Are tablets up to the task of accurate color testing?

Finally getting around to posting a follow-up to a follow-up to John The Math Guy’s recent series on color gamut size, colorblindness and tablet displays. I thought I might be able to at least shed a little more light on his question about the differences in color accuracy between some of these devices.

In his testing, John found no statistically significant difference in scores among different people taking the EnChroma colorblindness test on different devices. I found this somewhat surprising since, in my experience, even tablets with similar color gamuts tend to show colors with very different levels of accuracy.

iPad mini color gamut and Gretag Macbeth colors against sRGB in CIE1976

To show what I mean by that, I measured how two different tablets show the colors found in the Gretag Macbeth color checker chart.Nexus 7 color gamut and Gretag Macbeth colors against sRGB in CIE1976

As you can see, the iPad mini and Nexus 7 each produce very different colors, even for those colors that are actually inside their gamuts.

For example, even though the iPad mini has enough gamut coverage to accurately display the Gretag chart’s deepest blue, it cannot do so without distorting the image in another way. This is because of data in the underlying image standard- most content today is encoded in the sRGB standard. If the iPad were to show that Gretag blue correctly, it would not have enough color saturation headroom left over to show you a different color if a deeper blue, say right at the bottom of the sRGB triangle, were called for.

A good real world example of this can be found in the picture below of my bloodhound, Louisa, racing down the beach at Carmel, CA. The middle of the sky in this image is right on the edge of the iPad’s color gamut, very similar to the Gretag blue in the charts above, while the deepest blues found in the ocean fall outside the iPad’s gamut.

Out of gamut colors at beach

If the iPad were striving for accuracy at all costs, it might map both colors right on top of each other at the edge of the gamut. There’d be no visible difference between the two in this case and the quality of the image would suffer but at least the sky would be accurate. In order to avoid this scenario, the designers of these devices have decided to compromise on accuracy so they can show a full range of color differences to the user.

They do this by remapping colors inward, away from the edges of the gamut, effectively compressing the gamut even further so that otherwise out-of-gamut colors can be seen. This is a good solution given the gamut limitations of the device since it results in more pleasing, if less accurate images.

As newer devices trend towards wider color gamuts this kind of compromise should become a thing of the past. In fact, tablet designers may be working on the reverse issue- how to avoid oversaturating images that were encoded for smaller gamuts.

Great, how does this relate to colorblindness again?

iPad mini vs Nexus 7 color accuracy comparison in CIE 1976

iPad mini vs Nexus 7 color accuracy comparison in CIE 1976

Taking another look at the Gretag results from the two devices plotted on top of each other, there clearly are major differences. But, in the reds and greens, two colors associated with a common form of color blindness, the devices are relatively close. So, the simple answer may just be that colorblindness tests do not require pinpoint accuracy to be effective, at least as basic screening tools.

Color of the year for 2013 falls outside sRGB gamut

Pantone Emerald 17-5641

Pantone recently announced their color of the year for 2013, a deep shade of emerald green that they call “Emerald 17-5641.” It’s a great color but there’s a catch- most displays cannot accurately show it.

Based on data from Pantone’s website, I was able to plot the color in CIE 1931 (xy). As you can see in the chart below, Pantone’s color is well outside the sRGB/rec.709 color gamut standard used by most HDTVs, the new iPad/iPhone and many desktop monitors. These devices will be stuck showing a version of Pantone’s emerald green that’s less saturated and probably a bit more yellow than the real thing.

Pantone Emerald 17-5641 vs sRGB, Adobe RGB 1998 and DCI-P3 color gamuts in CIE 1931

This is a perfect example of a popular real-world color that falls outside of the sRGB/rec.709 gamut. Unless you have a monitor that’s able to show wider color gamuts, like the DCI-P3 or Adobe RGB standards, you are missing out on a great color.

So you bought a 4K TV, now where is the 4K content?

Content is king. One of the biggest challenges for emerging display technology is content availability. Whether it’s 3D, 4K or wide color gamut, these new features simply aren’t worth much without access lots of great, optimized content.

As new 4K TV’s begin hitting store shelves this year, they are entering a content vacuum.

Standards bodies like the Consumer Electronics Association (CEA) and International Telecommunication Union (ITU) are still working out the precise definition of marketing terms like Ultra High Definition TV (UHDTV). Proposed standards could include support for eight million pixel resolution, extremely wide color gamut and 3D content. But, today, there is almost no content out there that takes full advantage of all of the exciting capabilities of the new sets.

And, unlike the transition to HDTV, there’s no government-mandated switch on the horizon to force broadcasters to get on board.

CIE 1931 rec.2020 vs rec.709

At least one set-maker is taking it upon themselves to solve this problem by delivering both the 4K content and hardware. Sony announced last week that it will loan a 4K Ultra HD video player loaded with UHD content to buyers of their new 84” UHD television. The selection of 4K content on this player is fairly limited for now, but as more titles are released, this approach could help drive adoption of high resolution and wide color gamut formats.  I wouldn’t be surprised if other set makers started following suit, though Sony does have an inherent advantage, owning a movie studio.