sYCC Color Space
© 2005 KenRockwell.com

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INTRODUCTION

Now that everyone knows what sRGB means (ITU-R BT.709 primaries, 2.2 gamma, 6500º Kelvin, no luma spec but try 30 fL for consistency with video) we have a new sYCC space in some digital cameras.

"YCC" is video terminology that's been around for decades. It's new only to photographers. It's old hat to video design engineers. It's already inside your TV, every JPG and every color video device ever made. A variant has been in Photoshop forever as the Lab (CIE L*a*b*) color space.

YCC transmits or represents the three Red, Green and Blue (RGB) channels as a luminance (Y) channel (Y) and two color-difference channels, Cr and Cb. I explain color-difference signals below.

sYCC is simply YCC created from sRGB. YCC is just a shorter way of saying YCrCb.

The advantage of YCC over RGB is that it can reduce overall file size by reducing resolution of the color channels without altering the apparent resolution image, since the main Y (luminance) signal is untouched. This allows better images for the same file size.

You'll most likely see this as a TIFF format option. I think it was even on my antiquated Nikon D1H.

sYCC saves file size for otherwise huge TIF files. It makes no sense to select it for JPG since JPG already incorporates it for free.

You don't lose anything in a digital camera, since the real resolution is limited by the Bayer-pattern of the CCD anyway. sYCC allows more efficient use of memory space with no loss of quality.

HISTORY and BACKGROUND

Back in the early 1950s some very brilliant men designed an analog data compression scheme which we know today as color TV. It allows the transmission of the three discrete Red, Green and Blue (RGB) color channels in just one TV channel. This was hot stuff back in the 1950s: many other proposals suggested taking up three TV full channels or transmitting R, G and B one after the other with a big synchronized filter wheel spinning in front of a black-and-white TV!!! Even cooler, these guys did it all with vacuum tubes and this system is still exactly what we watch today. Even your DVD player's component outputs are usually marked Y, Cr and Cb.

These clever men realized that people see details much more clearly in black-and-white than as variations of color. Is it easier to read a sign in B/W or in red and blue? Exactly: we can't see much detail as differences in colors of the same brightness. When we can see details it's because of brightness differences between the colors.

The original 1940's black-and-white signal is called Luminance, or "Luma," and denoted as "Y."

By combining R, G and B into one channel as Luma, or Y, only one channel needs to be transmitted at full resolution.

Two color difference channels are created by comparing the colors to the Luma signal. These are comparison, or difference, channels. We call them color difference signals. The two color difference channels are called Cr and Cb.

We put R, G and B through a matrix to get Y, Cr and Cb. Vacuum tubes did it with analog circuits. Digital cameras do it with matrix multipliers.

The Cr and Cb color difference channels can be transmitted or stored with much lower resolution and no one notices. Television has done this continuously since 1954! One of the two color difference channels in broadcast TV has only one-tenth the resolution of the Y channel! Even Super VHS tape has only one-tenth the color (chroma) resolution as luma resolution, and everybody loves it. Television transmits the details in B/W and paints fuzzier colors over the base B/W image for color and it looks great. This is what we've been watching unchanged since the 1950s.

YCC gives you full resolution for test charts and almost everything that matters. YCC looks great and is in use today for every kind of broadcast television, analog and digital. Even DV cam and every JPG uses it. It's everywhere.

You can't see anything in native YCC. It has to be converted back to RGB for display.

YCC works fine in full resolution, even for the two color channels. YCC merely provides the framework for efficient reduction of data if the designers desire. For instance, in Photoshop's Image Ready save-for-web, colors are full resolution at JPG quality settings of 51 and higher, and at lower resolution at 50 and lower.

Mathematical Asides

For you mathematicians, you get Y, Cr and Cb from R, G and B this way:

Y   = 0.299R + 0.587G + 0.114B (Luminance or B/W signal)
Cb = 0.564(B - Y) + 1/2 full scale (a color difference signal)
Cr = 0.713(R - Y) + 1/2 full scale (the other color difference signal)

This gets complex fast when you start taking gamma correction, scaling and everything else into account. These are beyond the scope of this article.

In broadcast television and composite video one passes the Cr and Cb channels through bandwidth limiting filters and then on to modulate the I and Q quadrature channels of a dual-sideband suppressed subcarrier. This subcarrier is added to the Y signal and broadcast, or sent in and out the yellow-colored RCA jack on your video gear.

Much more excellent and practical information about digital imaging and video is contained in the video application notes at Tektronix.

The specification and definition for sYCC may be seen here. It gives you the precise coefficients but skips defining YCC.

As an interesting aside, the color primaries of sRGB space are ITU-R BT.709, the same colors as HDTV but not regular TV!