The big picture (Colorspaces VIII)


A few posts ago, I said that while the colorspaces looked random, they really weren’t, and that there was underlying order. The structure cannot be easily seen just by looking at the numbers themselves, but at how the numbers are obtained.

The story begins sometimes in the 1950s, were transmitting color TV images started to be the next logical step. Someone (not sure who was first, but it may have been Valensi, in the 1930s) proposed that TV color should be encoded in a perceptually friendly way [1]. It was known for a while that the retina had four types of sensors, rods for brightness with no color information, and three other types corresponding to red, green, and blue, but also that in, and beyond the retina, information travels as brightness, yellow-blue and red-green differences [2,3].

Read the rest of this entry »

YCrCb and friends (Colorspaces VII)


JPEG, MPEG, and other compression algorithms all use a colorspace other than RGB. The colorspaces they use are such that most of the perceptually useful information is concentrated into one component, essentially brightness, and the color information diffused into the remaining components. Furthermore, we hope that we can heavily quantize the color information. JPEG separates the image into brightness and two other components: brightness is coded full resolution, but the two other components are downsampled 4:1. Yet, it’s not visible in the reconstructed image, because our eyes are good at brightness, but not at chrominance.

But that’s not a surprise. All the colorspaces we’ve seen so far do this, and (are believed to) have the same properties, grosso modo.

Read the rest of this entry »

YDbDr (Colorspaces VI)


Also an analog colorspace, YDbDr was used in SÉCAM (quentielle ccouleur à mémoire), the French analog TV standard that was also used in Germany, Greece and the Middle east, among others. YDbDr, like the others, use the luminance, Y, and two chrominance components: Db, Dr (for différence bleue and différence rouge).

Read the rest of this entry »

YUV and YIQ (Colorspaces V)


Not all colorspaces are inherently digital, and in fact, most were first conceived as, and for, analog means. Let’s have a look at two of those colorspaces, YUV and YIQ. The YUV colorspace was used for the European analog TV standard, PAL, while YIQ was used for the North American and Japanese standard, NTSC.

Read the rest of this entry »

Xerox YES (Colorspaces IV)


It seems pretty much everyone tried their hands at designing a colorspace. One such example—essentially forgotten today—is Xerox’s YES colorspace. I am not sure what problem this colorspace tries to solve:

\begin{bmatrix}  0.253 & 0.684 & 0.063\\  \frac{1}{2} & -\frac{1}{2} & 0\\  \frac{1}{4} & \frac{1}{4} & -\frac{1}{2}\\  \end{bmatrix}  \begin{bmatrix}  r\\  g\\  b\\  \end{bmatrix}  =  \begin{bmatrix}  Y\\  E\\  S\\  \end{bmatrix}

The first line, yielding Y, gives something close to the brightness, but it differs from most other colorspaces that will use 0.299, 0.587, and 0.114 to compute brightness. Are Xerox YES’ values gamma corrected? Or relative to some other space than RGB?

Read the rest of this entry »