As web publishing and commerce continue to rise in importance, a way of managing colors from the source to the destination becomes an essential feature of a browser. Specifically, the source image that is kept on a web server will look different on various displays that access it as part of the html stream, unless something is done to compensate for the various viewing conditions. ColorSync is an Apple Computer technology that is widely adopted in the print and graphic arts industries, and it is designed to perform the necessary tasks of compensation for different devices. The combination of a browser and ColorSync means that not only can people visit sites such as art archives and see what the artist intended, they can order products from online catalogues and not be surprised with the color when it arrives in the mail.
ColorSync relies on device profiles
to be able to account
for different device characteristics—these are files which have
a format specified by the International Color Consortium—they
contain data for a given device or class of devices.
The following file formats support embedded ICC profiles:
Thus, a browser accepting files in these formats can detect embedded profiles and use them to match from the source device colors to the display. Support for this type of matching already exists in Microsoft Internet Explorer 4.0.1.
In addition, with extensions to the HTML <img> and <body> tags, profiles can be associated with images instead of having to be embedded within. Details of the additional HTML tags and a sample site testing browser use of images with profiles can be found at Apple’s ColorSync Web site.
The benefits of profile association vs. embedding is that the profile can be downloaded once and used for multiple images on a page, or in a session. It also gives web site developers some flexibility, they don’t need to rewrite all their images to embed profiles, they can make some simple additions to their HTML instead. GoLive’s CyberStudio 3 allows for content developers to use either profile association or embedding to enable color management.
The new code that has been added implements color management for images that contain or reference ICC profiles, using ColorSync. It is not a public release of ColorSync code, rather it is code that calls ColorSync. ColorSync has been a component of Mac OS for some time now—in the future it will run on other OSes as well. The new code checks for the existence of ColorSync on the OS, and if so will use its services as the browser encounters images with ICC profiles. As noted, images can either contain or reference ICC profiles. Therefore we first had to add smarts to the image decoders (e.g. JPEG) to look for embedded profiles. Secondly, we had to add new code to make requests over the net for ICC profiles referenced by URLs (or local files). This code keeps track of whether a profile has been received already and skips the download if so. Finally, code and user interface elements have been added to turn on/off these new features via preferences.
Apple and Mozilla/Netscape engineers discussed many of the issues involved in adding this functionality, with a rough spec drafted and revised. The timing was such that if the additions were to have any chance of being picked up into timely future versions of the browser, it had to be done quickly—so as the opportunities for outside help faded Apple bit the bullet and started writing code, after further consultations with various module owners.
Since ColorSync now runs (alpha stage) on more than just the Mac platform, we decided to place as much of the functionality in the cross-platform image library as possible, with calls to the front end being used only as a hedge against our being able to deploy ColorSync everywhere Mozilla deploys. As the code currently stands, both the Mac and Win FEs have the same set of new IMGCB entry points, and the code therein is also identical. The XPFE has stubs for these IMGCB entry points.
Here’s what has been done so far:
What’s not there yet (for the future?)
Changes were made mostly to the image lib, expanded structs and new calls to a set of routines in a new file: icc_profile.c. I also had to change some code in layout, to parse the new <img> and <body> tags. Finally, in addition to the front end code and additional IMGCB entry points, we added new ‘ppob’ resources to the Mac resource file. This enables the new preferences, code was added to the image lib to read them.
The list above mentions color world caching and profiles in PNG. The PNG spec for embedded ICC profiles has just been adopted by the PNG working group. It is available for download at the following sites:
For this project, code was added to the JPEG and GIF reader routines in the imagelib—similar code would have to be added to the PNG reader. As far as "color world" caching goes, this is a similar optimization to image caching (in memory), and it means keeping around table information built from a set of ICC profiles with the idea of being able to use the information again. This is a good idea if the source (input) and destination (output) profiles are expected to be the same for multiple images—a valid assumption in some cases. There is code in the new file that is designed to implement this, but it is not turned on yet.
Finally, here is a brief, undetailed list of some other things that could possibly be done:
If you’re interested in helping further ColorSync support in Mozilla, by all means contact me or others in Mozilla.org. Thanks.
Eric Broadbent
ebb3@apple.com
August 31, 1998