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ClearType

ClearType

Overview
ClearType is a trademark for Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

's implementation of subpixel rendering
Subpixel rendering
Subpixel rendering is a way to increase the apparent resolution of a computer's liquid crystal display or Organic Light Emitting Diode display by rendering pixels to take into account the screen type's physical properties...

 technology. ClearType attempts to improve the appearance of text on certain types of computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...

 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
COMDEX
COMDEX was a computer expo held in Las Vegas, Nevada, each November from 1979 to 2003. It was one of the largest computer trade shows in the world, usually second only to the German CeBIT, and by many accounts one of the largest trade shows in any industry sector...

 exhibition. The technology was first introduced in software in January 2000 as an always-on feature of Microsoft Reader
Microsoft Reader
Microsoft Reader is a Microsoft program for the reading of e-books, originally released in August 2000.Microsoft Reader is available for download from Microsoft as a free program for computers running Windows. It can also be used on a Pocket PC, where it has been built into the ROM since Windows CE...

, which was released to the public in August 2000.
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Encyclopedia
ClearType is a trademark for Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

's implementation of subpixel rendering
Subpixel rendering
Subpixel rendering is a way to increase the apparent resolution of a computer's liquid crystal display or Organic Light Emitting Diode display by rendering pixels to take into account the screen type's physical properties...

 technology. ClearType attempts to improve the appearance of text on certain types of computer display
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...

 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
COMDEX
COMDEX was a computer expo held in Las Vegas, Nevada, each November from 1979 to 2003. It was one of the largest computer trade shows in the world, usually second only to the German CeBIT, and by many accounts one of the largest trade shows in any industry sector...

 exhibition. The technology was first introduced in software in January 2000 as an always-on feature of Microsoft Reader
Microsoft Reader
Microsoft Reader is a Microsoft program for the reading of e-books, originally released in August 2000.Microsoft Reader is available for download from Microsoft as a free program for computers running Windows. It can also be used on a Pocket PC, where it has been built into the ROM since Windows CE...

, which was released to the public in August 2000. ClearType was later introduced as an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 feature in Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...

, where it was kept turned off by default. In Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

 and Windows 7 ClearType is turned on by default. In Microsoft Office 2007
Microsoft Office 2007
Microsoft Office 2007 is a Windows version of the Microsoft Office System, Microsoft's productivity suite. Formerly known as Office 12 in the initial stages of its beta cycle, it was released to volume license customers on November 30, 2006 and made available to retail customers on January 30, 2007...

 and later versions, Internet Explorer 7
Internet Explorer 7
Windows Internet Explorer 7 is a web browser released by Microsoft in October 2006. Internet Explorer 7 is part of a long line of versions of Internet Explorer and was the first major update to the browser in more than 5 years...

 and later versions and Windows Live Messenger
Windows Live Messenger
Windows Live Messenger is an instant messaging client created by Microsoft that is currently designed to work with Windows XP , Windows Vista, Windows 7, Windows Mobile, Windows CE, Xbox 360, Blackberry OS, iOS, Java ME, S60 on Symbian OS 9.x and Zune HD...

, ClearType is also 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
Windows Presentation Foundation
Developed by Microsoft, the Windows Presentation Foundation is a computer-software graphical subsystem for rendering user interfaces in Windows-based applications. WPF, previously known as "Avalon", was initially released as part of .NET Framework 3.0. Rather than relying on the older GDI...

 text-rendering engine.

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
Anti-aliasing
In digital signal processing, spatial anti-aliasing is the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution...

 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
Font hinting
Font hinting is the use of mathematical instructions to adjust the display of an outline font so that it lines up with a rasterized grid. At low screen resolutions, hinting is critical for producing a clear, legible text...

 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
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...

detail) for another (light and dark or luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...

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 bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...

s) 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.

