Microsoft ClearType - A Technology to Enhance Screen Fonts

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Introduction to ClearType

ClearType is the Microsoft implementation of su***xel rendering technology, aiming to improve the appearance of text on certain types of computer display screens, especially LCD flat panel monitors.

ClearType, a trademark of Microsoft, was first announced at the November 1998 COMDEX exhibition. The technology was first introduced in software in January 2000 as an always-on feature of Microsoft Reader, which was released to the public in August 2000. ClearType was later introduced as an operating system feature in Windows XP, where it was kept turned off by default. In Windows Vista, ClearType is turned on by default. In Microsoft Office 2007 and Internet Explorer 7, ClearType is turned on by default, even if it is not enabled throughout the operating system. ClearType is also an integrated component of the Windows Presentation Foundation text-rendering engine.



ClearType Font Smoothing Technology in Windows XP

ClearType is a form of sub-pixel font rendering that draws text using a pixel's red-green-blue (RGB) components separately instead of using the entire pixel. When the pixel is used in this way, horizontal resolution theoretically increases 300 percent.

Picture elements on an LCD screen are actually comprised of individual horizontally oriented red, green and blue sub-pixels. For instance, an LCD screen that has a display resolution of 800 x 600 pixels actually has 2400 x 600 individual sub-pixels. The human eye is not capable of differentiating colors on such a small scale, so a combination of these three primary colors can emulate any intermediate color. Sub-pixel font rendering takes advantage of this by antialiasing at the sub-pixel level instead of at the pixel level.

Because a standard cathode-ray tube (CRT) screen uses an electron beam to excite pixels, and does not have specific pixels at a specific location, you do not experience the same benefits that you experience on an LCD screen when you use a CRT screen. However, because ClearType still includes antialiasing support, you may see some improvement when you enable ClearType on a CRT screen.

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ClearType and human vision

ClearType and similar technologies work because human vision is much more sensitive to variations in intensity than it is to variations in color. The human eye can discern contrasts in intensity about three times better than it can discern contrasts in color. Thus, when ClearType sacrifices color accuracy in order to increase luminance detail, the overall effect—as seen by human eyes—is an improvement for most people.

According to MSDN website, Microsoft acknowledges that

Text that is rendered with ClearType can also appear significantly different when viewed by individuals with varying levels of color sensitivity. Some individuals can detect slight differences in color better than others.

This opinion is shared by the font designer Thomas Phinney, program manager for fonts and core technologies at Adobe:

There is also considerable variation between individuals in their sensitivity to color fringing. Some people just notice it and are bothered by it a lot more than others.

The hinting expert Beat Stamm, who works on ClearType at Microsoft, agrees that ClearType may look blurry at 96 dpi, which is a typical resolution for LCD displays in 2008, but adds that higher resolution displays improve on this aspect:

WPF uses method C, but few display devices have a sufficiently high resolution to make the potential blur a moot point for everybody. Some people are ok with the blur in Method C, some aren’t. Anecdotal evidence suggests that some people are fine with Method C when reading continuous text at 96 dpi (e.g. Times Reader, etc.) but not in UI scenarios. Many people are fine with the colors of ClearType, even at 96 dpi, but a few aren’t. [...] To my eyes and at 96 dpi, Method C doesn’t read as well as Method A. It reads "blurrily" to me. Conversely, at 144 dpi, I don't see a problem with Method C. It looks and reads just fine to me.

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ClearType Tuner PowerToy

An alternative to Microsoft's popular online ClearType Tuner, Windows XP PowerToy lets you activate and tune your ClearType settings via the Windows Control Panel.

• download - ClearType Tuner PowerToy
• location - http://download.microsoft.com
• file details - setup.exe – 2.47MB
• requirements - Windows XP
• installation - Select the download location. Choose 'Run' or save the file to a local drive and run it from there.
• use - Access the tuner from the Windows Control Panel.

Using the tuner

1. To start the tuner, select Start and then Control Panel. If you are in Category View, select Appearance and Themes, and then ClearType Tuning.
   
