ringe
Posts: 1
Joined: Mon Oct 05, 2015 9:32 am

Touch sensitivity and speed

Mon Oct 05, 2015 9:38 am

I would like to use the official touch screen for drawing.
To this end, I've bought a touch screen stylus, which seems to work well.

However, the screen movement does not keep up with the actual movement.

With a mouse, I can adjust acceleration and speed, how about the Pi Display?

I need to have the cursor follow the actual position of the finger/stylus, even when I move quickly across the screen.

Does anyone have a tip?
I'm running Jessie.

Mark_T
Posts: 149
Joined: Sat Dec 27, 2014 10:54 am

Re: Touch sensitivity and speed

Tue Oct 06, 2015 6:02 pm

You can't get zero latency, its a question of how much latency you can tolerate. The capacitive
touch IC will be doing some processing, sending it on to the RPi, then that has to be translated into cursor position
and style decisions, then that gets painted on a frame buffer, then that framebuffer is typically swapped with the active
one before it is scanned and sent to the LCD, and finally the LCD pixels take a few milliseconds to respond
to the voltage change. All of this can easily mean 3 or 4 frame's delay...

User avatar
PeterO
Posts: 5068
Joined: Sun Jul 22, 2012 4:14 pm

Re: Touch sensitivity and speed

Tue Oct 06, 2015 6:14 pm

ringe wrote: I need to have the cursor follow the actual position of the finger/stylus, even when I move quickly across the screen.

Does anyone have a tip?
I'm running Jessie.
The touch screen returns absolute positions unlike a mouse that returns movements. An acceleration algorithm can be applied to the movements but not to the absolute positions.

PeterO
Discoverer of the PI2 XENON DEATH FLASH!
Interests: C,Python,PIC,Electronics,Ham Radio (G0DZB),1960s British Computers.
"The primary requirement (as we've always seen in your examples) is that the code is readable. " Dougie Lawson

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