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
John Platt
John Platt is a Principal Researcher in the Machine Learning department at Microsoft Research. Platt has worked for Microsoft since 1997. Prior to Microsoft, Platt had served as Director of Research at Synaptics....

 designed an improved version of the algorithm. Dick Brass
Dick Brass
Dick Brass is a technology investor and executive, and a former newspaper reporter and editor.Brass attended Cornell University, where he was an editor of The Cornell Daily Sun and member of the Quill and Dagger society...

, a Vice President at Microsoft from 1997–2004, complained that the company was slow in moving ClearType to market in the portable computing field.

How ClearType works


Normally, the software in a computer treats the computer’s display screen as a rectangular array of square, indivisible pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

s,
each of which has an intensity and color that are determined by the blending of three primary color
Primary color
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....

s: red, green, and blue. However, actual display hardware usually implements each pixel as a group of three adjacent, independent subpixels, each of which displays a different primary color. Thus, on a real computer display, each pixel is actually composed of separate red, green, and blue subpixels. For example, if a flat-panel display is examined under a magnifying glass, the pixels may appear as follows:
In the illustration above, there are nine pixels, but there are 27 subpixels.

If the computer controlling the display knows the exact position and color of all the subpixels on the screen, it can take advantage of this to improve the apparent resolution of the images on the screen in certain situations. If each pixel on the display actually contains three rectangular subpixels of red, green, and blue, in that fixed order, then things on the screen that are smaller than one full pixel in size can be rendered by lighting only one or two of the subpixels. For example, if a diagonal line with a width smaller than a full pixel must be rendered, then this can be done by lighting only the subpixels that the line actually touches. If the line passes through the leftmost portion of the pixel, only the red subpixel is lit; if it passes through the rightmost portion of the pixel, only the blue subpixel is lit. This effectively triples the horizontal resolution of the image at normal viewing distances; but the drawback is that the line thus drawn will show color fringes (at some points it might look green, at other points it might look red or blue).

ClearType uses this method to improve the smoothness of text. When the elements of a type character are smaller than a full pixel, ClearType lights only the appropriate subpixels of each full pixel in order to more closely follow the outlines of the character. Text rendered with ClearType looks “smoother” than text rendered without it, provided that the pixel layout of the display screen exactly matches what ClearType expects.

The following picture shows a 4× enlargement of the word Wikipedia rendered using ClearType.
The word was originally rendered using a Times New Roman 12 pt
Point (typography)
In typography, a point is the smallest unit of measure, being a subdivision of the larger pica. It is commonly abbreviated as pt. The point has long been the usual unit for measuring font size and leading and other minute items on a printed page....

 font
Typeface
In typography, a typeface is the artistic representation or interpretation of characters; it is the way the type looks. Each type is designed and there are thousands of different typefaces in existence, with new ones being developed constantly....

.


In this magnified view, it becomes clear that, while the overall smoothness of the text seems to improve, there is also color fringing of the text.

An extreme close-up of a color display shows (a) text rendered without ClearType and (b) text rendered with ClearType. Note the changes in subpixel intensity that are used to increase effective resolution when ClearType is enabled—without ClearType, all sub-pixels of a given pixel have the same intensity.

The above shows an animation with some lines of text. In the frame with the orange circle present, the text is rendered using Cleartype (RGB subpixel rendering), and in the one without it, the text is rendered using normal (full pixel greyscale) antialiasing.

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.

According to MSDN website, 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 by the font designer Thomas Phinney, program manager for fonts and core technologies at Adobe Systems
Adobe Systems
Adobe Systems Incorporated is an American computer software company founded in 1982 and headquartered in San Jose, California, United States...

