Glyphviewer Documentation

What is Glyphviewer?

Glyphviewer is a web application that analyses web font files.

  • It lists the characters supported by a font using its glyphs. This is the raison d'être for the application.
  • It displays information present in the font's header, such as developer and license details.
  • It provides a test bed for users to enter their own message, and see it displayed using the font.

For those unsure about the meaning of "glyph", "font", and "web font", a small glossary is provided at the end.

Glyphviewer Usage

Glyphviewer analyses font files, not fonts per se. (Computer fonts often consist of multiple files.) There are several formats supported

  • TrueType (.ttf)
  • OpenType (.otf)
  • Web Open Font Format (.woff). This is a font format designed for the web, as font data is compressed by default. Since this results in smaller font sizes, fonts are quicker to download.
  • Web Open Font Format 2 (.woff2), which manages even better compression (and thus even quicker download times). This is a new standard as of March 2018.

There are two places where font files can be hosted for analysis.

  • The default behavior for Glyphviewer is to analyse font files stored on the Local server hosting this application. When a user first encounters this page, the application randomly selects one from its file system, analyses it, and displays the results to the user. Users can choose other font files stored locally, as long as Local is checked; they just select an item from the File drop down list. All fonts are listed by file name, so File will display (say) "DejaVuSans.woff" rather than "DejaVu Sans".
  • More adventurous users can choose a font somewhere else on the Internet by checking Remote instead. They write the full URL for the font file (including the extension) in the URL control. For example, here's an URL for the Lato web font ready for copying-and-pasting. However, the remote font must be accessible for sharing under Cross-origin Resource Sharing (CORS).

Other controls present are:

  • Shows characters in font: If checked, all characters in the font are presented as a table to the user (with the exception of any line breaks and control characters present inside). The advantage is that users can discover which characters and languages are supported by the font. Moreover, people can inspect individual glyphs for legibility and appearance. The disadvantage is that the resulting page can be quite large.
  • Arrange characters as Unicode blocks: If checked, Glyphviewer splits characters into different tables according to their Unicode blocks. If not checked, all characters are displayed in one gigantic table. If Shows characters in font is not selected, this checkbox is ignored altogether.

To finish, users hit the Submit button.

On Success

If Glyphviewer succeeds, it will generate a page with the following sections.

  • General information: This data is taken from the font's header; it generally includes details such as name, designer, and copyright information. The last piece of data is extremely useful to know, as it may indicate whether the font is legally free to use or not.
  • Testbed: This provides an area where users can play around with the font. Fields of interest are:
    • Enter some text to try out the font: This is a multi-line field for changing the message (because "Hello World!" gets boring quickly).
    • Font size (in pixels): This sets the size of the message text in pixels. But to actually resize the text, one has to click...
    • Try it: This rewrites and redisplays the message.
    • Reset: This resets the message to the default text and size.
  • Supported characters: if Shows characters in font is selected, Glyphviewer will generate a table (or tables) of the characters provided by the font. For maximum utility, characters will be displayed using the actual glyphs in the font, and each character is displayed with its Unicode code point underneath. However, some fonts include characters such as the humble "NULL" (U+0000) which were never intended for display. Rather than writing them to the screen, Glyphviewer lists them instead.

On Failure

Glyphviewer will display an error message when it fails. There are several reasons for failure.

  • The URL cannot be resolved to some resource on the Internet.
  • The URL points to an actual resource on the Interwebs, but it's not a font file.
  • The URL points to a font file, but it is too "big". This limitation is deliberate. Imagine if some smartarse puts all 22MB of Arial Unicode MS on their web server, and directed Glyphviewer to analyse it. This would slow down this server. That's why the application refuses to read very large files.
  • The font is genuine, but essential information such as the header is missing. (This shouldn't happen, as it's against the specification).
  • The font is a symbol font, a relic from the days before Unicode. Glyphviewer will refuse to analyse it out of disgust.

Cross Origin Resource Sharing

There is one situation which will result in a warning (rather than an outright failure). It occurs when the user attempts to analyse a font from a remote server whose Cross-origin Resource Sharing (CORS) policy refuses resources to be served by third-party sites. Glyphviewer will be able to analyse the font's header for information such as designer and copyright information. CORS doesn't prevent a resource being accessed by its lonesome. The problem will be when Glyphviewer - located on a particular server - tries to display the characters in the font when accessing it from a secondary, remote server. Many browsers - such as IE and Firefox - will refuse to display the characters using the font, and thus use some "fallback" font for that browser. Since this may mislead users as to the appearance of the font, Glyphviewer will warn them about this situation.

