Initial touch latency in Windows 8
This topic describes initial touch latency and sustained report latency.
Initial touch latency
This is defined as the first touch event that is reported in relation to the first physical contact in time. Sometimes the firmware delays reporting the first touch contact up to a few frames in order to gain better accuracy by using certain kinds of temporal filtering. This causes fast motion panning to have a delayed start, and the end result is a far smaller range than that of physical finger motion, leading to a sluggish panning experience.
Responsiveness is critical to the end user touch experience. Immediately report the first touch registered even when it is less accurate, and indicate so in the report. The subsequent reports can still use the previously buffered samples for filtering to gain accurate reports. This way the buffering delay can be eliminated altogether.
Note This scheme requires a gestural recognition engine to be aware of the potential inaccuracy of the initial touch.
- Panning - A large initial touch latency can cause sluggishness in the panning experience, as the range of the panning is much reduced.
Sustained report latency
This is defined as the total time duration from the touch contact on the touch screen (finger down) to the visual response on screen. It can be caused by a number of factors compounded together (hardware, firmware, driver, input stack, apps, graphics subsystem, and so on). This latency manifests itself as a moving lag of touch points on screen when the physical touch moves. For the purpose of this report, the focus should be a minimal latency in milliseconds.
- Manipulation - Failure to have a small enough sustained latency can cause visual delay of the tracking which causes an unsatisfactory effect in scenarios of manipulation.