|23 Jun 2009||#1|
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Engineering Changes to ClearType in Windows 7
One of the many passions held by Bill Gates is a passion for reading and so his desire to make reading on PCs a fantastic experience has been an effort ongoing for many years. In the 1998 COMDEX show, Bill Gates unveiled ClearType – hard to believe it was that long ago. Back when it was announced, very few of us had LCD monitors and those that did invested several thousand dollars in one that was 15” and 1024x768 (today one like that costs less than $100). The notions of smoothing and anti-aliasing have been around for many years and are common in the world of typography, animation, and games. ClearType took this to new levels by building on the properties of LCD panels. ClearType was subsequently included in Windows XP and continues in Vista and Windows 7—each release saw changes in the underlying technology, the fonts that support the technology, and the APIs available to developers. It is fair to say that over the years we have learned that there are a set of customers who simply find ClearType rendered screens less than appealing and wish to turn it off. We recognize this and want to make sure we provide the appropriate controls. ClearType is also part of the Windows platform and provides APIs callable and controllable by developers of applications. There is a conventional view that ClearType is a "visual preference" and through this post we want to show how there are elements that are such a preference but there are also elements that are APIs used by applications, just like applications can choose fonts, colors, and other attributes as required.* This post goes into the details of Windows 7’s implementation along with some history and background. Greg Hitchcock is the development lead on ClearType and has worked on it since the start. He’s also one of the most tenured members of the Windows 7 engineering team with only 6 folks having been at Microsoft longer -- Larry is one of them :-)!
Based on feedback, we want to clarify how font rendering works in Windows 7 and give some background on how we chose ClearType font rendering to be the default in Windows. For those that dislike ClearType and want to change the system default setting to bi-level rendering, as were defaults in Windows Millennium, the quick answer is:
ClearType is a technology developed to improve both the appearance of font rendering and reading performance on computer displays. As most people spend over 80% of their time on computers reading on the screen, improvements in this area greatly improve the overall experience of Windows. The ClearType technology has continued to evolve and the latest improvements have been made in Windows 7, as discussed in this earlier E7 post.
In simple terms, ClearType works by using the underlying geometry of colored sub-pixels in the display as if they were full pixels—gaining extra resolution while at the same time using principles of human vision to remove the perception of color artifacts. Further details on the technology and how it uses human visual perception are described here. More specifically, the ClearType technology is optimized for LCD panels with red, green, and blue (RGB) striped sub-pixels that are oriented vertically, although it performs reasonably well on CRT displays (especially those that are aperture grille based) and even LCD panels with horizontally oriented RGB stripes. Although this might seem counterintuitive, through informal studies, we’ve found that about 70% of users prefer ClearType even on these non-optimal displays. Of the 30% who preferred other rendering techniques, their biggest concern with ClearType in these non-optimal cases was the loss of text contrast.
Other Types of Font Rendering in Windows
Given the complex world of many display types and a wide variety of users and their visual systems, how did we go about implementing ClearType into Microsoft Windows? Microsoft did not rush headlong into making ClearType the default rendering. The technology was first released in 2000 with the Windows CE product. The Windows CE environment is usually quite controlled in terms of the hardware used, so it was quite easy to verify that ClearType worked properly on each device, and either tune ClearType or adjust the hardware to optimize the onscreen reading experience. The first release of ClearType on the Windows platform was with Windows XP in 2001.
Prior to Windows XP, two types of font rendering were supported in Windows. The first type of font rendering supported was bi-level rendering, more commonly referred to as “black and white” rendering, but sometimes also called aliased text. With bi-level rendering, two colors represent the font, the foreground color and the background color. This was the first type of rendering supported by TrueType when Windows 3.1 was released, and also the essential method of displaying fonts in bitmap form from Windows 1.0. Bi-level rendering, especially when generated from outline technologies like TrueType, is very difficult to optimize for low screen resolutions. Significant effort needs to be put into the font hinting for TrueType in order to get the best bi-level quality. It can reasonably take a skilled person 6 months to a year per font of hinting time to get this level of rendering detail. That would be multiplied by four for a four-member family. If the character sets are larger, as in some system fonts, it can take even longer.
Font Smoothing / Grayscale
The second form of rendering, known as font smoothing, became the default rendering in Windows 2000 and was first released as an option for Windows 95 in the Plus! Pack. Font smoothing is a hybrid grayscale anti-aliasing technique designed to improve the contrast of fonts over traditional anti-aliasing techniques. There are two factors that differentiate font smoothing from more traditional text anti-aliasing.
