microsoft

ClearType is Microsoft's implementation of subpixel rendering technology in rendering text in a font system. ClearType attempts to improve the appearance of text on certain types of computer display screens by sacrificing color fidelity for additional intensity variation. This trade-off is asserted to work well on LCD flat panel monitors.

ClearType was first announced at the November 1998 COMDEX exhibition. The technology was first introduced in software in January 2000[1] as an always-on feature of Microsoft Reader, which was released to the public in August 2000. ClearType was later introduced as a built-in 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, Internet Explorer 7 and Windows Live Messenger, 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 anti-aliasing works on a principle where the text is first rendered with a few tweaks in horizontal hinting and six times the horizontal size, then the red channel is shifted two pixels right, the blue channel is shifted two pixels left, and the result is scaled down six times horizontally with a box filter.

Background

Computer displays in which the positions of individual pixels are permanently fixed by the design of the hardware—such as most modern flat panel displays—can show saw-tooth edges when displaying small, high-contrast graphic elements such as text. ClearType uses anti-aliasing at the subpixel level to allegedly reduce visible artifacts on such displays when text is rendered, making the text appear "smoother" and less jagged. ClearType also uses very heavy font hinting to force the font to fit into the pixel grid. This increases edge contrast and readability of small fonts at the expense of font rendering fidelity and has been criticised by graphic designers for making different fonts look similar.

Like most other types of subpixel rendering, ClearType actually involves a compromise, sacrificing one aspect of image quality (color or chrominance detail) for another (light and dark or luminance detail). The compromise can improve text appearance when luminance detail is more important than chrominance.

ClearType is applied only to text that is rendered as such by user and system applications. Other graphic display elements (including text that has already been converted to bitmaps) are not altered by ClearType. For example, text in Microsoft Word will be rendered on the screen with ClearType enhancement, but text placed in a bitmapped image in a program such as Adobe Photoshop will not be modified. In theory, the method (called "RGB Decimation" internally) can be applied to enhance the antialiasing of any digital image.[2]

ClearType is not used when printing text. Most printers already use such small pixels that aliasing is rarely a problem and, in any case, they don't have the addressable fixed subpixels ClearType requires. Nor does ClearType affect text stored in files. ClearType only applies any processing to the text while it is being rendered onto the screen.

ClearType was invented in the Microsoft e-Books team by Bert Keely and Greg Hitchcock. It was then analyzed by researchers in the company, and signal processing expert John Platt designed an improved version of the algorithm.[3] Dick Brass, a Vice President at Microsoft from 1997 to 2004, complained that the company was slow in moving ClearType to market in the portable computing field.[4]

ClearType and human vision

ClearType and similar technologies work on the theory that variations in intensity are more noticeable than variations in color. Thus, when ClearType sacrifices color accuracy in order to increase luminance detail, the overall effect—as seen by human eyes—should be an improvement for most people.[citation needed]

According to MSDN website,[5] Microsoft acknowledges that "[t]ext 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[6] by the font designer Thomas Phinney, program manager for fonts and core technologies at Adobe Systems:[7] "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,[8] agrees that ClearType may look blurry at 96 dpi, which was a typical[9] resolution for LCDs in 2008, but adds that higher resolution displays improve on this aspect:[10]

WPF [Windows Presentation Foundation] uses method C [ClearType], 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.

Display requirements

ClearType and allied technologies require display hardware with fixed pixels and subpixels. More precisely, the positions of the pixels and subpixels 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 subpixels 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 actually looks worse than type rendered without it. Some flat panels have unusual pixel arrangements, with the colors in a different order, or with the subpixels 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, are 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.

Sensitivity to display orientation

Because ClearType utilizes the physical layout of the red, green, and blue pigments of the LCD screen, it is sensitive to the orientation of the display.

ClearType in Windows XP supports the RGB and BGR sub pixel structures. Rotated displays, in which the subpixels are arranged vertically rather than horizontally, are not currently supported.[11] Using ClearType on these display configurations will actually reduce the display quality. The best option for users of Windows XP having rotated LCDs (Tablet PCs or swivel-stand LCDs) is using regular anti-aliasing, or switching off font-smoothing altogether.

The software developer documentation for Windows CE states that ClearType for rotated screens is supported on that platform.[12]

Vertical sub-pixel structures are not supported in Windows XP.[13]

Implementations

ClearType is also an integrated component of the Windows Presentation Foundation text-rendering engine.

ClearType Font Collection

As part of Windows Vista's release, Microsoft released a set of fonts, known as the ClearType Font Collection, thought to work well with the ClearType System:

Fonts included by some, but not always part of the set:

ClearType in GDI

ClearType can be globally enabled or disabled for GDI applications. A control panel applet is available to let users tune the GDI ClearType settings. The GDI implementation of ClearType does not[14] support sub-pixel positioning.

ClearType tuning

Microsoft 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.[15] The tool can also be used for tuning Windows Vista and Windows 7 ClearType settings, on Windows 7 it comes preinstalled and is located inside Control Panel\Appearance and Personalization\Display\Adjust ClearType text. 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[16] 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.[17][dubious – discuss] WPF supports aggressive caching of pre-rendered ClearType text in video memory.[18] 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.

ClearType in DirectWrite

The font rendering engine in DirectWrite supports rendering to sub-pixel positions, as demonstrated at PDC 2008. [19]

Patents

ClearType is a registered trademark and is protected by the following U.S. patents:[20]

As of late 2008, another patent is still pending.[20]

References

  1. ↑ "First ClearType screens posted". Microsoft Typography. 2000-01-26. Retrieved 2008-03-20. 
  2. ↑ Betrisey et al., "Displaced Filtering for Patterned Displays", Proc. Society for Information Display Symposium, 2000
  3. ↑ Platt, J.C., "Optimal Filtering for Patterned Displays", IEEE Signal Processing Letters, 7(7) 2000
  4. ↑ Microsoft’s Creative Destruction
  5. ↑ Windows Presentation Foundation ClearType Registry Settings
  6. ↑ "ClearType, in XP and Vista". Typophile. Retrieved 2010-01-22. 
  7. ↑ "Thomas Phinney". MyFonts. 1999-02-22. Retrieved 2010-01-22. 
  8. ↑ "Beat Stamm". MyFonts. 1999-02-22. Retrieved 2010-01-22. 
  9. ↑ "Dpi: Definition and additional resources from ZDNet". Dictionary.zdnet.com. Retrieved 2010-01-22. 
  10. ↑ "ClearType, in XP and Vista". Typophile. Retrieved 2010-01-22. 
  11. ↑ brandon.furtwangler blog » Blog Archive » Tablets and cleartype, and a requested feature of Avalon
  12. ↑ Working with ClearType Fonts
  13. ↑ Tablets and cleartype, and a requested feature of Avalon at Brandon Furtwangler blog
  14. ↑ Windows Presentation Foundation ClearType Overview
  15. ↑ "Microsoft's ClearType Tuner PowerToy". Retrieved 2007-09-27. 
  16. ↑ Tips for improving your WPF text rendering experience
  17. ↑ WPF ClearType anti-aliasing cannot be turned off
  18. ↑ MSDN Library : .NET Development : WPF : ClearType Overview
  19. ↑ Kam VedBrat, Leonardo Blanco (2008-10-28). "PC18: Introducing Direct2D and DirectWrite". Microsoft. 
  20. ↑ 20.0 20.1 "Microsoft Intellectual Property and Licensing: ClearType". Retrieved 2008-12-02. 

External links