Smearing

Smearing is the perception of motion blur that reduces the sharpness and detail of the image in VR. Smearing along with strobing is part of the judder phenomenon in head-mounted displays. Because smearing reduces the visual quality of the display and can even cause simulator sickness, it can ruin a user's immersion.

The effect comes from the way displays show moving images. Each frame is held on screen for a period called the persistence time, and when the eye moves relative to the display during that time, the lit pixels slide across the retina. In a 2013 series of posts on head-mounted display perception, Michael Abrash of Valve described judder as "a combination of smearing and strobing that's especially pronounced on VR/AR HMDs". Oculus VR cited the elimination of motion blur and judder as the reason its Oculus Rift DK2 used a low persistence OLED display, and later headsets such as the HTC Vive and Valve Index also light their pixels in short pulses.

Reviewed 27 September 2026. Checked every claim against the two 2013 Abrash Valve blog posts, his 2014 Steam Dev Days slides, the 2014 Oculus DK2 announcement, the Valve Index headset page, the Kreylos black smear post, and the Zhao et al. 2022, Chen et al. 2018 and Gou et al. 2018 papers. About review dates.

How it works

Smearing occurs when each pixel moves across the retina while it is lit. The longer the pixels are lit (full persistence) and the more movement of the HMD (quickly turning your head), the more smearing occurs.

Abrash explained the mechanism with space-time diagrams. A display updates each pixel once per frame, and the pixel keeps a constant color for as long as it is lit. When the eye is fixed on a point and the image is static, this causes no problem. When the eye moves relative to the display, however, the illuminated area of each pixel "sweeps a constant color across the retina for however long it's lit", producing a smear. At the start of the next frame the object is drawn again at its correct position, so the image snaps back and the pixels start to slide again.

The eye can move relative to the display while still seeing clearly in two situations: when it tracks a moving virtual object, and when it stays fixed on a virtual or real object while the head turns, held in place by the vestibulo-ocular reflex. Abrash noted that the head-turn case can involve relative velocities of hundreds of degrees per second between the eye and the display, and so very long smears. A 2022 study of VR displays by researchers at the U.S. Food and Drug Administration described the smooth-pursuit case in the same terms: because the display image is static within a frame, the relative motion of the tracking eye "results in motion blur that is highly associated with the temporal response of the display".

Size of the smear

What sets the length of the smear is the absolute time for which a pixel is lit, multiplied by the speed of the eye relative to the display, not the fraction of the frame for which it is lit. Abrash gave a worked example: a leisurely head turn is around 100 degrees per second, and a head turn of 120 degrees per second on a 60 Hz full-persistence HMD moves the image two degrees per displayed frame. On the Oculus Rift development kit of the time that was about 14 pixels, and on a display approaching the resolving power of the human eye a two-degree arc would cross hundreds of pixels. For the same reason, the effect gets worse as pixel density rises, because the smears grow longer relative to pixel size and more detail is lost.

The FDA study quantified this with validated simulations of the HTC Vive and HTC Vive Pro. For a line moving at 40 degrees per second, the blurred line width (full width at half maximum) dropped from more than 0.4 degrees with a 100 percent emission duty cycle, as on conventional flat-panel displays, to less than 0.15 degrees with the 17 percent duty cycle used in those two headsets.

Pixel response time

Slow pixel switching adds a second source of blur. Abrash wrote that the slow LCD switching times of the Rift development kits made pixel updates follow a ramped curve, which made smears "longer and smoother" and masked strobing, at the cost of even more lost detail than full persistence with fast-switching pixels. Display engineers measure the combined effect as the motion picture response time (MPRT). In a 2018 review, Chen, Lee, Lin, Chen and Wu showed that once liquid crystal response time is much shorter than the frame time, MPRT is set mainly by the frame rate (about 0.8 times the frame time), and that an LCD with a response time under 2 ms has an MPRT comparable to an OLED at the same frame rate. In the experiment they cite, an LCD test cell with a 1.29 ms average gray-to-gray response time reached an MPRT of 6.88 ms at 120 fps against 6.66 ms for an OLED.

Why HMDs are affected more

Smearing appears on other displays too, for example when tracking text while scrolling a phone, but Abrash gave three reasons it is more visible in a head-mounted display. The field of view is much wider, so objects can be tracked for longer; the head can turn far faster than the eyes can normally track a moving object without a saccade, while the vestibulo-ocular reflex still keeps vision clear; and virtual images in an HMD appear to be in the world rather than on a surface, so the visual system expects more of them.

Relation to judder and strobing

Abrash used "judder" for the combination of smearing and strobing, which he compared to "a choppy motion blur". Strobing is the perception of multiple simultaneous copies of an image, and it can start when an image moves more than about five to ten arc-minutes between successive updates. On full-persistence displays the smear mostly hides the strobing. Abrash wrote that both artifacts might contribute to eye fatigue or motion sickness, but described that point as speculative.

Removing the smear exposes the strobing. In his follow-up post Abrash reported that on low persistence prototypes the object the eye is tracking does not strobe, because its pixels land in the same place on the retina each frame, but the rest of the scene can. His team also found a "visual instability" effect on a low persistence prototype, in which a rapid gaze shift made the whole virtual room seem to move; it vanished when the display was switched back to full persistence.