First, traditional anti-aliasing works by overscaling the font outline data and then downsampling. Font smoothing uses the same technique, except that it applies font hints prior to overscaling the outline data. Although we don’t have enough space here to fully describe font hinting, I can simplify it enough to say that it often uses a method called “grid fitting” to snap the vertical and horizontal edges of the font outlines so that they are aligned with the pixel grid. In this situation, most horizontal and vertical stems of a font, when overscaled, cover 100% of the underlying pixels, and when downsampled return the text foreground color, which is usually black. Diagonal and round features of the font will not have full coverage of the pixel and thus will return some shade of gray, reflecting the coverage of the underlying pixel. It should be noted that when text displays the “jaggies” (or more formally aliasing) this usually occurs on round or diagonal parts of the glyphs—exactly the areas receiving gray coverage with this method. This way of anti-aliasing is beneficial due to the higher contrast of the stems in the font at a slight cost of some spatial accuracy.
The second differentiating factor is that the fonts determine the exact size that the font smoothing turns on or off. Most fonts that provide this level of information turn on grayscale anti-aliasing below 9 pixels per em (PPEM.) That is the equivalent of 7 points on a 96 PPI screen. Above 9 PPEM, anti-aliasing is turned off until the main stems of the font are around two pixels wide, which is around 13 to 20 points, depending on the typeface. Once the main stems are two pixels wide, anti-aliasing remains on as the sizes increase. Two pixel wide stems are usually chosen because there is usually enough “backbone” of foreground colored pixels to keep the stem contrast high.* If the font does not have specific sizes for font smoothing, system defaults are used. There are independent defaults for both regular and bold typefaces. So although font smoothing was the default, most fonts, when displaying text at typical reading sizes, would render them bi-level.
Defaults for Font Rendering
With the addition of ClearType to Windows XP, there were now three types of font rendering available (Bi-level, Font Smoothing, ClearType). During the time period that Windows XP was being developed there was a clear transition underway from desktop systems with CRT monitors to laptops and desktops with LCD displays. Since this transition was far from complete, we felt that the default value for font rendering in Windows XP should be grayscale font smoothing, the same as Windows 2000. OEM’s who provide Windows XP on their systems could change this default, and in fact, by the time Windows XP SP 2 shipped, many of them had set the default to ClearType. It should be noted that OEMs can always change these settings as part of configuring a PC.
In Windows Vista, the system’s default font rendering was changed to ClearType. It is important to clearly understand what is meant by default font rendering. In Windows, the default font rendering is the rendering used when the application does not choose an explicit type of rendering. Some have confused this default value to mean that all applications must use this value. This view is inconsistent with the way text APIs worked when introduced in Windows 95’s font smoothing. In GDI, the API for choosing the current font has the rendering type explicitly as input. It is expected that there are situations where the application knows best what type of rendering should be used. For example, in displaying a print preview page with small text, traditional font smoothing might be the best choice for rendering. Likewise, if an application was targeting on-screen reading, it might be best to use ClearType as the rendering for that application. In some situations, like remote terminal services, the application might choose to use bi-level rendering to reduce the bandwidth of text information that needs to be sent to a remote client.
There are many examples where applications decide one way or another to use rendering other than the system default—just as applications that choose to use different fonts, colors, sizes, or other text attributes.* The most typical example is in applications that wish to have reproducible layout and flow of documents.* By being specific about which way to render text, the applications can be certain of how text will flow across different PCs.* Another common example, as mentioned above, is Print Preview where the ability to properly render representations of higher resolution output, particularly for small text sizes, is much improved.* We recognize that for some it is counter-intuitive that an aspect viewed as a “display” property is something that applications can choose to “over-ride”.* We’ve designed rendering so that the default case is to respect the setting, but applications, including Windows itself, might have elements that require explicit rendering techniques.
Although each application can make the choice on a per-font basis of which rendering to use, the majority of applications choose the default rendering. Therefore, making the decision to change the default for Windows Vista was not taken lightly. The trends in the hardware displays were strongly showing a rapid movement from CRTs to LCD-based displays as we have shown in earlier blog posts based on the Windows XP and Windows Vista real-world telemetry. Even though there were still CRTs in use, feedback from Windows XP customers was positive on the quality of ClearType rendering on CRTs. After we made the choice, the feedback on the decision to enable ClearType as the default for Windows Vista was overwhelmingly positive.
Even with the default rendering set to ClearType, there are some scenarios that can change the default. If an OEM is providing Windows on their hardware, they can change the default. In some situations, and this was more common with font smoothing in Windows 95, the hardware may not meet the minimum requirements for the rendering technology. In the case of both font smoothing and ClearType, a minimum of sixteen bits per pixel display resolution is required. (When rendering to an off-screen bitmap in GDI, it is important that it not be the default color depth of 1 bit per pixel if you desire to capture ClearType text.) In some cases, when optimizing for system performance, font smoothing (both ClearType and grayscale) can be disabled. In a similar fashion, using Remote Desktop to connect to another computer or session usually disables ClearType by default.