   
   
   
   
2. If you are in Classic View, just select ClearType Tuning.
   
   
   
   
   
3. Start the Wizard or select the Advanced tab.
   
   
   
   Follow the steps and use the context sensitive help if needed.

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Enable & Customize ClearType

This step-by-step article describes how to enable ClearType to sharpen fonts on your Windows XP display. ClearType enhances your display by smoothing the edges of screen fonts. ClearType works especially well on Liquid Crystal Display (LCD) devices, including flat screen monitors and laptop computers.


How to Enable ClearType

To use ClearType for screen fonts:

1. Click Start, click Control Panel, click Appearance and Themes, and then click Display.
2. On the Appearance tab, click Effects.
3. Click to select the Use the following method to smooth edges of screen fonts check box, and then click ClearType in the list.

How to Customize ClearType

You can adjust the contrast setting for ClearType for your personal preference. Although there are no tools or settings in Windows XP for doing this, the following Microsoft Web site includes tools to turn ClearType on or off, and to adjust the contrast:
http://www.microsoft.com/typography/...peactivate.htm

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ClearType in GDI

ClearType can be globally enabled or disabled for GDI applications. A control panel applet is available to let the users tune the GDI ClearType settings. The GDI implementation of ClearType does not support sub-pixel positioning.


ClearType tuning

Windows, out of the box, only allows ClearType to be turned on or off. However, there are other parameters that can be set via a ClearType tuner utility that Microsoft makes available as a free download from its site. The tool can also be used for tuning Windows Vista ClearType settings. Switching off ClearType disables ClearType completely, as expected, unless the application uses its own ClearType controls, separate from the OS level ones.

The tuner has wizard and advanced modes that adjust the same parameters visually or by direct selection:

1. Cleartype on/off
2. RGB or BGR sub pixel structure, though BGR is quite rare, so the default works for most monitors
3. Contrast

Microsoft Reader has a similar ClearType tuner that only affects that program.



ClearType in WPF

All text in Windows Presentation Foundation is anti-aliased and rendered using ClearType. There are separate ClearType registry settings for GDI and WPF applications, but by default the WPF entries are absent, and the GDI values are used in their absence. WPF registry entries can be tuned using the instructions from the MSDN WPF Text Blog.

ClearType in WPF supports sub-pixel positioning, natural advance widths, Y-direction anti-aliasing and hardware acceleration. However, due to the resolution-independent architecture of WPF, ClearType cannot be optionally turned off in WPF applications. WPF supports aggressive caching of pre-rendered ClearType text in video memory. The extent to which this is supported is dependent on the video card. DirectX 10 cards will be able to cache the font glyphs in video memory, then perform the composition (assembling of character glyphs in the correct order, with the correct spacing), alpha blending (application of anti-aliasing), and RGB blending (ClearType's sub-pixel color calculations), entirely in hardware. This means that only the original glyphs need to be stored in video memory once per font (Microsoft estimates that this would require 2 MB of video memory per font), and other operations such as the display of anti-aliased text on top of other graphics — including video — can also be done with no computation effort on the part of the CPU. DirectX 9 cards will only be able to cache the alpha-blended glyphs in memory, thus requiring the CPU to handle glyph composition and alpha-blending before passing this to the video card. Caching these partially-rendered glyphs requires significantly more memory (Microsoft estimates 5 MB per process). Cards that don't support DirectX 9 have no hardware-accelerated text rendering capabilities.

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ClearType Overview

The ClearType available in Windows Presentation Foundation (WPF) is the latest generation of ClearType which has several improvements over version found in Microsoft Windows Graphics Device Interface (GDI).


Sub-pixel Positioning

A significant improvement over the previous version of ClearType is the use of sub-pixel positioning. Unlike the ClearType implementation found in GDI, the ClearType found in Windows Presentation Foundation (WPF) allows glyphs to start within the pixel and not just the beginning boundary of the pixel. Because of this extra resolution in positioning glyphs, the spacing and proportions of the glyphs is more precise and consistent.

