Sunday, 3 March 2013

Low-level Graphics on Raspberry Pi (part four)

In the part three we saw how to plot individual pixels in the framebuffer. Now let's turn the plot-pixel code into a reusable function.

First we need to move some of the variables outside of the main function, so we can access them in the new function - then we just move the pixel plotting code from the main into the new function and make main to call it. We will also move the 'draw' code into another function to separate it from the main and make it easier to read.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>

// 'global' variables to store screen info
char *fbp = 0;
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;

// helper function to 'plot' a pixel in given color
void put_pixel(int x, int y, int c)
    // calculate the pixel's byte offset inside the buffer
    unsigned int pix_offset = x + y * finfo.line_length;

    // now this is about the same as 'fbp[pix_offset] = value'
    *((char*)(fbp + pix_offset)) = c;


// helper function for drawing - no more need to go mess with
// the main function when just want to change what to draw...
void draw() {

    int x, y;

    for (y = 0; y < (vinfo.yres / 2); y++) {
        for (x = 0; x < vinfo.xres; x++) {

            // color based on the 16th of the screen width
            int c = 16 * x / vinfo.xres;
            // call the helper function
            put_pixel(x, y, c);



// application entry point
int main(int argc, char* argv[])

    int fbfd = 0;
    struct fb_var_screeninfo orig_vinfo;
    long int screensize = 0;

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

    // Get variable screen information
    if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
      printf("Error reading variable information.\n");
    printf("Original %dx%d, %dbpp\n", vinfo.xres, vinfo.yres, 
       vinfo.bits_per_pixel );

    // Store for reset (copy vinfo to vinfo_orig)
    memcpy(&orig_vinfo, &vinfo, sizeof(struct fb_var_screeninfo));

    // Change variable info
    vinfo.bits_per_pixel = 8;
    if (ioctl(fbfd, FBIOPUT_VSCREENINFO, &vinfo)) {
      printf("Error setting variable information.\n");

    // Get fixed screen information
    if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
      printf("Error reading fixed information.\n");

    // map fb to user mem 
    screensize = vinfo.xres * vinfo.yres;
    fbp = (char*)mmap(0, 
              PROT_READ | PROT_WRITE, 

    if ((int)fbp == -1) {
        printf("Failed to mmap.\n");
    else {
        // draw...

    // cleanup
    munmap(fbp, screensize);
    if (ioctl(fbfd, FBIOPUT_VSCREENINFO, &orig_vinfo)) {
        printf("Error re-setting variable information.\n");

    return 0;

Now save the file as fbtest4.c, compile with make fbtest4.c and execute ./fbtest4 - you should see the same vertical color bars at the upper half of the screen as in part three. Making the put_pixel function should help to come up with new ideas for images to draw...

[Continued in part five]

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