Why Create a Custom Video Mode

In most cases, X starts and works correctly at a reasonable resolution. If you just want to change to another common resolution (such as 1024 x 768 rather than 1280 x 1024), you can do so by editing the Modes line in the Display subsection of the Screen section of XF86Config. Remove an unwanted resolution, add a new one, or change the ordering. (XFree86 tries the resolutions in order, from left to right.) If your desired resolution appears before the one you're seeing and if it's a standard resolution, chances are your monitor can't handle it—or at least, X thinks that your monitor can't handle it, given the HorizSync and VertRefresh values you gave in the Monitor section. If you're sure your monitor is capable of handling the video mode, double-check those values and restart X if you change them.

Sometimes, though, the default video modes just aren't enough. You might want to create a custom video mode for several reasons, including:

Correcting Display Centering or Image Size One common problem, particularly on multiboot systems, is that the display may not be centered or may not be the proper size. (In this context, size refers to the physical size of the image, not its resolution in pixels.) For instance, if you adjust the display to be centered in Windows, and if you then boot to Linux, the display may appear physically shorter and shifted to the left. You can correct this matter by tweaking your configuration, as described in the upcoming section, "Tweaking a Modeline." If you're otherwise satisfied with the resolution and refresh rate, this is the simplest type of change you can make. Many monitors can remember multiple settings, though, which may obviate the need for this sort of adjustment.

Custom Resolution You may want to run X at an unusual resolution. For instance, I have a 19-inch monitor, and the standard resolution for a monitor of this size is 1280 x 1024; however, this resolution creates pixels that are slightly wider than they are high, which can distort graphics. To get pixels that are closer to square, I use a custom 1360 x 1024 display. You can set just about any resolution you like, though, within the limits of your monitor's and video card's capabilities.

Custom Refresh Rate Depending on your resolution, chances are that your monitor is capable of delivering a slightly higher overall refresh rate than a standard video mode delivers. XFree86 selects the highest refresh rate available from the modes it has at hand, but it's usually possible to get a slightly higher refresh rate from a system. For instance, on my monitor, the standard 1280 x 1024 display runs at 85Hz; but by creating a custom 1360 * 1024 display, I was also able to boost the refresh rate to 105Hz, reducing eye strain from looking at the display.

You should realize that these reasons for creating a custom video mode apply mostly to traditional cathode ray tube (CRT) monitors, not to the newer LCD monitors that are becoming popular on desktops and have long been the standard on laptops. LCDs have a fixed number of pixels, and so work best at a specific resolution, which is almost always a standard size. When they work with analog outputs, they support a very limited set of refresh rates, again including standard values, and there's little benefit to providing a different refresh rate to LCDs; they don't change their scan rate in the way that CRTs do. The main exception to this rule would be if you have an LCD

with an unusual display size, but such screens are quite rare.

If you want to proceed with creating a custom video mode, the next step is in learning a bit more about modelines, which are numerical descriptions of the video display. You'll then be able to use a tool to help compute an appropriate modeline, insert it into your XF86Config file, and use it. You can then tweak the modeline so that it creates a display of the appropriate size and placement.

0 0

Post a comment