camera can capture, or a specific image file can carry. Monitors have colors that cannot be represented in paper, and the other way around. Out of gamut means a color that cannot be represented precisely, so it has to be substituted by something else. We call this clipping. It can cause different color values to look identical, which may ruin tones and gradients. sRGB is important, because most TVs and monitors are reasonably close to sRGB, or near identical to sRGB when calibrated. Adobe RGB is important, because high gamut monitors are close to it. ProPhoto RGB is so wide that it cannot even be displayed using today's technology, as far as I know. It's useful when you want to make extensive tonal editing. However, for display purposes it has to be narrowed down to something smaller.

MichaelKingsfordGray You get an instrument called a colorimeter, and measure various RGB values that the monitor can display. Then create a software profile, which tells your operating system and/or image editing software what corrections it needs to make. I do not propose this, it is how photographers and designers do it.

Hundersoft You're right, that's a mistake in the video. And usually you add 128 to the Cb and Cr values to get a non-negative range.

Yes and no. :) 1 byte can represent 256 different values. That is all it can do. The meaning of those bytes is what we want. We can say it is unsigned integer, then it is 0 to 255. We can say it is signed integer, then it will be -128 to 127 as you say. But it can be anything else, if we want it to. For example if you build some science experiment and want to log data about it for long period of time, like temperature, which you know will be between -10 and +50 degrees with granularity of 1/4 degree, you can store it in a byte... 00000000 will be -10, 00000001 will be -9.75, 00000010 will be -9.5... It's only about how you define your format according to what you want to store (and have a means to code and decode the information into that format). So in this case, I would say it's neither from -128 to 127 nor from -127 to 128. Its from "more yellow" to "more blue".

***** in this particular case it's -128 to 127. I'm talking about the part where he was unsure about which one. -128 to 127 is because we interpret the byte using two's complement. If we use sign-magnitude, we will have -127 to 127 with 2 different zeroes: 0 and -0.

0 is 0, not positive. If you are encoding a signal as a signed integer, you would typically ignore the extra negative value, so that the positive and negative limits are the same. Otherwise your signal would end up with a DC offset.

mfaizsyahmi. Well, the next video is going to be about JPEG, which uses the YCbCr color space.

mfaizsyahmi. Well, I guess, but RGB isn’t really that ubiquitous. What happens is that people are less aware of other color spaces. Everything that is printed is CMYK, but how many people do you think know about it? Not many, I’d think. Judging by the fact that the vast majority (or so I believe) of photos on the Internet are stored as JPEG, I’d say YCbCr is far more common than RGB. Although... computer monitors display images as RGB, so even though JPEG images are YCbCr, what you see in your screen is always RGB...

***** and not to forget about RCT and YCoCg. both are "similar" to YCbCr, but are integer reversible, allowing for a lossless to/from RGB (at the cost of needing a larger number range). there are also other YUV spaces, which are like YCbCr (and will generally look "about right" if mixed up with YCbCr), and which mostly differ in that the calculations are tweaked. one example is tweaking the calculations to make them cheaper to calculate, for example: Y=(5*R+8*B+3*B)>>4 U=((B-Y)>>1)+128 V=((R-Y)>>1)+128 (where 'x>>y' means essentially 'y/(2^y)', or diving by a power of 2, but is a lot cheaper to calculate than using a '/', nor does it have the implication of producing a fractional output...).

I hadn't thought about that in a long time, ingenious.

MichaelKingsfordGray Our color spaces are not in any reasonable sense an approximation to our receptor stimulation responses. That we use three primaries, and are also trichromatic, is about as close as the approximation gets. A receptor based color space would look qualitatively similar to CIE-1931; most of the colors would be missing, and the visual color gamut would form strange shapes (this is due to the overlap of cone sensitivities). If you include such colors, colors would likely not have unique coordinates (because color perception begins with the opponent color process, which works off of comparative signal strength rather than pure signals).

MichaelKingsfordGray I'm not exactly a specialist, just interested in the subject. "What is the colour space for human tetrachromats?" That would simply be LMS color space.

MichaelKingsfordGray Ah, my apologies. Tetrachromatic vision is extremely rare--as far as I know there's only one confirmed case. I'm not sure it's been studied enough to create a color space model for it.

therealEmpyre That's right, except that it wasn't YCbCr, but YIQ (NTSC) or YUV (PAL). They are similar to YCbCr, but not identical.

***** If you're using luminosity and hue, you also need to add saturation (HSL or HSV color space). Saturation makes the difference between muddy, pastel and vibrant/cartoonish colors.

