Friday, 20 August 2010

PNG embedded diagrams

In the previous post I mentioned embedding the XML of mxGraph models into PNG. The PNG specification includes a zTXt element, which enables very large sections of compressed text to be placed in the PNG encoding. mxGraph and onwards contain functionality to embed the model XML in created PNGs and to decode PNGs created in this way, using the Java code base. The GraphEditor example of the Java Swing client now saves in this PNG+XML format (.png suffix) by default to demonstrate the idea. We felt this is a somewhat more intuitive way of storing and transferring diagrams than text XML, since suddenly every user has the tools to look at the diagram, without the need for mxGraph. It provides a simple means for the server side to display diagram thumbnails without having to process the diagram and removes the necessity to synchronise XML formats and their associated images.

You can still stick with text XML if you prefer, but we'll sulk if everyone doesn't use this shiny new feature. This does bring up the issue of whether the JavaScript display looks the same as the generated server diagram. By default, they are identical, but customisation of the JavaScript can cause duplication of implementation on the server technology to get image generation to look the same. The majority of cases where this occurs happen because of new shapes being added to the JavaScript client programmatically. Which is another important reason for adding the stencils mentioned in the last post.

We are going to add an additional option to add the text without compression, not everyone will want their servers unzipping every time a diagram is opened and it provides a mechanism to view and extract the XML using a text editor, if required. Given the text is likely to form the minority of the file size, we're inclined to make this the default save format.

If you have other cases where there is code duplication necessary (other than I/O), we'd be interested to hear.

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