Xlib tutorial part 3 -- Colour

by Alan at Mon 26th Jan 2009 1:00AM EST

Hello,

The world is not very interesting if we are stuck in black and white. X has a sophisticated colour management system to be able to handle colour on displays that can only handle 16 colours at a time up to those that can handle millions or billions.

In our case, we just want three new colours. We're going to assume that the colours we name already exist on the server. For instance, on a true colour system they automatically will. X requires us to have a colormap so that we can map colours to integer values. So we'll put create some new variables. One to store the colormap and two to store the colours. Don't worry about the XColor structure right now. We'll get back to it later.


	...
	Colormap cmap;
	XColor xc, xc2;
	...

Then we'll stick in some code to do colour lookup after we get the screen number, but before we create the window. We'll also remove the three lines where we call BlackPixel or WhitePixel


	...
	cmap = DefaultColormap(dpy, screen_num);
	XAllocNamedColor(dpy, cmap, "DarkGreen", &xc, &xc2);
	background = xc.pixel;
	XAllocNamedColor(dpy, cmap, "LightGreen", &xc, &xc2);
	border = xc.pixel;
	XAllocNamedColor(dpy, cmap, "Red", &xc, &xc2);
	values.foreground = xc.pixel;
	...

So, nine new lines and three lines from the last section removed. We have to remove the lines to get the black or white pixel values for the default screen or they may overwrite our allocated colours.

Things to try:

Try choosing various other colours from rgb.txt . Do a web search if you can't find it on your system. It should be in /usr/lib/X11/rgb.txt or /usr/share/X11/rgb.txt .

Comments on Xlib tutorial part 3 -- Colour
by Mike at Tue 5th May 2009 5:16PM

Because XSetForeground takes an int, just passing an RGB value seems to work just fine. Other than converting text to an RGB value and supporting old systems (or ones that don't use 24bit color?) what purpose does XAllocNamedColor serve?
by Alan at Wed 6th May 2009 1:49PM

Hi Mike, Supporting non 24 bit colour systems is one of the purposes of XAllocNamedColor. It's quite possible that, in the future, we could have systems supporting 48 bit colour. To specify those, you could use a string to describe the actual colour and it would give you a long int as a handle. Another purpose is supporting writable colours. Using XStoreColor(s), you can change many pixels all at once from one (set of) colour to another. You don't see it used a whole lot, but it is possible.
by Megane at Wed 23rd Sep 2009 3:06AM

Nice tutorial. I've tried to change colour,but when I copied & replaced new code,it doesn't seems to work. It reported an error. So when I deleted "XAllocNamedColor(dpy, cmap, "Red", &xc, &xc2); values.foreground = xc.pixel;" everything seems to be normal. Program work fine,and I see green background,but still,i can't get green border(it's still default). Why? I'm running Ubuntu 9.04 Thanks!
by Alan at Wed 23rd Sep 2009 3:32PM

Hi Megane, It would be very useful to know what the error report was. Likely the error told you that the colour name you tried to use was not a colour it was familiar with. You can look in rgb.txt (like in /usr/lib/X11/rgb.txt) to get the list of colours it is familiar with. The "best" way to change colour is to copy colour names from that file and use the to replace the strings Red, LightGreen, DarkGreen in the above code snippet. What other code were you trying to place there?
by Megane at Mon 5th Oct 2009 5:19PM

Hey Alan. The error was next: in function main :33 error: 'values' undeclared (first use in this function) :33 error: Each undeclared indentifier is reported only once for each function it appears in. But,when i delete,foreground,everything seems to work fine. Maybe that causes the error,because there is no foreground in this example?
by Alan at Fri 9th Oct 2009 2:31PM

Hi Megane, Oh, I see. I guess when I was creating the code fragments, I was building on the code I had used in the previous two installments and had made sure there was a values variable to put stuff into as I went. I notice that the link to the finished code is missing from this page. It should be http://xopendisplay.hilltopia.ca/xtut3.c Hopefully, reading that will make sense to you. Let me know if you are having other troubles.

Comments are closed.