The following two examples show how glyphs may begin on any sub-pixel boundary when sub-pixel positioning is used. The example on the left is rendered using the earlier version of the ClearType renderer, which did not employ sub-pixel positioning. The example on the right is rendered using the new version of the ClearType renderer, using sub-pixel positioning. Note how each e and l in the right-hand image is rendered slightly differently because each starts on a different sub-pixel. When viewing the text at its normal size on the screen, this difference is not noticeable because of the high contrast of the glyph image. This is only possible because of sophisticated color filtering that is incorporated in ClearType.

Text displayed with earlier and later versions of ClearType:-



The following two examples compare output from the earlier ClearType renderer with the new version of the ClearType renderer. The su***xel positioning, shown on the right, greatly improves the spacing of type on screen, especially at small sizes where the difference between a sub-pixel and a whole pixel represents a significant proportion of glyph width. Note that spacing between the letters is more even in the second image. The cumulative benefit of sub-pixel positioning to the overall appearance of a screen of text is greatly increased, and represents a significant evolution in ClearType technology.

Text with earlier and later versions of ClearType:-





Y-Direction Antialiasing

Another improvement of ClearType in Windows Presentation Foundation (WPF) is y-direction anti-aliasing. The ClearType in GDI without y-direction anti-aliasing provides better resolution on the x-axis but not the y-axis. On the tops and bottoms of shallow curves, the jagged edges detract from its readability.

The following example shows the effect of having no y-direction antialiasing. In this case, the jagged edges on the top and bottom of the letter are apparent.

Text with jagged edges on shallow curves:-



ClearType in Windows Presentation Foundation (WPF) provides antialiasing on the y-direction level to smooth out any jagged edges. This is particularly important for improving the readability of East Asian languages where ideographs have an almost equal amount of horizontal and vertical shallow curves.

The following example shows the effect of y-direction antialiasing. In this case, the top and bottom of the letter show a smooth curve.

Text with ClearType y-direction antialiasing:-





Hardware Acceleration

ClearType in Windows Presentation Foundation (WPF) can take advantage of hardware acceleration for better performance and to reduce CPU load and system memory requirements. By using the pixel shaders and video memory of a graphics card, ClearType provides faster rendering of text, particularly when animation is used.

ClearType in Windows Presentation Foundation (WPF) does not modify the system-wide ClearType settings. Disabling ClearType in Windows sets Windows Presentation Foundation (WPF) antialiasing to grayscale mode. In addition, ClearType in Windows Presentation Foundation (WPF) does not modify the settings of the ClearType Tuner PowerToy.

One of the Windows Presentation Foundation (WPF) architectural design decisions is to have resolution independent layout better support higher resolution DPI monitors, which are becoming more widespread. This has the consequence of Windows Presentation Foundation (WPF) not supporting aliased text rendering or the bitmaps in some East Asian fonts because they are both resolution dependent.

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Display requirements

ClearType and allied technologies require display hardware with fixed pixels and su***xels. More precisely, the positions of the pixels and su***xels on the screen must be exactly known to the computer to which it is connected. This is the case for flat-panel displays, on which the positions of the pixels are permanently fixed by the design of the screen itself. Almost all flat panels have a perfectly rectangular array of square pixels, each of which contains three rectangular su***xels in the three primary colors, with the normal ordering being red, green, and blue, arranged in vertical bands. ClearType assumes this arrangement of pixels when rendering text.

ClearType does not work properly with flat-panel displays that are operated at resolutions other than their “native” resolutions, since only the native resolution corresponds exactly to the actual positions of pixels on the screen of the display.

If a display does not have the type of fixed pixels that ClearType expects, text rendered with ClearType enabled may actually look worse than type rendered without it. Some flat panels have unusual pixel arrangements, with the colors in a different order, or with the su***xels positioned differently (in three horizontal bands, or in other ways). ClearType needs to be manually tuned for use with such displays (see below).

Similarly, displays that have no fixed pixel positions, such as CRT displays, may be harder to read if ClearType is enabled.

Additionally, when images are prepared to be display-independent (that is, when they are prepared for distribution, and not just for display on the computer with which they were prepared), ClearType should be turned off if rendered text is part of the image. For example, screenshots should always be prepared with ClearType turned off. Image-editing programs such as Adobe Photoshop or Corel Paint Shop Pro bypass ClearType when rendering text directly, for precisely this reason.