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Easy graphics: A beginner's guide to SVGAlib
By Jay Link
Are you looking for a simple graphics package for your Linux system? If
so, look no further. SVGAlib provides an easy way to create graphical
applications and eliminates the rigmarole of the X Windowing System. If
you have even the most rudimentary grasp of programming in C, then you can
use SVGAlib.
What is SVGAlib?
SVGAlib is a low-level graphics library for Linux. It augments the C
programming language, which doesn't provide support for graphics.
But there are lots of graphics programs written in C!
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Yes, but they all rely on external library functions. C itself can only
give you text. That's because all graphic functions are system dependent
and non-portable. Unfortunately, graphic routines coded for one
operating system will not work under another unless they are completely
rewritten. For example, graphics originally written for DOS or Windows are
useless under Linux.
To code graphics in C under Linux, it is necessary to use an external set of functions which are native to Linux. SVGAlib is one such set.
How is SVGAlib different from the X Windowing System?
The X Windowing System (XFree86) is actually a server. This server must be started prior to using any applications that require X. Furthermore, the X server is unabashedly system-intensive (i.e., it's piggy with your resources) and it might (depending on your graphics card) prohibit you from using your virtual terminals (Alt 1-6).
SVGAlib, on the other hand, requires no such preparation. You do not "run"
SVGAlib like you run the X server. SVGAlib is simply a binary C library
called by C programs, just like all the other libraries in /lib and /usr/lib. If SVGAlib is properly installed, the average user shouldn't even be aware of its existence. Finally, SVGAlib will not affect your virtual terminals, leaving you free to run multiple applications like always. You can have normal text on one terminal and graphics on another.
Drawbacks
There are many more applications available for X Windows than there are for SVGAlib, due to the fact that the X Windowing System is cross-platform (it runs on a variety of UNIXs). Only Linux uses SVGAlib. Also, poorly written SVGAlib applications can mung up your console, requiring a reboot. Finally, you shouldn't switch back and forth quickly between two
consoles using SVGAlib graphics or you risk screen corruption (forcing another reboot).
However, it is a myth that SVGAlib is a security risk. While SVGAlib apps must be setuid root, that privilege is given up immediately after execution. There is no need to be concerned.
In summary, despite the aforementioned problems, SVGAlib's speed and ease of use make it attractive in many situations. Especially if you just want to doodle on the screen.
Examples
To use SVGAlib, you must reference it in your C program. Simply #include
<vga.h> . Here's about the easiest SVGAlib program there is:
<![CDATA[
#include <stdio.h>
#include <vga.h>
int main(void)
{
vga_init();
vga_setmode(5); /* G320x200x256 */
vga_setcolor(4);
vga_drawpixel(10, 10);
sleep(5);
vga_setmode(0); /* TEXT */
return(0);
}
]]>
This will paint a single red pixel on your screen. After five seconds, it will reset your console to text mode and will exit.
Note our first statement, vga_init() . This relinquishes root status and initializes the SVGAlib library. The second line, vga_setmode(5), sets the screen to mode 5, which is 320x200x256. That is to say, your screen becomes a grid which is 320 pixels wide, 200 pixels high, and supports 256 colors.
Alternatively, you could write
vga_setmode(G320x200x256). Either statement is acceptable. Our next command, vga_setcolor(4), makes red the current color. We can choose any value from 0 to 255. More colors are available with other commands but we'll stick with these basic colors for this example. Finally, we paint our pixel at coordinate 10, 10. This is eleven spaces right of the screen's left border, and 11 spaces down. It's 11, not 10, because the coordinate grid starts at 0. Coordinate 0,0 is in the upper left-hand corner. Vga_setmode(0) returns the screen to text mode. Vga_setmode(TEXT) is identical to vga_setmode(0). It's always nice to do this at the end of your program. Otherwise, you'll make life difficult for your users.
To compile this code, use the regular gcc compiler. You'll also need to link to SVGAlib with the -lvga command. Lastly, I suggest using -O3, the best level of optimization. So here's our command:
gcc -O3 -o sample sample.c -lvga
Then, to make it usable by non-root accounts, type:
chmod u+s
To execute, just type:
sample <or whatever you named it>
The complete set of SVGAlib commands is documented in the SVGAlib man page. We won't go into all of them here, though. Instead, we'll write our second sample program using a faster set of SVGAlib functions: vgagl.
Type "man vgagl", and you'll see that vgagl is "a fast, framebuffer-level graphics library based on SVGAlib." Basically, it gives you advanced graphics functions, such as the ability to draw shapes with one statement.
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