Reducing smearing

Smearing can be eliminated by either having a very high refresh rate or using a low persistence display, the more practical method.

Higher refresh rate

Raising the frame rate shortens the time each frame is held and so shortens the smear. Abrash wrote in 2013 that 100 Hz is "nowhere near enough" and 200 Hz would be a significant improvement but still not enough; he estimated that the sweet spot for 1080p at a 90 degree field of view is probably somewhere between 300 and 1000 Hz, with higher rates needed at higher resolutions. At 1000 Hz, full persistence is only 1 ms, short enough to eliminate judder in most cases, but he considered such displays, data links and rendering rates impractical. Chen et al. found that the benefit of a higher frame rate gradually saturates: for an LCD with a 10 ms response time, going from 30 to 60 fps sharply reduces MPRT, while further steps to 120 and 240 fps give smaller gains.

Low persistence

A low persistence display lights each pixel for only a small part of the frame, at higher intensity to compensate. With little eye movement during that short flash, the pixel barely moves across the retina. Abrash wrote that persistence "below roughly 2 ms, maybe less at 1080p with a 90 degree FOV" should almost completely eliminate the smear component of judder, and that experimental prototypes confirmed it. In his 2014 talk at Valve's Steam Dev Days, he listed low pixel persistence of no more than 3 ms as "necessary to avoid blurring with eye motion", together with a refresh rate high enough to avoid flicker, for which 95 Hz seemed sufficient.

On OLED panels the short pulse comes from the emission cycle of each pixel; on LCDs it comes from switching the backlight quickly. Chen et al. describe reducing the duty ratio, the on-time ratio of the backlight in a scanning or blinking backlight, as the usual way to bring MPRT below 2 ms, with a brighter LED backlight to make up for the lost light. Gou and colleagues reported in 2018 an LCD for VR that reaches an MPRT of 1.5 ms when driven at 90 Hz with a 17 percent duty ratio, which they described as comparable to a cathode-ray tube.

The FDA study concluded that reducing the emission duty cycle below 20 percent helps mitigate motion blur in VR headsets, and that once a headset uses such short pulses, raising the refresh rate from 90 to 120 Hz does not significantly change its motion performance; with continuous emission, by contrast, going from 60 to 120 Hz gives a clear improvement. The authors also noted that content frame rate should match the display refresh rate, since repeated frames add blur from "replicated shadow images".

Persistence in VR headsets

The table lists persistence figures that manufacturers have published or researchers have measured for several headsets.

Headset	Display	Refresh rate	Persistence or duty cycle	Source
Oculus Rift DK2 (announced 19 March 2014)	Low persistence OLED, 960 x 1080 per eye	Not stated in the announcement	Low persistence, used "to eliminate motion blur and judder"	Oculus VR
HTC Vive	PenTile OLED, 1080 x 1200 per eye	Up to 90 Hz	17 percent duty cycle, pulse width about 1.9 ms (measured)	Zhao et al.
HTC Vive Pro	PenTile OLED, 1440 x 1600 per eye	Up to 90 Hz	17 percent duty cycle, pulse width about 1.9 ms (measured)	Zhao et al.
HTC Vive Pro 2	RGB LCD, 2448 x 2448 per eye	Up to 120 Hz	5 percent duty cycle, emission time 0.42 ms per frame at 120 Hz (measured)	Zhao et al.
Valve Index	Dual 1440 x 1600 LCDs, full RGB per pixel, global backlight	80, 90, 120 or 144 Hz (144 Hz experimental)	Illumination period of 0.330 ms to 0.530 ms depending on frame rate (0.330 ms at 144 Hz)	Valve

Valve states that the Index's shorter illumination period keeps imagery "just as sharp when your head is in motion as when you're standing still" and calls it a 5x improvement over first-generation PC VR headsets.

Black smear

A related artifact on OLED headsets is known as "black smear". Oliver Kreylos, writing in 2014 about the Oculus Rift DK2, explained that although OLED pixels switch orders of magnitude faster than LCD pixels, they cannot switch instantly. When the eye follows a bright object on a dark background during a head turn, the pixels at the object's leading edge that must change from dark to bright appear darker for a moment, leaving a dark smear; a fainter "white smear" appears at the trailing edge. He noted that the effect is hardly visible on an ordinary OLED screen and becomes obvious only when the display is fixed to the head, because of the vestibulo-ocular reflex.

The fix Kreylos described, which he attributed to Oculus, is overdrive: a pixel whose brightness must change between frames is driven past its target value so that, after its response lag, it lands close to the intended brightness. In the DK2 the overdrive factors were 0.1 for rising brightness and 0.05 for falling brightness. Because a pixel cannot be driven below black or above full white, overdrive fails at the extremes, so Kreylos also compressed the brightness range slightly; for the DK2's factors this cost 13.85 percent of contrast, which he found hardly noticeable in use.

See also

• Judder
• Strobing
• Persistence
• Low persistence
• Refresh rate
• Motion-to-photon latency
• Screen door effect