Reblogged from MaximumPC
Posted 05/18/10 at 06:03:33 PM
by Dr. Raymond Soneira
Color gamut, which is the range of colors that a display can produce, is undoubtedly the most misunderstood and exploited spec—precisely because it’s natural to believe that the range should be as large as possible. While that’s true for most specs (even when they’re exaggerated), it’s definitely not the case for the color gamut.
The color gamut that you want on all of your PC monitors, laptops, HDTVs, and even smartphones is the same color gamut that was used when the content you’re viewing was created. If a different gamut is employed, you’ll see different colors than you’re supposed to see.
Virtually all consumer content is created using industry standards that specify the exact color gamut to be used. For computers and digital cameras it’s sRGB. For digital HDTVs, it’s called ITU-R BT.709 (often referred to as Rec.709). Fortunately, both of these standards specify the same exact gamut. Yes, there are other color gamuts for specialized applications (more about that later), but sRGB and Rec.709 cover virtually all consumer content, and that is the color gamut you want on all of your displays. The color gamut in these standards specifies the exact color coordinates for the three red, green, and blue primary colors, which are used to produce all color mixtures on screen.
Now that you’re versed on what color gamut is, let’s share what it isn’t—as illustrated by six examples of manufacturer misinformation.
Bigger Isn't Better
One common misconception frequently exploited by manufacturers is that a wider color gamut indicates a better display that produces more realistic colors. This is absolutely wrong. A larger gamut will simply make all of the screen colors for standard production content appear more saturated than they ought to appear. Indeed, displays claiming more than 100 percent of the standard color gamut simply can’t show colors that aren’t in the original source image. Expanded gamuts are just gimmicks that make consumers think they’re getting something better.
The Perils of Recalibration
If you do get a display with a larger color gamut, it’s necessary to reduce the gamut back to the sRGB/Rec.709 standard values by adjusting color saturation via a user control. Unfortunately, if the display isn’t calibrated at the factory to match the standard color gamut, it’s unlikely you’ll be able to visually adjust it properly yourself. This kind of adjustment typically requires professional calibration using instrumentation.
NTSC? Never!
The often-quoted NTSC Color Gamut is from 1953. It’s also obsolete and irrelevant. Computers, digital cameras, and HDTVs use the sRGB/Rec.709 color spaces, and specs should refer to them instead of NTSC. As explained above, values greater than 100 percent of the standard color gamut aren’t desirable—unless you like punchy, unrealistic, oversaturated colors.
Adobe RGB or Not to Be?
As stated above, there are specialized color gamuts for specialized applications, and some of these are larger than the sRGB/Rec.709. Adobe RGB, one of the more common ones, is used by imaging professionals and you’ll find it as an option on some digital cameras and scanners. Just be aware that if you use the Adobe gamut, you will also need a display that produces the Adobe gamut, and only a small fraction of consumer displays can do this. If you display an image produced with an Adobe gamut on a monitor with a standard sRGB/Rec.709 gamut, the colors will be incorrect and oversaturated.
How the Eyes Play Tricks
Adobe RGB is a larger gamut than sRGB/Rec.709, but be aware that for most applications, gamut size doesn’t matter very much. The further out you go in color space, the less frequent the colors appear in nature, so the human eye doesn’t notice that they’re not quite right except in rare circumstances (like when viewing a full-screen rendering of a very red tulip). When faced with a gamut beyond their rendering range, displays simply wind up reproducing the closest most saturated color they can under the circumstances.
Bit-Depth Misconceptions
Manufacturers will also dupe consumers by advertising useless and misleading specs about the number of screen colors produced by their displays. Screen color counts have absolutely nothing to do a with display’s color gamut, though manufacturers will attempt to tie them together. In reality, a display’s maximum number of colors is a function of the total number of intensity-level combinations that the device can produce.
Let’s do the math. Standard 24-bit color has eight bits per primary color, and eight bits generate 256 intensity levels. Because there are three primary colors, the number of possible color combinations is 256³—16.8 million colors. Now, if a manufacturer uses 12-bit color processing internally within the same display, there are (in theory) 4,096 intensity levels and 68.7 billion possible colors. Sounds impressive, yes, but the display’s color gamut remains the same as before and the additional number of colors doesn’t mean anything visually.
Still not convinced? First, remember that essentially all consumer content is 24-bit color. Thus, the source images have only 16.8 million colors, and the display can’t “invent” intensities and color combinations that don’t exist in the original.
Second, true onscreen 24-bit color does a good job of meeting the human eye’s color and brightness discrimination abilities. You can read more about that here under “Digital Granularity”:www.displaymate.com/ShootOut_Part_3.htm.
Third, be aware of the real reason why additional processing bits are necessary. Onscreen intensity levels are not supposed to be linear. Rather, they should follow a standard gamma curve with a nonlinear mathematical 2.2 power-law exponent (meaning the screen brightness for any sub-pixel varies as s²·², where s is the input signal intensity level 0-255). The extra processing bits are necessary just to get the display to produce the gamma curve accurately on screen.
Ending Display Fraud
It’s both shocking and sad that display specs have been exaggerated to the point of meaninglessness. And you’re not the only one who suffers—innovative manufacturers that develop new and better display technologies can’t trumpet their hard work with superior performance specs. Instead, they’re forced to play the game or lose significant business.
The National Institute of Standards and Technology (NIST) could help, but its display division was terminated in 2009. The only realistic solution that I see is the creation of an organization (that is completely independent of the manufacturers) to develop a set of straightforward, objective standards for measuring and advertising display specs. Manufacturers that meet those standards would be allowed to advertise their specs with a special controlled trademark, like the EnergyStar program. Consumers would learn to only trust specs with that trademark.
I proposed this back in 2003, but it went nowhere because too many manufacturers resisted the idea. But it’s high time for this solution to finally be implemented—or just imposed. It’s in everyone’s interest except for the subset of manufacturers that can only compete using fraud.
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