: "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 worked on ClearType at Microsoft, agrees that ClearType may look blurry at 96 dpi, which was a typical resolution for LCDs in 2008, but adds that higher resolution displays improve on this aspect:

The above image demonstrates the same portion of text without and with ClearType rendering (as opposed to Standard and ClearType in the previous image). This and the previous image demonstrate the blurring introduced. For many observers this blurring is beneficial. Some observers, however, do not find the use of ClearType beneficial. According to a study at the University of Texas, ClearType "may not be universally beneficial". The study notes that maximum benefit may be seen when the information worker is spending large proportions of their time reading text (which is not necessarily the case for the majority of computer users today). Additionally, over one third of the study participants experienced some disadvantage when using ClearType. Whether ClearType, or other rendering, should be used is very subjective and it must be the choice of the individual, with the report recommending "to allow users to disable [ClearType] if they find it produces effects other than improved performance".

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).

Displays that have no fixed pixel positions, such as shadow mask
Shadow mask
The shadow mask is one of two major technologies used to manufacture cathode ray tube televisions and computer displays that produce color images. The other approach is aperture grille, better known by its trade name, Trinitron. All early color televisions and the majority of CRT computer monitors...

 CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

 displays, may be harder to read if ClearType is enabled. However, on CRT displays with a similar pixel arrangement as flat-panel displays, such as aperture grille
Aperture grille
An aperture grille is one of two major technologies used to manufacture color cathode ray tube televisions and computer displays; the other is shadow mask....

 and Trinitron
Trinitron
Trinitron is Sony's brand name for its line of aperture grille based CRTs used in television sets and computer display monitors. One of the first truly new television systems to enter the market since the 1950s, the Trinitron was announced in 1966 to wide acclaim for its bright images, about 25%...

 CRT displays, it can also result in a slightly improved readability.

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
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...

 currently supports the RGB and BGR sub pixel structures. Rotated displays, in which the subpixels are arranged vertically rather than horizontally, are not currently supported. Using ClearType on these display configurations will actually reduce the display quality. The best option for users of Windows XP having rotated LCD displays (Tablet PCs
Microsoft Tablet PC
A Microsoft Tablet PC is a term coined by Microsoft for tablet computers conforming to a set of specifications announced in 2001 by Microsoft, for a pen-enabled personal computer, conforming to hardware specifications devised by Microsoft and running a licensed copy of Windows XP Tablet PC Edition...

 or swivel-stand LCD displays) is using regular anti-aliasing, or switching off font-smoothing altogether.

The software developer documentation for Windows CE
Windows CE
Microsoft Windows CE is an operating system developed by Microsoft for embedded systems. Windows CE is a distinct operating system and kernel, rather than a trimmed-down version of desktop Windows...

 states that ClearType for rotated screens is supported on that platform.

Vertical sub pixel structures are not supported in Windows XP.

ClearType in GDI


ClearType can be globally enabled or disabled for GDI
Graphics Device Interface
The Graphics Device Interface is a Microsoft Windows application programming interface and core operating system component responsible for representing graphical objects and transmitting them to output devices such as monitors and printers....

 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


Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

, 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
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

 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
    RGB color model
    The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...

     sub pixel structure, though BGR is quite rare, so the default works for most monitors
  3. Contrast


Microsoft Reader
Microsoft Reader
Microsoft Reader is a Microsoft program for the reading of e-books, originally released in August 2000.Microsoft Reader is available for download from Microsoft as a free program for computers running Windows. It can also be used on a Pocket PC, where it has been built into the ROM since Windows CE...

 has a similar ClearType tuner that only affects that program.

ClearType in WPF


All text in Windows Presentation Foundation
Windows Presentation Foundation
Developed by Microsoft, the Windows Presentation Foundation is a computer-software graphical subsystem for rendering user interfaces in Windows-based applications. WPF, previously known as "Avalon", was initially released as part of .NET Framework 3.0. Rather than relying on the older GDI...

 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
Hardware acceleration
In computing, Hardware acceleration is the use of computer hardware to perform some function faster than is possible in software running on the general-purpose CPU...