Changing the Default Rendering in Windows 7
Windows 7 maintains the same defaults as Windows Vista. There are several ways for the user to change the default values for font rendering in Windows 7. For those that want the default rendering to be bi-level, the user interface for this selection is in the performance section of the Control Panel. From the root of the control panel you can access it by selecting System and Security –> System –> Advanced System Settings –> Performance (Settings…). An easier way is to enter “Appearance” into the start menu search, and then select “Adjust the appearance and performance of Windows.” The setting that should be changed under the custom option is: Smooth edges of screen fonts, as shown in the figure.
The option of no font smoothing as the default value is considered to be an uncommon setting, so it is a little more difficult to find than other settings. If the user prefers to change the default font rendering selection to the Windows grayscaling anti-aliasing technique described earlier, in Windows 7 that is now done through the ClearType Tuner.
The quality of the ClearType text can be optimized for you and your monitor. The ClearType Tuner is a new control panel component for Windows 7. Because there are differences in monitor characteristics and differences between readers’ eyes, there are font rendering options that can only be optimized by a reader looking at text on their monitor. The ClearType Tuner uses various samples of ClearType, presented in the form of an eye-test, to make fine grained adjustments to the ClearType algorithms. Each wizard page tunes a parameter such as monitor gamma (relationship between voltage and brightness), your sensitivity to color artifacts, and your preference for letter heaviness.
In order to switch between ClearType and grayscale, the setting “Turn on ClearType” on the opening page of the ClearType Tuner can be toggled.
Either way, the user is taken through the rest of the ClearType Tuning wizard for two reasons; if an application explicitly enables ClearType rendering, it is useful for that experience to be tuned, and some graphics platforms have more fine tuning of the rendering for both gray rendering and ClearType.
Font Design and Font Rendering
The availability of higher resolution font rendering techniques like ClearType has had a significant impact on the design of fonts for onscreen reading. From the early days of the printing press, as new technologies and printing styles were developed, typefaces were redesigned to take advantage of those technologies. For example, many typefaces still in use today incorporate “ink traps” into the design so that ink would not clog up key features of a glyph. This is an aspect of making specific design choices in the font in order to work the best with the technology. In traditional typeface design, the term font refers to a typeface at a given size. So a 10 point Times New Roman would be a different font from a 24 point Times New Roman. In the days of metal cast typography, each of these sizes were designed by a punch cutter to be optimized for the medium for which they were to be used, often with changes in stem contrast, x-height, or character spacing for a given size. The advent of photo typesetting in the mid-twentieth century was a step backwards in this regard, as it used one size as a type master, and then used optics to scale that master size to any other presentation size.
Microsoft Windows has taken the more traditional approach to digital outline fonts, and through a combination of font hinting and new typeface design we attempt to optimize each size for the medium for which they were intended. With Microsoft’s initial release of TrueType for Windows 3.1, the traditional typefaces Times New Roman, Arial, and Courier New were used as core fonts. In the creation of these fonts, one master size was chosen for the outline data, usually something around 10 or 12 point, and, similar to the technique used in photo-typesetting, the outlines could be scaled to any requested size for a given display resolution. But, going back to the more traditional ways, each size was carefully examined and changes were made to the basic design through font hinting—including changes to critical features like stem contrast, x-height, or glyph spacing. As earlier mentioned, hinting fonts to be tuned for a low-resolution medium like full pixels on a 96 PPI screen was very time consuming. To help in this process for Microsoft Windows, we commissioned or designed in-house new outline typefaces designs that were optimized for the world of 96 PPI bi-level rendering. These custom fonts include Tahoma, Verdana, Georgia, Trebuchet MS, and even Comic Sans MS. These fonts still needed to be hinted to tune the individual sizes, but because the typeface was designed with the medium in mind, it was a more straightforward process and less time consuming.
Even with typefaces tuned to the display medium, 96 PPI pixels on a screen are still larger than many of the features we’d like to show in a typeface—and that is where ClearType helps us. Therefore, with ClearType, it made sense to commission a new set of fonts that were optimized for this new medium. Now the existing fonts for Windows still work well with the technology, but this project was an attempt to get the very best design for onscreen reading using ClearType. This led to a new set of fonts that shipped and were tuned for Windows Vista. The ClearType Collection fonts of Calibri, Cambria, Consolas, Corbel, Candara, Constantia, the new user interface font Segoe UI, and the Japanese font Meiryo were designed for this medium. As part of the engineering work on these font projects along with the default setting of ClearType, we decided in the hinting process to do the fine, size-specific hinting only for ClearType, and not for bi-level rendering. This allowed us to focus our efforts on the fine levels of detail and quality for the vast majority of customers.
