On anti-aliasing
Published June 26th, 2003 in GeneralTomas Jogin wrote a bit about anti-aliasing and ClearType in Windows. I thought I’d chip in with my €.02.
The first thing I noted when I first laid eyes on OS X was the nice fonts. If you have a graphic card capable of OpenGL, OS X uses its Quartz Extreme engine to render the fonts and windows, which automatically provides a very nice anti-aliasing of fonts.
Windows 2000 and later also has anti-aliasing, but it’s not as good as the one in OS X. The main problem is that Windows doesn’t anti-alias fonts below a certain size, whereas OS X anti-aliases pretty much everything. (I have a vague memory of a registry setting where you can fiddle with this size threshold. I’ll look into it and report if I find anything.)
Windows 2000 and later Windows XP has a function called ClearType, which provides a sort of anti-aliasing. It is, however, not enabled out of the box, and for a good reason.
ClearType isn’t really supposed to be used on CRT monitors — it uses a technology called sub-pixel font rendering, which only provides an advantage if you have an LCD or TFT monitor.
On a CRT, a pixel is a point of light where three beams converge. These beams are red, green and blue and vary in strength to give the pixel a certain color.
An LCD monitor, however, has no cathode ray tube that shoots beams. Instead, each pixel on the screen is made of three individual diodes: red, green and blue. This means that an LCD screen with a resolution of 1024×768 really has a resolution of 3072×768.
Sub-pixel font rendering works by lighting individual pixel elements on an LCD screen, making the font smoother by effectively tripling the horisontal resolution.
On an LCD screen this looks nice and smooth. On a CRT screen, the results vary wildly — I find it less than good-looking, for reasons displayed in these pictures.
The top text uses “Standard” Windows anti-aliasing — meaning “none,” since the font size is too small to anti-alias, according to Windows. The bottom one uses ClearType. Now, let’s have a look up close and personal.
Here you see what sub-pixel font rendering looks like on a CRT screen: even though the text is black, you get various red, green and blue-tinted pixels. A CRT has no sub-pixel elements, so it gives the entire pixel an average color. The pixels on the left side of the letters are slightly red, whereas the ones to the right are slighly blue — this is the order in which LCD screens have their sub-pixel elements arranged: red-green-blue.
Below is the same image, but in gray-scale, approximating how it would look like if Windows used proper anti-aliasing:
If you take a few steps back and compare the images, you will probably find that the bottom one looks best on a CRT.
Update: I should also note that on an LCD screen, you can’t see these color artifacts at all. But on a CRT, I find that ClearType makes the text look blurred and hazy and a bit hard to focus on, due to the color artifacts.
Further reading
- Splitting the pixel: How sub-pixel font rendering works and Sub-pixel rendering implementation details.
- Daring Fireball: Anti-aliasing
- Daring Fireball: Anti-anti-aliasing
- Daring Fireball: Bitmap like it’s 1989
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Like you said, results vary on CRT monitors. Sometimes you get better readability and sometimes you get worse.
In your pictured example (not the zoomed-in one), I think the italicized text is easier to read in the Clear Type enabled version, while the rest of the text is hard to read in both, wether Clear Type is enabled or not; too pixely in the first case, too fuzzy in the other.
I have a professional line Trinitron monitor at home, and while that is a CRT screen just like the “regular” CRT at my office, I get better Clear Type results on the one at home.
The worst about the nice, anti-aliased fonts in Jaguar is when you do webdesign in it, and later get to see what your page looks like in Windows.
ClearType is smooth and nice. OS X’s antialiasing is all blurred up. I can hardly read the smaller font sizes with OS X’s antialiasing.
The Trinitron CRT is arranged in RGB columns, whereas the normal CRT is RGB colors are not in a column, they are shifted each row. This may be why trinitron has much better results with cleartype than regular CRT.
Cleartype looks best on a digital LCD running at native resolution, where each pixel corresponds to a single RGB component, but on CRT (even trinitron and analog LCD), one pixel can occupy more than one RGB component.
Just to clarify that ClearType is only available in Windows XP, not in 2K.
What if not ClearType is the “Smooth edge of fonts” facility available then in Windows 2000 ?
Is this anti-aliasing but at pixel, not sub-pixel level ?
Does anybody know about imaging anti-aliasing scanned facsimile documents, and the optimisation thereof for LCD screens ?
Windows XP has non-Cleartype anti-aliasing available for people who use CRT monitors. It is an improved version of “Smooth edges of screen fonts” in previous versions of Windows, like one commenter pointed out is in 2000. If you want this to be an accurate article, it wouldn’t hurt to look into that.
The only problem I see with anti-aliasing is that for small fonts it only serves to make them harder to read. On my monitor, the first, non anti-aliased example is much easier to read IMHO.
So, to keep the fonts from giving readers headaches, you just make them larger, so that a 12pt anti-aliased font appears larger on screen than a 12pt aliased font, which is what KDE and other Linux GUIs do. Except that, of course, this is a waste of screen space.