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C access to touchscreen events via simple graphics I/O

scudieremb

I recently bought your 7" touchscreen and it works great for a default screen.  But I want to write a graphic touchscreen display to control and display various RPi I/O.  How do I intercept and interpret touch events in my C code?  Also is there a simple app that lets me design a static graphic display with labeled buttons to accomplish entering and displaying the input values to the RPi I/O?  Perhaps something akin to the raspi-config command but with graphics & touch input instead of just text & mouse clicks.

  • Hi,

    You'll be there forever reinventing the wheel to do this at such a low level. It's not necessary, you can get real-time performance by running a web browser in a kiosk-type mode (full screen with no address bar and so on). Then, you have access to HTML to create whatever style user interface you wish, and you can have your application in whatever language you feel comfortable in. In other words, you can separate the user interface from the low-level code. The touchscreen will "just work", with zero coding from you, since it will interact with the web browser through the operating system and user interface just as it always does.

    The "interface" between your web page and your code is actually a web server. You can create a simple web server using Node.js (or Python or whatever; I find it easier in Node.js) and pass events to your C code; a simple way is to make your C code an executable which accepts switches/parameters to perform actions. Then, the Node.js code can send those parameter to your C code.

    If you want to see how to do such a thing, check out this wave miner webdsp project source code, because it does exactly that; there is some Node.js code, and there is some C code. Together it all provides a web user interface to the C code.

    Another approach (if you want an all-C method) would be to use a library called Qt. It is commonly used for user interfaces.

    Both of these approaches use Linux running on the Pi.

    If on the other hand you are not running Linux at all (e.g. going 'bare metal') then you've instantly made life difficult for yourself, because you'll need to find or develop your own drivers for the display, since you can no longer take advantage of the existing Linux drivers which require Linux, and which will provide the OS with a framebuffer so that the windowing system (I forget what it's called) can just write pixels into memory pretty much. Better to spend the time quickly picking up a bit of HTML and Node.js (it is JavaScript) because it will speed up your development tremendously. You can still often get real-time performance from the C application (provided it is architected well) even in user code (or drivers for further performance increase but that's a more advanced topic).

  • I ahve doen a few Graphical UIs on RPI touch screens and there were two easy ways to go: python and tkinter with a simple script to autostart on boot.
    Or HTML, much more often Html with some CSS runniongg in a frameless browser(chromium or FF in kiosk mode) was the best solution and was very flexible especially if the screen size might change later.
     You can directly code integrate html code into your C program and call the backend functions from there. In my application i used python as the backend and had a separate python script running the gui as a webserver that just got called as https://127.0.0.1 with the browser started on boot.
    You can do the same just with C as the backend.