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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LG L1730SF touch screen display in Ubuntu Linux 8.04 Hardy Heron, Part 1

A few years ago I bought a 17″ touch screen display device LG L1730SF. The product itself was nice, and very affordable at the time. However, it wasn’t easy at all to get it working with Ubuntu Linux 6.06 Dapper Drake. I wrote a somewhat detailed instructions about my setup and published them on these web pages. To my surprise, the article got many positive comments and questions even several years after. Apparently touch screens in Linux was a hot issue (at least to geeks like me) and there was not enough information available for troubleshooting.

In August 2008 I went through it all again after updating to Ubuntu Linux 8.04 Hardy Heron. However, this time I was better prepared, having a few years of experience on Ubuntu. This article brings the instructions more up-to-date and answers some questions I didn’t have a clue about at the first time. Although Hardy is already a year old and major computer manufacturers are publishing computers with built-in multi-touch capabilities almost daily, I still think these instructions should be valid for all DIY guys for a few years, since it is a long term support (LTS) version of Ubuntu.

Note: I have written this article 8 months later from my memos. It was not straightforward to get the touch feature working, thus some unnecessary adventures had to be made. However, I decided to include them all here so that others can learn from my mistakes. Moreover, with a complex issue like this I think it is better to elaborate too much than too little – you can easily skip the chapters you don’t care about. I have also tried to explain what I am trying to do and why at each step, as many Linux/Ubuntu newbies need these things to be explained so that they can learn (I went through that myself a few years ago). As a result, the article is quite long and you don’t actually need to perform all the steps that I describe. These steps are marked in the text.

Introduction

In this article I assume that you have already setup Ubuntu Linux 8.04 Hardy Heron to your computer using keyboard, mouse and another display device. I will focus on explaining how to configure the system to operate with LG L1730SF display and its touch feature. Note that a keyboard and a mouse should be connected to the computer when you are going through the setup according to this guide. After you’re finished you can use the computer without them via the touch feature and virtual keyboard. Read the rest of this entry »