ClearType Fonts in Windows 7
A reasonable question for us to ask ourselves is what is the experience like in Windows 7 when bi-level or hybrid font smoothing is chosen as the default?
As mentioned earlier, not all applications will choose to render with the default settings. Microsoft Office and Internet Explorer will default in some cases to using ClearType rendering. Some applications that use fonts tuned for ClearType and not bi-level rendering may choose ClearType rendering to maintain the benefits of the font designs. Some applications need higher precision glyph widths like sub-pixel positioning or “natural width ClearType,” and would reflow if they were changed to bi-level or grayscale rendering. Other applications like Adobe Reader have their own built-in text rendering engine that is independent of the Windows graphics platforms. Likewise, platforms like Java on Windows also use their own rendering techniques.
In some situations with the Windows 7 Explorer, ClearType rendering will remain on so that Segoe UI will keep its optimal design. Changing the system font from Segoe UI to some other font could be problematic, leading to issues like reflowing dialog box entries, missing text due to wrapping, unlabeled buttons, etc. We know many would value global changes to the fonts used by Windows, but to maintain to reliably across resolutions, DPI, and languages to name a few issues means we cannot have total flexibility on the system font settings at this time.
Given the challenges of turning off ClearType, there are a few mitigations in the fonts to handle some scenarios where ClearType is not available. In the ClearType font Calibri, since it is the default font for Microsoft Office, an unusual technique was used to attempt to improve the quality of the font rendering when font smoothing grayscale was selected. In this case, as opposed to the normal situation where font smoothing was disabled at lower text sizes to remove the blur, at these lower sizes the font enabled grayscale in order to improve the character shape. Also, at a few key sizes, the Calibri font had some bitmap fonts embedded in the outline file. These bitmaps kick in when bi-level rendering is requested. These bitmaps were intended to handle the case where Calibri was being used in a Remote Terminal situation and the default for Remote Terminal was not set to ClearType for performance reasons.
ClearType Research on Performance
As mentioned earlier, one of the goals behind ClearType is to improve the performance of reading text on computer screens. We have supported several areas of research looking into measuring this work. The research is done at universities and published in peer-reviewed journals. We have another Microsoft blog, that among other things related to fonts, also describes some of the research work on reading performance. Since those blog entries give more detail and background, we’ll just describe some of the performance highlights.
Another research question we’ve asked ourselves is why do some people prefer bi-level rendering over ClearType? Is it due to hardware issues or is there some other attribute that we don’t understand about visual systems that is playing a role. This is an issue that has piqued our curiosity for some time. Our first attempt at looking further into this involved doing an informal and small-scale preference study in a community center near Microsoft. This was done with two identical laptops, one with ClearType and one with bi-level rendering. They were placed side by side and participants were asked which version they preferred. This was done with three different samples. Here were the results:
* Prefer ClearTypeComments:
Further examination of those who prefer bi-level rendering is of great interest to us and we continue to research this topic and to work with university researchers as well. We expect to see published papers on this topic in the future.
Going forward, much of our research is in finding ways to make the highest quality text rendering more accessible to everyone. Each visual system has its own characteristics, and just as the ClearType tuner allows us to tune the algorithm for display characteristics, it would also be nice to tune for visual system characteristics. For example, in the United States 7% of the male population is color blind. We believe that we can improve the ClearType algorithm so that text for a colorblind reader is even better than for a reader without colorblindedness. Researching ways to improve text rendering for those with high color sensitivity and lower visual acuity would be just as important for us.
Making the screen the best possible place to read is an exciting opportunity for us.* It blends the engineering challenges of working with many display technologies and human visual systems with the artistic challenge of creating a beautiful set of glyphs, where every subtle typographic nuance is important.* In doing this, we need to keep in mind how the science of reading must guide us in making the experience optimal for us—humans. Each rendering technology has advantages and disadvantages for different people; depending on the application in use there are tradeoffs involved. Many of these issues go beyond the ability for people to easily discern choices. Our job is to work hard to provide a great platform for developers as well as tools that people can use to make choices and control how they use their technology. Our goal should be that the out-of-box experience just works. We think that, most of the time, we’ve accomplished this and we also recognize this area is complex and there is a wide spectrum of feedback.
The team at Microsoft working on these problems has been together since 1990, developing fonts and font-rendering solutions, and working to get a better understanding of the science of reading. The team is made up of engineers, type designers/artists, and psychologists and we work with many other experts throughout Microsoft in attempting to tackle this tough, yet vitally important task. You spend over 80% of the time at the computer reading, so it should be as pleasant an experience as possible. The following article from IEEE Spectrum describes some of the issues we deal with related to the technology, art, and science of text.
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