Linux FrameBuffer

I found a 4" touchscreen for RPi in my cupboard the other day.  I actually purchased it 3 years ago and had forgotten about.  I thought I should try it out and see what it can do.  It turns out to be a Waveshare 480x320 touchscreen.  Initially I wanted to use it for output/display.  I certainly don't want to run Xwindows on it so I needed to find out what is possible.

Installation

I plugged it in to an RPi 2B and installed the driver software. I run RPi servers headless and I was pleasantly suprised to see that the RPi console startup messages  are displayed.  It looks nice but isn't useful as the screen / characters are very small and I don't have the ability to type in to the console or login.

Display Text

To send output to a terminal it is easiest to do if the device is logged in.  Using raspi-config we can tell linux to login to the console as pi at startup.  We can now easily display messages, e.g.:
  echo ‘Hello little screen’ > /dev/tty1

To clear the screen or display text at specific positions we use control characters.  I have a fondness for these from my DEC PDP-11 programming days when a DEC VT100 was considered an excellent screen and I used it extensively.  I can clear the screen with:
 echo -e '\e[2J\e[1;1H' > /dev/tty1

Images and videos

It is surprisingly simple to play videos on the frame buffer with vlc:
 vlc starwars.avi > /dev/fb0
VLC can actually display images such as:
 vlc AtTheBeach.jpg /dev/fb0

As everything is a file in linux I can take a screen snapshot with:
 cat /dev/fb0  > screenshot.raw
and then redisplay it later with:
 cat screenshot.raw > /dev/fb0

However the best way to display images is to install the package fbi (framebuffer image) which gives you lots of image capabilities.

C Programs using the Framebuffer

A great tutorial is provided by Raspberry Compote explaining how to use the framebuffer in a C program.  It makes the whole process fairly straightforward.

Step 1 is to interrogate Linux to obtain characteristics of the device, in our case a 480x320 32bpp screen.

This is followed by a program to create a memory map for the buffer in RAM and write pixels to it.  Again it isn't very complicated and you end up with a pattern on your screen.

After that you need to investigate formats, palettes and colours in a little detail so that you can use the best approach for your application.  In practice, for your own programs an 8 bit format is preferred.  You have 16 basic colours provided and you can define the remaining 240 colours in the palette as needed.

Image formats and processing can become very complicated so C programs are probably best suited to displaying patterns, but it is great to have a bit-mapped screen available.

As a quick test I wrote a C program to display a grid on the screen and incorporated with the colour display.

Lvgl

In this post we are concentrating mainly on outputting to the framebuffer, however, its value lies in being a touch screen.  For touch screen usage we will be using lvgl (little versatile graphics library). As an introduction I found a great demo which uses lvgl to write a number of objects ("widgets") to the screen, for example text and buttons.  A subsequent blog will describe progress on touch related functions.