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This is a quick document I produced while figuring out how to use the framebuffer device to produce graphics on the Linux console. I don't claim to be an expert in this area, but when I looked for a general guide to doing this, I found absolutely nothing, except the header file <linux/fb.h>

Therefore, this is meant to be a jump-start guide for those who want to begin using graphics in the linux console. Hopefully someone out there will write a decent graphical browser, so I don't have to start up X every time I want to surf!

As always, no warranty, express or implied...etc. Give me a break, I took my first look at this device less than 24 hours ago! Any corrections, money, gifts, etc. (I wish!), just contact me
You may freely distribute this document, as long as you do not change it in any way, and credit me as author. In particular, I encourage Linux distributions to include this in their packages, as I feel there isn't half as much programming information in most distributions as there ought to be. Copyright James Dabell, May 1999.
This document was last updated June 11th, 1999. (Corrections from Joerg Beyer & Ian Thompson-Bell)

Part One:

First of all, configure your system for the framebuffer console device. It may come in handy. Note that the device is only available for graphics cards that implement VESA 2.0. Fortunately, virtually all recent cards out there do this.

When you have got high-res textmodes, then you can start experimenting. You will have a device /dev/fb0 that you can look at like any normal file. To take a screenshot, all you have to do is

cat /dev/fb0 > ~/sshot
        

And you will have a pretty big file with the contents of your graphics card's memory inside. Now, if you clear the screen, and type

cat ~/sshot > /dev/fb0
        

You should have a display that looks exactly like before. Of course, the second you start typing the display reverts to normal.

Part Two:

So now, we can move on to using the device in a C program. Take a look at the following code:

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
int main()
{
        int fbfd = 0;

        fbfd = open("/dev/fb0", O_RDWR);
        if (!fbfd) {
                printf("Error: cannot open framebuffer device.\n");
                exit(1);
        }
        printf("The framebuffer device was opened successfully.\n");

        close(fbfd);
        return 0;
}
        

All in all, a pretty simple program. We open the framebuffer device file using the low-level open(), because the buffered, higher level fopen() is unsuitable for our needs.
Next, we test the file descriptor to make sure that we were successful, and print a message out that tells the user what happened.
Note that if you #include <errno.h>, then you will be able to find out exactly why the open() call fails. Check the man page for open() for more information on this
Finally, we clean up by closing the file descriptor and returning.

Part Three:

Using the newly created file descriptor, we can perform operations on the framebuffer with the ioctl() function. This function is used to talk to special device files in ways that are particular to that device. In this case, we can use it to obtain information on the video card. We can also use the file descriptor to map the file into memory, and use a pointer to access it, which is more efficient and easier on us. This is done using the mmap() function. If you haven't used this before, here is some sample code:

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

int main()
{
        int fbfd = 0;
        struct fb_var_screeninfo vinfo;
        struct fb_fix_screeninfo finfo;
        long int screensize = 0;
        char *fbp = 0;
        
        fbfd = open("/dev/fb0", O_RDWR);
        if (!fbfd) {
                printf("Error: cannot open framebuffer device.\n");
                exit(1);
        }
        printf("The framebuffer device was opened successfully.\n");

        if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
                printf("Error reading fixed information.\n");
                exit(2);
        }

        if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
                printf("Error reading variable information.\n");
                exit(3);
        }

        screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
        fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0);
        if ((int)fbp == -1) {
                printf("Error: failed to map framebuffer device to memory.\n");
                exit(4);
        }
        printf("The framebuffer device was mapped to memory successfully.\n");

        munmap(fbp, screensize);
        close(fbfd);
        return 0;
}
        

As you can see, we have to #include an extra header file to deal with mmap(). We use the information returned by the ioctl()s to figure out how much memory to map. The members of fb_var_screeninfo that are used are xres, yres, and bits_per_pixel.
Note that there is also what is known as the virtual screen size, which can be utilised for scrolling, etc, but that is beyond the scope of this document. However, if you are planning on using scrolling, you will probably want to use xres_virtual and yres_virtual to calculate how much memory to map.
Finally, remember to munmap() the memory you have mapped for use with the framebuffer.

