Xlib tutorial part 2 -- Lines

Hello,

A blank black window is not very interesting. We want to place some thing in the window. Well, just like in real life you need a pen or pencil to draw on a blank piece of paper, our program needs a pen to draw on our window.

First we'll add a new GC pen, and a XGCValues structure for choosing what style of pen we want.

	...
	GC pen;
	XGCValues values;
	...
	

Then, after we've created the window we can create the pen,

	...
	/* create the pen to draw lines with */
	values.foreground = WhitePixel(dpy, screen_num);
	values.line_width = 1;
	values.line_style = LineSolid;
	pen = XCreateGC(dpy, win, GCForeground|GCLineWidth|GCLineStyle,&values);
	...
	

This particular pen that we've chosen, draws with a white ink. Makes one pixel wide lines and draws the line solidly. There are a lot of options on that you can choose. Look at the man page for XGCValues to see all the different pieces of style information you can choose. For our purposes the three we've got is enough.

	...
	/* as each event that we asked about occurs, we respond.  In this
	 * case, if a piece of window was exposed we draw two diagonal lines
	 */
	while(1){
		XNextEvent(dpy, &ev);
		switch(ev.type){
		case Expose:
			XDrawLine(dpy, win, pen, 0, 0, width, height);
			XDrawLine(dpy, win, pen, width, 0, 0, height);
			break;
		case ConfigureNotify:
			if (width != ev.xconfigure.width
					|| height != ev.xconfigure.height) {
				width = ev.xconfigure.width;
				height = ev.xconfigure.height;
				XClearWindow(dpy, ev.xany.window);
				printf("Size changed to: %d by %d
", width, height);
			}
			break;
		case ButtonPress:
			XCloseDisplay(dpy);
			return 0;
		}
	}
	...
	

The complete code can be found in xtut2.c

This is an extra 13 lines. This program is not nearly as easy to do in HTML.

Things to try:

• Using the window manager, try resizing the window, do the diagonal lines change right away?
• Try resizing and then exposing part of the window (by covering it with another window and then moving it away). Does anything look weird?
• Make a copy and try drawing different lines in the window.
• Try drawing outside the window. Does it work?
• Try drawing white on white or black on black.
• Create a second pen. One black and one white. Try drawing with one then the other and back again.
• Investigate the XGCValues structure. Try setting different values for function, plane_mask, line_width, and line_style. If you're ambitious you can try using the dash_offset and dashes members. If you set any values other than foreground, line_width and line_style, make sure to add to the GCForeground|GCLineWidth|GCLineStyle bitmask. ( GCFunction for function and GCPlaneMask for plane_mask. ) See the XGCValues man page for other details.