Judder is a mixture of smearing and strobing in a HMD. Judder becomes apparent when the display is moved quickly, such as when you rapidly turn your head. The phenomenon can significantly reduce the visual quality of a VR and AR displays and cause simulator sickness. The Judder is caused by low refresh rate or high persistence of the display. Smearing occurs when each pixel moves across the retina while it is lit. After smearing, the slight jump causes strobing.
How to Reduce Judder
Judder can be reduced by increasing the refresh rate of the display. When refresh rate is high enough, smearing and strobing will not longer be visible to the human eyes, effectively eliminating the phenomenon. To truly eliminate smearing, refresh rate of 1000 Hz is needed. Currently this is almost impossible to do due to technical limitations. Another, more practical, method is to use low persistence display. In a low persistence display, each pixel is lit for a very short amount of time, less than 2 ms. Because lit pixel will no longer move across the retina, smearing dissipates. While smearing is gone, strobing can become more visible (smearing no longer hides it). Having higher refresh rate will solve the strobing issue.
Timewarp and Judder
Dropped frames can cause judder. When the engine takes too long to render a frame, the frame will be missed, causing judder. This can be mediated by using timewarp. While using timewarp to solve this problem works well in a rotational-only system, problems can a raise when positional / translational tracking is introduced.
When using timewarp, moving objects such as an animated character can also cause judder. The notice-bility of the judder depends on the size, number and speed of the moving object.