screen door effect
What is the screen door effect?
The screen door effect is the occurrence of thin, dark lines or a mesh appearance caused by the gaps between pixels on a screen or projected image. The effect -- which is similar to looking through the mesh or flyscreen on a screen door -- is commonly seen in older low-resolution displays and in virtual reality (VR) headsets.
What causes the screen door effect?
The gaps between pixels in a display cause the screen door effect. The most common liquid crystal display (LCD) technology uses small (thin-film) transistors (TFTs) to block or allow light through from a back light. Each transistor uses a filter to allow a single color -- red, green or blue (RBG) -- through. These subpixels work together to give the illusion of every color in a small square area, called a pixel. These pixels can't be right next to each other; a small gap is needed between them to allow for manufacturing and wiring. In older displays, these pixels could be quite large, making the gaps between them noticeable. In modern high-DPI displays, the pixels and gaps are so small and tightly packed that it's very difficult to perceive and almost eliminates the screen door effect.
The subpixel arrangement can cause the screen door effect. Traditionally, the red, green and blue subpixels in a display are the same size and equally spaced. In other displays, mainly televisions, the order is changed to blue, green and red, causing color-fringing. In some display technologies, most notably Samsung's PenTile displays, the subpixels are unequal in size and in a different arrangement. This can cause some people to perceive the screen door effect.
Lower-cost digital projectors also use an LCD to create the image. The screen door effect can occur if the projected image is too large compared to the resolution. Digital light processing (DLP) projectors use microscopic mirrors to generate the image, but again, the gaps between the mirrors can cause the screen door effect. High-quality projectors may use micro-lenses to correct for possible pixel gaps.
In VR and augmented reality (AR) headsets, a normal LCD screen is magnified and focused by lenses, which can cause the screen door effect to become more prominent. Therefore, the screens in VR headsets must be extremely high resolution and have a high DPI. Because the screen door effect is a matter of perception, it can vary from person to person and from headset to headset.
How to minimize the screen door effect in VR and XR headsets
Three main factors contribute to the screen door effect in VR/AR headsets: the screen, the lenses and the person.
The screen is built into the headset and can't be changed or adjusted. Choosing a headset with a higher resolution and DPI display can minimize the screen door effect.
A focusing lens sits between the display and a person's eye in a VR headset. The headset fit, location and pupil distance should all be adjusted so they match the person. The eye should line up with the middle of the lens. Looking through the lens off-axis can cause fringing and magnify the screen door effect. The lenses should also be clean. If a person wears corrective lenses, they may need to find a pair of glasses that fit properly with the headset.
The screen door effect is a visual perception by a person. By focusing attention on the content rather than the display itself, the screen door effect can become less noticeable. In simple scenes with big blocks of a solid color, the screen door effect will be more prominent; viewing content with greater graphical fidelity and complexity will lessen the effect.
See also: 360-degree VR, enterprise uses for virtual reality, use cases strengthen with AR, VR