Display: 5.5-inch 1080p LCD
The HTC One X9 comes with a 5.5-inch Super LCD with a resolution of 1920 x 1080, equating to a pixel density of 401 PPI. Like most 1080p smartphones I've used, the sharpness and clarity of this display is great. 1080p is now the standard resolution for mid-range smartphones, so the One X9 doesn't really have an advantage over its competitors here, though that's not an issue considering some flagships still opt for the less power-intensive 1080p displays over 1440p.
Aside from the resolution, there's not a whole lot to like about the One X9's display. At 415 nits, peak brightness falls below the ideal 500 nits mark for a smartphone, and this is particularly disappointing considering this phone uses an LCD. Contrast and black levels are both average for an LCD, while viewing angles are below average.
The worst aspect of this display is color performance. Sometimes I forgive poor color performance if the display uses a wider-than-sRGB gamut, as this merely increases top-end saturation, but this is not the case with the One X9's display. This display's gamut actually sits below full sRGB, with 91.5% coverage, so the issues with accuracy are not from a basic increase in saturation.
Instead the issues lie with terrible mid-tone performance. Blue, green and yellow mid-tones are oversaturated significantly, which leads to colors being crushed towards the high end and loss of detail as well as color balance. When you look at the One X9's display, something appears to be 'off' from first glance, and the explanation for this is poor accuracy through the mid-range. On top of that, the display can't reach full green or blue saturation levels.
By default, the display has a cold color temperature like most smartphones these days. This can be corrected through a slider in the One X9's software: adjusting it three notches to the warm end pulled back the color temperature to around the 6600K mark. Unfortunately, this slider does little to address the rest of the display's color inaccuracies, and there is no other way to pull the display in line with sRGB standards.
It's disappointing here that HTC couldn't bring the same qualities from the HTC 10's LCD, or even last year's HTC One M9, down to the price point of the One X9. What we're left with is an inaccurate display that gets trounced by the cheaper Motorola Moto G4 Plus, which packs an LCD of the same size and resolution.