HeyHeyHarmonicaLuke Yes, lower resolution = larger pixels.

Penny Lane And that upsets you... I can tell...

Penny Lane Buy a book then, or sign up for a uni-course that goes through this.

Völundr Frey Good lord, what have I done to you that you treat me like that? You never know who people are on the internet, do you? So the benefit of the doubt is more prudent than anywhere. I happen to have a PhD in computer science. So how do you think you basically telling me to go read a book makes you sound like? And based on _what_? The sentence "That was a rather brief treatment of the subject." Seriously.

It's meant to be brief. They're giving a general overview of the subject, not a university lecture.

Penny Lane For future reference: don't criticize anything on youtube, or you will be bitten.

klutterkicker Because that system wouldn't have any way to represent any kind of gray. No 25% gray, no 50% gray, no reddish-gray, no grayish-blue, nothing. To cover all of those colors, you'd need to add a third dimension. You could call it, say, "saturation", and boom: you've just invented the HSV color space. HSV is a valid and useful color space, it's just different from YCbCr.

klutterkicker Because that's not enough. RGB is a three-dimensional color space. If you want to transform it to an equivalent representation that doesn't lose too much information, it also needs to be 3D. Even CMYK is 3D, despite having 4 variables - the K is not linearly independent and therefore does not add a new dimension. Your example of Y plus hue is lacking the dimension of saturation. You could represent a bright red (max Y, hue at red, implicit max saturation), but not a desaturated pale whiteish red (max Y, hue at red, low saturation). And by the way, those color spaces usually can NOT represent all colors we are able to perceive.

Gareth Dean there non-visible.

***** also they are not well defined. for example how do you use a scale baste on blueness to show a color that is more blue than blue.

***** And let's not get started on octarine.

MichaelKingsfordGray That movie.. Oh the pain... the pain...

***** You'd be surprised. Ultraviolet is reported by those with Aphakia (No lens in the eye) as the blue-sensing cones in the eye are sensitive to these wavelengths but they are excluded from the eye by the lens. Likewise our red receptors are sensitive to some of the IR spectrum but as the wavelength gets longer you need a higher intensity for the light to be visible, eventually reaching the point where damage occurs to the eye. This is a natural result of the way our color receptors work.

octavio echeverria Y: 100%, M: 50%, C: 0%, K: 0%. Also, it might seem funny at first, but red is 100% Y and M. EDIT: Wait, that's shown in the video anyway, whoops.

Jari Komppa Yeah that's also one of the reasons why YCbCr was made, so it could be backward compatible with old B/W TVs :)

And B&W TV worked so well because the eye is more sensitive to brightness/lightness than it is to colour.

almost no one uses HSV or HSL

Alex Shannon YPbPr and YCbCr are the same colourspace, YPbPr is analog and YCbCr is digitalized (scaled and rounded)

It's not blacK?

@@TiagoTiagoT read the comment you're replying to and you will get your answer.

***** You can specify in OpenGL a color value using any type from a byte, to a int, to a double, and in GLSL, you can use highp floats for color(which is a waste). The graphics card will always output in whatever the monitor uses, which is virtually always 24 bit color - unless you have some kind of weird deep color monitor. It's a nice feature, IMO.

BinaryHistory​ GEGL... Not GLSL... Not OpenGL...

***** Yes, I know.

BeastOfTraal And how it connected to angle..

Skisful Hue, Saturation and Value, another system to describe color in a numeric way. Munsell, an american painter described this system first.

BeastOfTraal HSV and its relative HSL are just transforms of RGB. They're defined in terms of RGB in ways that make it easier for us to pick out the kind of RGB colour we're after.

***** HSV was the first system to be used in NTSC broadcasting for describing color for color TV's in America. Thing is, all color TV's use RGB at the Cathode Ray Tube. HSV was the human interface.

Dirk Simons Actually, no. NTSC uses YIQ colour space, which is a rotated version of YUV (which is itself basically an analogue version of YCbCr). It's rotated so that the blue-orange axis can be given more bandwidth than purple-green (which gets a more aggressive low-pass filter), giving more precision for skin tones, while still saving some bandwidth compared to sending both at the same resolution, as is the case with PAL. I picked that up in the course of doing some research into how analogue TV worked when trying to write a shader to implement the PAL system and reproduce the glitches.

Yes they sent me some 😉

lucky :P

andrew b Not hearing it.

it's quiet but noticable. confused me for a few seconds :P