[XForms] slider control

[jon]

In a few weeks I will be working on a home automation system using
xforms front ends driven via touch screen.

I have done a quick and dirty test with a raspberry Pi board with a
23inch touch screen, all works well so far.  

Is it possible to replace the slider control with a different graphic.
I want to make a sliders with triangles or other shapes at the centre.
I guess what I want is a pixmap slider or similar functionality.

Thanks,
Jon