Glyphviewer should not encounter this situation when handling remote fonts delivered from Content Delivery Networks (CDN) such as Google Fonts. That's because these sites are designed to allow their resources to be shared by other sites. There are several ways to do this, but the most common way is to serve them with these particular HTTP headers:

Access-Control-Allow-Origin: *

If Glyphviewer detects these headers when downloading fonts from a remote server, it will assume that the browser can display them in the browser, and no warning will be presented to the user. This is not an issue for Glyphviewer when handling fonts hosted locally, because obviously the font and the server shares the same domain; CORS would not apply. However, enabling CORS on your own server may be useful if you are able to do it, especially when debugging remotely. For example, if you are serving your fonts from a nginx server that you control, you could use configuration declarations like the following:

    location /static/glyphviewer/fonts/ {
        add_header 'Access-Control-Allow-Origin' "*" always;
        add_header 'Access-Control-Allow-Credentials' 'true' always;
        add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
        add_header 'Access-Control-Allow-Headers' 'Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With' always;
        if ($request_method = 'OPTIONS') {
            # Tell client that this pre-flight info is valid for 20 days
            add_header 'Access-Control-Max-Age' 1728000;
            add_header 'Content-Type' 'text/plain charset=UTF-8';
            add_header 'Content-Length' 0;
            return 204;

CORS - although a hindrance to Glyphviewer - is generally a good thing. It can be problematic if a "resource" such as a hostile piece of JavaScript is injected into a page from a third party website. That's why some browsers (such as Firefox and IE) refuse to load resources from third party websites unless certain criteria are met.

For more information on CORS, please see enable cross-origin resource sharing, especially if you want to add CORS support to your webserver. You could also look at the Mozilla Developer Network article on the subject.


Apart from issues arising from CORS, these are several limitations built in to Glyphviewer.

  • Glyphviewer can only analyse OpenType, TrueType and Web Open Font Format 1 and 2 font files. Other font files such as Embedded OpenType (.eot) and Scalable Vector Graphics (.svg) cannot be handled by this application.
  • The application has a font size limitation built in (as stated before).
  • The application may be slow when analyzing a remote font, as the server has to access it to create a temporary copy.

Earlier versions had a serious limitation. The application could only show glyphs for characters from the Basic Multilingual Plane (U+0000 to U+FFFF). Characters out of this range were not displayed. Fortunately, this limitation has been removed from Glyphviewer as of version 0.4 (2016).

Installation and Dependencies

If you have Python, the quickest way to install Glyphviewer (and its dependencies) is via the following command:

pip install glyphviewer

The next stage is to add "glyphviewer" to your INSTALLED_APPS list in The stage after that is to add then desired URL to one of the files.

The final stage is to populate the directory with fonts where you display your chosen font or fonts. The Glyphviewer application comes with its own set of fonts adapted from the Free UCS Outline Fonts; they have been converted to WOFF form for quicker download. These can be moved into the correct directory via the following shell command:

python collectstatic

The font files wil then be moved into the {STATIC_ROOT}/glyphviewer/fonts/ directory, and the browser will read the files from the {STATIC_URL}/glyphviewer/fonts/ folder.

Note: it is not mandatory to run the collectstatic command. However, if there are no font files in the above directory, then Glyphviewer will only be able to read remote files.

The HTML template files in this application have been redesigned to work with the Mezzanine CMS. The redesign removed any explicit references to particular stylesheets found with earlier versions. The app (and the fonts) are released under a GNU general public license. If you wish to do any changes, pop over to the GitHub repository for the app.

Glyphviewer (as of version 0.8 and beyond) depends on the following software:

  • Python 3.x
  • Django 2.x or later
  • The FontTools package for parsing TrueType, OpenType, and WOFF files.
  • Numerical Python (numpy), which is used by FontTools.
  • Brotli, a package which handles the compression algorithm used by WOFF2.
  • Chardet, the universal character encoding detector. Font header information often stored in binary form, and chardet assists in picking the 'best' decoding for representing this information as a string.

Note: earlier versions of Glyphviewer depended on WoffTools, a package (built on top of FontTools) for parsing WOFF files. However, FontTools has WOFF support, and WoffTools has not been worked on for several years, so this dependency has been removed. Earlier versions of Glyphviewer also required Numerical Python (numpy) to be installed "by hand", since there were problems installing it with "pip". Fortunately, "pip install glyphviewer" will now provide all dependencies automatically.

