xlib blues (XFree86 - Linux)

Applications Programming
1

My goal is to open a window in the X-server and plot a series of pixels. However, I find many examples I have tried to draw to a window fail and the window remains blank. Therefore, I am here requesting information. I do not know c, and am a beginner. Currently, I am using a mix-match of code, running against the gcc compiler finding errors and fixing them as I go along. I have experienced segmentation faults, but the following code produces a minimal amount of error.

/* window.c --This program opens a window on the display
* Use "gcc -o window window.c -L/usr/X11R6/lib -lX11
* to compile this code.
*/


#include <X11/X.h>
#include <X11/Xlib.h>
#include<stdio.h>
#define width	320
#define height	200

int main(void)
{
Display *display;
Window window, rootwindow;
int screen;
GC gc;
XImage *img=NULL;
int x;
int y;

display = XOpenDisplay(NULL);
screen = DefaultScreen(display);
gc = DefaultGC(display, screen);
rootwindow = RootWindow(display,screen);
window = XCreateSimpleWindow(display, rootwindow,
                0, 0, width, height, 1, 0, 0);
XMapWindow(display, window);
XImage *xi;
unsigned long pixelvalue1;


xi = XCreateImage(display, 0, 8, ZPixmap,
                   0, malloc(width*height), width, height, 
		   8, 0);

pixelvalue1 = 65535;
for (x = 0; x < width/2; x++)
	{
		for(y = 0; y < height/2; y++)
			{
				XPutPixel(xi, x, y, pixelvalue1);
			}
	}
printf("putting image\n");

XPutImage(display, window, gc, xi, 0, 0, 0, 0, width, height);
sleep (5);
return (0);
}

The result of this code, is to open a window (which it does) for 5 seconds/cycles, and also plot a series of pixels half the length of the width and height specified.

However, what actually happens is it opens a window with a black background, plots nothing, and I am left frustrated with little resources. I suspect the problem may lie in either XCreateImage, or XPutPixel commands. I suspect I have not got the right values for either command.

Can anyone here offer a simple solution to my problem of wanting to plot pixels to a window?

Finally - I would like to elabroate on my plans...

I intend to further the pixel plotting using double buffering, for a simple animation routine I written in PHP graphics. As animation is not available in PHP, this is the reason for myself attempting c coding without any prior knowledge of the c language. However, I do have a basic grasp of BASIC (if, for, while etc), and constant/variable declaration (int, long, double etc).

Please can someone enlighten me with a code snippet as to how I can accomplish my goal of plotting individual pixels to the screen.

Much thanks,

Alux.

2

Well...I'm :slight_smile: almost there...

For some reason I cannot access any other color than blue. I would also like to apologise for posting a Linux question in the Unix forum. :confused:

However, my code has advanced and I would like to share the program I wanted to write.

/*--------------------------------------------------*
*Open a window on the XFree86 system and draw to it.*
*Created by Robbie Dave after learning about Xlib.h *
*---------------------------------------------------*
Compile with: gcc -o dave dave.c -L/usr/X11R6/lib -lX11
*/

// Include the Xlib.h file so as to use X functions.
#include <X11/Xlib.h>
#define NIL (0)       // A name for the void pointer
//This is the 'main' function in c programs.
int main(void) { 

// Initialise the variables.
Window win;
int win_width = 640;
int win_height = 480;
int win_x;
int win_y;
int screen_num;
GC gc;
XImage *xi=NULL; 
XColor system_color_1; 
XColor exact_color;
Colormap davescol;
int x; 
int y; 
int c;

// Get a pointer (*display) to the X display.
Display *display = XOpenDisplay(NIL);

/*
Obtain the screen number using our *display pointer.
The display pointer does not need the asterisk (*).
*/
screen_num = DefaultScreen(display);

/*
Now we have the screen number and display pointer,
we get the GC (graphics context) of the X system.
*/
gc = DefaultGC(display, screen_num); 

// I assume these are the window offsets set to 0.
win_x = win_y = 0;

// Create the window
win = XCreateSimpleWindow(display, RootWindow(display, screen_num), win_x, win_y, win_width, win_height, 0, BlackPixel(display, screen_num), BlackPixel(display, screen_num));

//Make the screen appear and flush events.
XMapWindow(display, win); 
XFlush(display); 

// Capture the screen for drawing.
xi = XGetImage(display, win, 0,0, win_width, win_height, AllPlanes, ZPixmap); 

//Obtain a colormap to get colors.
davescol = DefaultColormap(display, screen_num); 

// Enter a color into the colormap to use later.
Status rc = XAllocNamedColor(display, davescol, "red", &system_color_1, &exact_color);

	//These two x, y for loops used to plot pixels to window.
	for (x = 0; x < win_width; x++) 
		{ 
		for(y = 0; y < win_height; y++) 
			{
			//Plot a pixel. 
			XPutPixel(xi, x, y, exact_color.pixel);
			/*Experiement with the x, y formula
			for different animations.
			*/
			exact_color.pixel = ((x*x)+(y*y))*0.05;

			if (exact_color.pixel > 65535) 
			{exact_color.pixel=65535;}
			}
	
	}

//This for loop is used for animation.
for(c = 8; c < 64; c++) 
	{ 
	//Animate the colors
	XAddPixel(xi, c);
	//Try to control the animation.
	XFlush(display);
	XSync(display, 1);

	/*
	After plotting to every pixel in the window,
	actually display what you've drawn.
	*/
XPutImage(display, win, gc, xi, 0, 0, 0, 0, win_width, win_height); 
	}

//Clean up the memory a bit, but not much.
XDestroyImage(xi);
XCloseDisplay(display); 
return (0);
}

This program is a simple animatiion that compiles without error with gcc. I am really proud of my work, I just wish I had access to more than one color. For some reason, I can only use blue.

EDIT: After reading yet another Xlib tutorial I discovered how to allocate colors. I still do not understand how to utilize these colors for above animation, but I am getting there...

Ah vell...