NAME

nsdejavu - DjVu browser plugin

SYNOPSIS

/usr/local/lib/djview4/plugins/nsdejavu.so

WARNING

Using nsdejavu as a plugin in 2020 is very difficult because popular browsers either do not support netscape plugins (chrome), or removed support for netscape plugins other than flash (firefox). We regret this decision because djview/nsdejavu is a perfect example of very well behaved plugin that would have been easy to support.

DESCRIPTION

The shared library nsdejavu.so uses the Netscape browser plugin API to display DjVu images in in a number of popular web browsers. It works by invoking a standalone viewer out-of-process with the special command line option -netscape. The plugin first searches a program named djview. If this program cannot be found, it searches for djview4 and finally djview3. It is always possible to override this search strategy by setting the environment variable NPX_DJVIEW to the full path of the desired executable.

MIME TYPES AND EXTENSIONS

Typing the URL of a recognized DjVu document in your web browser should automatically invoke the DjVu browser plugin. Each browser uses different methods to determine that a particular URL is in fact a DjVu document. Web server normally provide a MIME type to web browsers. The official MIME type for DjVu documents is image/vnd.djvu. For compatibility with ancient versions of the DjVu viewer, it is common to use instead the experimental MIME type image/x-djvu or image/x.djvu. Web servers should be configured to send the proper MIME type for DjVu documents. Most web browsers also recognize files ending with .djvu or .djv as DjVu files.

An easy way to check if an http server is giving an appropriate content-type is to invoke the following command with a URL corresponding to an actual DjVu file on the server.

: curl -u URL | grep Content-Type

The result should be one of the following, preferably the first.

: Content-Type: image/vnd.djvu
Content-Type: image/x.djvu
Content-Type: image/x-djvu

Any other MIME type indicates a server misconfiguration.

CGI-STYLE FLAGS

The behavior of the DjVu browser plugin can be specified by augmenting the URL using a syntax similar to that used by the CGI programs. This syntax is described by the following template:

: http://.../file.djvu?djvuopts&keyword=value&keyword=value&...

The DjVu browser plugin only recognizes keywords that appear after the word djvuopts. The keywords recognized by each viewer are listed in the corresponding man page. Unrecognized keywords are ignored. The most common keywords are:

page=pagename

Specify which page is displayed by name or by ordinal number.

zoom=zoomfactor

Set the zoom factor. Legal values for zoomfactor are:

number	Magnification factor in range 10% to 999%.
one2one	Select the "one-to-one" mode.
width	Select the "fit width" mode.
page	Select the "fit page" mode.
stretch	Stretch the image to the plugin window size.

BROWSER COMPATIBILITY

The DjVu browser plugin has been tested with several popular web browsers: Netscape 4 and 6; Gecko based browsers such as Mozilla, Galeon and Firefox; KHTML based browsers such as Konqueror; and Opera. Please read the browser documentation to find out where the plugin library should be installed.

EMBEDDING DJVU IMAGES IN HTML PAGES

You can integrate DjVu content on an HTML web page with either the <embed> or the <object> tag. This method will work even if your web server does not support the DjVu MIME type. The CGI style flags can be directly used as attributes of the embedding tag. The following example shows the W3C standard syntax with the OBJECT tag:

: <object data="myfile.djvu" type="image/vnd.djvu"
width="100%" height="100%" >
<param name="page" value="iii">
<param name="zoom" value="stretch">
This browser cannot render djvu data.
</object>

And this is the customary syntax with the EMBED tag:

: <embed src="myfile.djvu" type="image/vnd.djvu"
width="100%" height="100%"
page="iii" zoom="stretch"></embed>

INTERFACING THE DJVIEW PLUGIN WITH JAVASCRIPT

Recent versions of the djview4 plugin can be controlled from the JavaScript interpreter of browsers implementing the Mozilla NPRuntime API. To access the plugin object, include the attribute id="pluginname" into the <object> or <embed> tag and use the JavaScript function getElementById("pluginname").

The plugin object implements two methods to retrieve and set the value of the options usually recognized as CGI-style flags. It also can evaluate a specified JavaScript expression whenever something changes in the status of the djview interface.

pluginobject.setdjvuopt("key",value): Set the value of the djvu option key to the character string value. This achieves the same effect as specifying option key=value among the CGI-style flags. For instance, values of the key page can be page IDs, page titles, page numbers, or page names.
pluginobject.getdjvuopt("key"): Return the value of the djvu option key as a string. The returned value is always a character string, even when the return is logically a number. Boolean values are returned as strings yes or no. Besides the usual CGI-style flags, this function recognizes the additional key pages and returns the total number of pages in the DjVu document. An empty string is returned when the key is not recognized.
pluginobject.onchange="code";: Ensure that string code is evaluated in the context of the plugin object whenever something changes in the djview graphical user interface. For instance, this evaluation happens when progressive refinements are painted, and when the user manipulates the image interactively.
pluginobject.version: Return a string describing the plugin version. This property can be used to test whether the djview plugin is scriptable in this browser.

Note that the scriptability feature may not be accessible until the djview plugin is fully loaded. Therefore it is advisable to check pluginobject.version from the JavaScript onload before calling any other method.

CREDITS

This program was written by Andrei Erofeev <andrew_erofeev@yahoo.com> and was then improved by Bill Riemers <docbill@sourceforge.net> and Léon Bottou <leonb@users.sourceforge.net>.