Why Glyphviewer?

I created Glyphviewer as a tool to examine the character repetoire of web fonts. It is easy to find fonts online; it is harder to find out exactly what characters they support. Almost all contain standard capital letters like "A", "B" and "C", and that's not hard to discover, because those are among the first characters to be advertised by the foundries. It's harder to find out how much support a font has for more unusual characters like (say) "ŋ". One could install individual fonts on the computer, and inspect each in a tool like "Character Map", but this is laborous and time-wasting.

So I created a tool that could analyse a font if it was located somewhere on the World Wide Web. Font makers often display their fonts using web pages, and even provide a place where people can type text and see it displayed using the typeface. All one has to do to analyse it is examine the attached cascading stylesheet for a link to the font file, and place its URL in Glyphviewer.

This application was designed in response to and in reaction to Google Fonts, which came out in 2011. The idea is wonderful: tens of web fonts available for access and download. A laudatory effort - except that it doesn't accurately show the full repertoire of characters for each font. Take Lato - a lovely font, and comes in 10 variants too. Now look at the character set Google Web Fonts displays for it. At the time, I was frustrated that "Ł" and "ł" were not displayed, even though the font supports them, and even though the creator goes by the name of Łukasz Dziedzic. Now Google Fonts displays them, but it took Google a couple of years to lift their game.


The primary inspiration for this is Mark Pilgrim, who wrote a blog post called Fuck the foundries in 2009 (before yanking it off the net two years later). His essay and/or rant got me interested in web fonts in the first place, as did his use of the utterly superb web font Essays 1743 in Dive into HTML5. Thanks to John Stracke for coming up with the "Essays" typeface in the first place.

I also have to acknowledge the developers of the libraries used in the making of Glyphviewer. A special shout out goes to the folk at TypeSupply who decided to open-source their code for WoffTools. The project wouldn't have been possible without them. I must also acknowledge the Free UCS Outline Fonts project for providing the fonts used in this application.


  • 0.1 (June 11th 2011) - Initial release.
  • 0.2 (June 28th 2013) - Added setup script to create a PyPI package. Removed bugs.
  • 0.3 (February 15th 2014) - Made compatible with Mezzanine and Bootstrap, changed styles for errors, now handles empty local directories.
  • 0.4 (January 16th 2016) - Now handles cases where remote fonts are inaccessible using CORS; removed the limitation that restricted characters to the BMP; added documentation for above.
  • 0.5 (March 31st 2017) - Updated to be compatible with Django 1.10. Now automatically install dependencies using pip
  • 0.6 (March 18th 2018) - Add WOFF2 compatibility.
  • 0.7 (December 6th 2020) - Updated for Python 3 compatibility.
  • 0.8 (December 10th 2020) - Better representation of font header information as text; better use of urllib for Python 3.


A file, or a set of files, that is used to specify the appearance of computer text. Common Windows fonts are "Times New Roman", "Arial" and "Courier New"; Mac OS X and Linux have their own equivalents. The "Times New Roman" font consists of 4 files: one for representing regular text, one for Bold text, one for Italic text and one for Bold Italic text.
Web font
A font that is designed to be downloaded with a web page only for the purpose of viewing it, and then discarded afterwards. Historically, most web designers assumed that users had a limited repertoire of fonts already installed on their machine, and would style accordingly. For example, it would be a safe bet to have either Helvetica or Arial on the viewer's computer, and if not, there would be some sort of sans-serif equivalent available instead. One problem with this is that it gets boring to see the same fonts all the time. So the W3C had a sit down with industry folk from Microsoft and Adobe and Mozilla and other companies, and worked out their own technology for embedding fonts on the fly.
Glyphs are the shapes used to represent characters in a font. For example, the character "I" is (more or less) shaped a horizontal line - in contrast to an "o" character, which resembles (roughly) a hollow circle. Terms like "more or less" and "roughly" are used here, because the same character would be generally represented by different glyphs in different fonts, and even within them. Continuing the example above, an "I" in a italic version of a font would be inclined relative to a regular version of "I", while an "I" in a serif font would have serifs coming out at the front and the bottom. In short, there are many ways to create a glyph for the same characters.

Copyright © Peter Murphy 2011-2024.

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