Why Fuchsia needs color management

There is no substitute for color management. Unfortunately, Android has a major color management problem.

Ultra HD (UHD) is a generational advance in consumer display standards. UHD marketing generally refers to the combination of high dynamic range (HDR) output, an expanded color space beyond sRGB, and a minimum resolution of 2160p (4K). While I intend to write an introductory HDR article in the future, today I want to focus on the many technologies required in order to correctly render UHD content.

There are only two color gamuts permitted for devices and content to be certified by the UHD Alliance: DCI-P3 and Rec. 2020. I am specifically referring to the gamuts, not the color spaces. In order to support an HDR-capable panel, a display vendor also has to utilize an HDR-capable DDIC (display driver). And in software, there will be a separate ICC profile specifically for HDR. (HDR for mobile may also require local tone mapping, but I’m not knowledgeable enough to know the details.)

Android gained "HDR" support in Nougat for Android TV. While strictly speaking this is true, I put HDR in quotes because there is no color management. What Google means is that Nougat supports the HDR10, Dolby Vision, and HLG electro-optical transfer functions (EOTFs), aka non-linear gamma. This almost works.

UHD content currently requires a display that targets the P3 color space. If you watch UHD content on a P3 display from, say, Google Play on Android TV, what actually happens is that desaturated frames are presented to the hardware because sRGB is the assumed color gamut, but the TV then oversaturates the image back to the correct P3 color space. The wrong color values get reversed, in other words. The net result will be a correct image, but only on P3 displays. All of the UI controls overlaid on top of the content will, however, be oversaturated.

While LG and Samsung — if LG even manages to correctly calibrate a panel to a target gamut for once — will claim that they are shipping HDR devices with the G6 and Galaxy S8, and the hardware will be technically capable, Android still has no color management. Want to watch a UHD movie in a separate window while multi-tasking with other apps at the same time? Too bad, all of your color and gamma will be wrong. Color (or HDR) modes are not a real solution, despite what Microsoft will tell you.

Extended sRGB, or e-sRGB, is a linear extension of the sRGB color space. This is an available option for some kinds of applications, such as OpenGL apps. Unfortunately HDR will probably break windowed e-sRGB apps on HDR displays due to the non-linear gamma. There is no substitute for color management, and all apps will require it.

It will be interesting to see whether the LG G6 is calibrated for sRGB or P3, because targeting one inherently compromises on the other. Neither strategy will work perfectly regardless, because Android is stuck on sRGB. sRGB remains the de-facto standard for most content, and this critically includes web content. Without color management, there is simply no way to simultaneously support multiple color gamuts. This is why Samsung calibrates to sRGB and includes a separate P3 color mode, even though almost no consumers will ever know or bother to change color modes. At best, this is a very annoying necessity.

The LG G6 will also suffer from color banding, due to insufficient color gradation steps. Even though its display hardware should be capable of 10-bit output, Snapdragon 821 only has an 8-bit display controller pipeline. Sadly, the iPhone 8 will probably be the first mobile device that can support UHD end-to-end, despite other hardware being HDR-capable dating back to 2016’s Note7.

It is not clear to me if adding color management to Android is even technically possible at this point. But for Fuchsia’s new rendering pipeline, there is probably no excuse not to design it with color management in mind from the outset. Mobile UHD devices require it, if they are ever going to actually work. Don’t let consumers down, Google.

 

Updates:

1) I'm extremely happy to say that Romain Guy himself says "there is no reason why color management cannot be added to Android." Awesome! Hopefully these problems will be addressed in Android O then :)

2) Color management is indeed a feature of Android O.