LG L1730SF touch screen display in Ubuntu Linux 8.04 Hardy Heron, part 3

In the 1st part we set up the drivers for the touch feature, and in the 2nd part we solved the weird jumpiness issue. Now we are ready to proceed to calibrating the display, and fine tuning the system for touch based usage in general.

Calibrating the touch input

The idea of the touch screen is that the user interface objects presented on the screen can be directly pointed at, using a pen or a finger. Ideally, when user sees even a single pixel at, say coordinates 100, 150 and then pushes the screen with a pen at this point exactly, the touch screen / mouse driver should register exactly the same coordinates and report them to the windowing engine.

However, in practice things are a bit different. For example, the resistive touch panel used e.g. in LG L1730SF is placed on top of the display panel. In real life things are not perfect, so there will be some offset in the display component framing, in the touch panel component framing, and in their relative placement on top of each other. Every manufactured device ends up being slightly different.

The result is that there will be some offset between the actual/perceived location of the touch, and the location reported to the windowing system that draws the mouse cursor and processes points and clicks. This is bad. For example, it can be very annoying if you are trying to hit a link on a web page, but always seem to miss it just a little!

Fortunately, this error can be fixed fairly well with a proper calibration. Calibration is a procedure based on showing a known location to be touched on the screen e.g. with an X mark, and then sampling the actual location that was touched. The difference between screen coordinates and mouse coordinates tells what kind of offset the particular display has. For example, when the user thinks he has touched precisely coordinates 100, 150 we may have received 108, 145 as mouse coordinates. Thus the difference is +8, -5. In order to correct the error, the offset can be removed by manipulating the mouse coordinates with the offset values found via calibration. This will be done already at the driver level, e.g. (+8, -5) could be subtracted from all input reported by the touch panel. The X windowing system would then get slightly different mouse coordinates that should match much better with the screen coordinates and perceived touch location.

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LG L1730SF touch screen display in Ubuntu Linux 8.04 Hardy Heron, part 2

Let’s continue with setting up the touch feature. In the end of part 1, we managed to rotate and mirror the display properly and just started calibrating it, when we encountered a problem where the cursor at first does not seem to follow the finger, and then suddenly jumps ahead. Let’s solve that next.

Note: You should only read this through, you don’t need to repeat everything I did. The actual solution is in the end of the text, so there’s what you need to do.

Since we now have two drivers in the system for the touch feature, usbtouchscreen for the display device and evtouch for converting the input events to a suitable format for X, the first question that comes to mind is, which one of the drivers is causing the jumpiness? We should go to the source of the information first, i.e. usbtouchscreen.

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