Part Four:

In this section we finally get to plot a pixel on the screen. First of all, we need to know in what format we should put the data. As it is the most common, and also the only type I have access to, I will be talking about the type of framebuffer that utilises packed pixels. All that is necessary to put a pixel on the screen, is to put bytes corresponding to the colours blue, green, red and transparency, in that order, at the correct location in memory, starting at 0 for x = 0, y = 0, and increasing by four bytes for every x, and y * the length of the line in bytes for every y. Standard graphics stuff; people used to the good ol' days back in mode 13h programming in DOS will catch on quickly. Anyway, here's the code:

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

int main()
{
        int fbfd = 0;
        struct fb_var_screeninfo vinfo;
        struct fb_fix_screeninfo finfo;
        long int screensize = 0;
        char *fbp = 0;
        int x = 0, y = 0;
        long int location = 0;

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

        /* Get fixed screen information */
        if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
                printf("Error reading fixed information.\n");
                exit(2);
        }

        /* Get variable screen information */
        if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
                printf("Error reading variable information.\n");
                exit(3);
        }

        /* Figure out the size of the screen in bytes */
        screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
        
        /* Map the device to memory */
        fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
                fbfd, 0);       
        if ((int)fbp == -1) { printf("Error: failed to map
        framebuffer device to memory.\n"); exit(4);
        }
        printf("The framebuffer device was mapped to memory successfully.\n");

        x = 100; y = 100;       /* Where we are going to put the pixel */

        /* Figure out where in memory to put the pixel */
	location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
	        (y+vinfo.yoffset) * finfo.line_length;

        *(fbp + location) = 100;	/* Some blue */
        *(fbp + location + 1) = 15;     /* A little green */
        *(fbp + location + 2) = 200;    /* A lot of red */
        *(fbp + location + 3) = 0;      /* No transparency */

        munmap(fbp, screensize);
        close(fbfd);
        return 0;
}
        

Now that you know how to plot a pixel, it becomes a trivial matter to write functions to draw lines, boxes, windows, etc. Hopefully, by now you are well on your way to writing that web browser for me - consider it payment for this tutorial ;).

<SHAMELESS_MODE>
Talking of payment, I get paid every time somebody buys a book this way. Sorry, but I'm a poor student, and let's face it, if you are reading this page, then you will probably be interested in them, if you don't have them already. I have personally read each of these, and can say with all honesty that they are great books.

Beginning Linux Programming:
This book really has it all for getting to grips with Linux programming. If you can already program in C, and want to know how to use some of the more specific Linux stuff, then get this book. It covers all sorts, from shell programming, through socket programming in C, to X programming using tcl/tk.

C: The Complete Reference:
This is the only book you should consider getting if you want to learn C. This was all I needed to get programming in C, and since it also doubles as a reference book, you will keep coming back to it again and again. It's by Herbert Schildt, one of the best authors on the planet when it comes to C. Forget the "learn C in 14 days" cons, get this. Now.

Genetic Algorithms in C++:
This is all you need to understand the theory behind, and implement Genetic Algorithms. Genetic Algorithms model themselves on nature, by "evolving" solutions based upon how well they perform in an environment (your program). It sounds complex, but I assure you that it is *really* easy to get into.
One thing though. All of the sample code is targetted at MS Windows, so if you are the kind of person who relies on the code in a book to get through it, and you are developing for Linux, then this book may not be for you. Personally, I didn't even need to look at the code before implementing my own stuff, the text is more than sufficient.

Internet Programming With Python
This book is co-authored by Guido Van Rossum, the guy who wrote the Python language. It also comes with a CD containing Python for many different platforms, as well as sample code. It covers CGI programs written in Python, and also a couple of subjects not usually mentioned in books of this kind, namely generating HTML (in a *very* nice way) and extending the language. It also covers the basics in a way that makes this book also good for learning the language from scratch.

Thanks.
</SHAMELESS_MODE>

posted on 2005-03-05 19:27 浙林龙哥 阅读(...) 评论(...) 编辑 收藏