Have you ever looked closely at the image from a digital projector and notice fine black lines between the pixels? What you're seeing is the space between each individual pixel. It's a type of fixed-pattern noise, more commonly referred to as a screen-door effect because it feels like you're looking at the projection through a fine mesh screen.
The screen-door effect is certainly not a new phenomenon — you've probably noticed it if you ever sat too close to the TV as a kid. While the issue of screen-door effect has all but been remedied for digital projectors and newer TVs due to improved technology and calibration techniques, it has re-emerged as an issue for some virtual reality headsets.
Some folks are simply more sensitive to the screen-door effect and can't help but find it distracting.
So why does the screen door effect happen? The screen-door effect occurs when the digital image is scaled so big that you can see the space in between the LEDs — whether that's caused by a lens (i.e. in a projector) or from being too close to the display (i.e. an old Radiation King TV). If you've ever looked at your phone's screen or computer monitor through a magnifying glass (and we'd recommend you do because it's super cool), you'll see the tiny grid of light-emitting diodes (LEDs), whether it's an OLED or AMOLED display.
To see the screen-door effect in your VR headset, actively to focus your eyes straight ahead at the display screen itself, then move your head around; the head-tracking technology will adjust the image to where you're looking while the LEDs remain stationary, making the grid of LEDs you're staring at all the more noticeable. You'll definitely notice it more if you're actively trying to notice it; however, some folks are simply more sensitive to the screen-door effect and can't help but find it distracting.
This is not to be mistaken for aliasing effect, which relates to the game graphics trying to render smooth curves with square pixels. As mentioned, the VR screen-door effect occurs due to a combination of the headset lenses used to refract the displayed images, and the close proximity of the display to your eyes.
You're dealing with a smaller screen, magnified.
Should you be concerned about the screen-door effect ruining your VR experience? Probably not. Concerns about the screen-door effect have been raised over the past few years based on hands-on experience with the Oculus Rift developer kits (DK1 and DK2). The screen-door effect was extremely noticeable and distracting at the time, but seems to have been fairly well addressed for the consumer editions of both the HTC Vive and Oculus Rift.
Sure, it's still there, in the same way it's always been there on TVs and smartphone screens — it's just more noticeable in VR headsets because your eyes are so close to the screen. Fortunately, it's not so distracting when you're in the heat of the action shooting zombies or blasting robots out of the sky with lasers. It is, however, much more noticeable on mobile VR headsets, such as a Samsung Gear VR. Again you're dealing with a smaller screen, magnified via lenses and positioned mere inches away from your eyes. As screen technology for both VR headsets and smartphone screens continue to advance towards 4K resolution (and beyond), the impact of the screen-door effect will be further mitigated.