Low-level Graphics on Raspberry Pi (part one)

The Raspberry Pi (RPi) comes built with hardware support - and supporting software programming libraries - for all the current state of the art standardised graphics goodies: OpenGL ES, OpenVG, EGL  etc. and considering the performance gains of using the VideoCore GPU over the ARM CPU, it definitely makes sense to utilise these libraries to their full extent.

However, one of the main ideologies of the Raspberry Pi Foundation - the people who conceived the crafty little appliance we now know as RPi - was to introduce new generations to 'what goes behind the scenes' of fancy applications and user-interfaces. In my opinion, this goes as well for the 'fancy' graphics libraries and technologies. Therefore I would like to think it makes sense to introduce also the lower level interfaces for programming graphics on the RPi (most principles and some of the code I will introduce apply to other systems as well - *see comment 4 March 2016).

Basic command-line and file editing skills expected - some understanding of C programming would not hurt...

The lowest level graphics interface on a Linux system is the framebuffer (also see linux/fb.h). The framebuffer device - like most devices on a Linux system - can be opened as a file. The file can then be accessed for example using ioctl calls.

A basic example to open the framebuffer device and query the current display settings:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>

// application entry point
int main(int argc, char* argv[])
{
  int fbfd = 0; // framebuffer filedescriptor
  struct fb_var_screeninfo var_info;

  // Open the framebuffer device file for reading and writing
  fbfd = open("/dev/fb0", O_RDWR);
  if (fbfd == -1) {
    printf("Error: cannot open framebuffer device.\n");
    return(1);
  }
  printf("The framebuffer device opened.\n");

  // Get variable screen information
  if (ioctl(fbfd, FBIOGET_VSCREENINFO, &var_info)) {
    printf("Error reading variable screen info.\n");
  }
  printf("Display info %dx%d, %d bpp\n", 
         var_info.xres, var_info.yres, 
         var_info.bits_per_pixel );

  // close file  
  close(fbfd);
  
  return 0;

}

Save (using your preferred text-editor) the above code to a file called fbtest.c (in your preferred directory/folder - I use a main directory called projects in the user's home directory and a couple of sub/directories... - then compile and link simply using the command (from command-line in the same directory the file is):

make fbtest

...this and it's output should look like:

pi@raspberrypi:~/projects/test/fbtest# make fbtest
cc fbtest.c -o fbtest
pi@raspberrypi:~/fbtest#

...and if you examine (list) the directory, you should see both the source file fbtest.c and the executable file fbtest:

pi@raspberrypi:~/projects/test/fbtest# ls -la
total 20
drwxr-xr-x  2 rst rst 4096 Jan 20 16:09 .
drwxr-xr-x 10 rst rst 4096 Jan 20 16:05 ..
-rwxr-xr-x  1 rst rst 5790 Jan 20 16:09 fbtest
-rw-r--r--  1 rst rst  839 Jan 20 16:08 fbtest.c

Now you can run the executable using the command:

./fbtest

...you should see output similar to this (based on your display configuration - this is for a Full HD LCD connected through the HDMI on RPi and default configuration):

The framebuffer device opened.
Display info 1920x1080, 16 bpp

[Continues in part two]