. However, due to the resolution-independent
Resolution independence
In computing, resolution independence is the concept that elements on a computer screen can be drawn at sizes independent from the pixel grid. This is done so that those with larger screens and more compact screens can still view the UI at the same size....

 architecture of WPF, ClearType cannot be optionally turned off in WPF applications. WPF supports aggressive caching of pre-rendered ClearType text in video memory
Video memory
Video memory is a term generally used in computers to describe some form of writable memory, usually RAM, dedicated to the purpose of holding the information necessary for a graphics card to drive a display device...

. The extent to which this is supported is dependent on the video card
Video card
A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...

. DirectX
DirectX
Microsoft DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay,...

 10 cards will be able to cache the font glyphs in video memory
Video memory
Video memory is a term generally used in computers to describe some form of writable memory, usually RAM, dedicated to the purpose of holding the information necessary for a graphics card to drive a display device...

, 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
DirectWrite
DirectWrite is a text-layout and glyph-rendering API by Microsoft. It was designed to replace GDI/GDI+ and Uniscribe for screen-oriented rendering and was shipped with Windows 7 and Windows Server 2008 R2, as well as Windows Vista and Windows Server 2008 DirectWrite is a text-layout and...

 supports rendering to sub-pixel positions, as demonstrated at PDC
Professional Developers Conference
Microsoft's Professional Developers Conference is a conference for software developers, normally Windows developers.It covers new and upcoming technology from Microsoft, and so only occurs in the years when there is something new to talk about...

 2008.

Patents


ClearType is a registered trademark and is protected by the following U.S. patents:
  • Subpixel rendering:
    • Method and apparatus for displaying images such as text
    • Mapping image data samples to pixel sub-components on a striped display device
    • Weighted mapping of image data samples to pixel sub-components on a display device
    • Methods and apparatus for performing image rendering and rasterization operations
  • Complex color filtering:
    • Mapping samples of foreground/background color image data to pixel sub-components
    • Method and apparatus for detecting and reducing color artifacts in images
    • Methods apparatus and data structures for enhancing the resolution of images to be rendered on patterned display devices
  • Subpixel font hinting and layout:
    • Methods and apparatus for performing grid fitting and hinting operations
    • Maintaining advance widths of existing characters that have been resolution enhanced
  • ClearType tuning:
    • Method and apparatus for improving the quality of displayed images through the use of user reference information


As of late 2008, another patent is still pending.

See also

  • Font rasterization
    Font rasterization
    Font rasterization is the process of converting text from a vector description to a raster or bitmap description. This often involves some anti-aliasing on screen text to make it smoother and easier to read...

  • FreeType
    FreeType
    FreeType is a software library written in C that implements a font rasterization engine. It is used to render text on to bitmaps and provides support for other font-related operations.-Details:...

  • OpenType
    OpenType
    OpenType is a format for scalable computer fonts. It was built on its predecessor TrueType, retaining TrueType's basic structure and adding many intricate data structures for prescribing typographic behavior...

  • Apple Advanced Typography
    Apple Advanced Typography
    Apple Advanced Typography is Apple Inc's computer software for advanced font rendering, supporting internationalization and complex features for typographers, a successor to Apple's little-used QuickDraw GX font technology of the mid-1990s...

  • Subpixel rendering
    Subpixel rendering
    Subpixel rendering is a way to increase the apparent resolution of a computer's liquid crystal display or Organic Light Emitting Diode display by rendering pixels to take into account the screen type's physical properties...

  • Adobe CoolType
    CoolType
    CoolType is a software technology, designed by Adobe Systems to increase the legibility of text on color LCDs like laptop or TFT monitors, especially to make reading long text, like E-Books, easier...

    , a similar technology
  • Quartz (graphics layer)
    Quartz (graphics layer)
    Quartz specifically refers to a pair of Mac OS X technologies, each part of the Core Graphics framework: Quartz 2D and Quartz Compositor. It includes both a 2D renderer in Core Graphics and the composition engine that sends instructions to the graphics card...


External links