[FW] [fly] is a method of generating images "on the fly" using [GD], and allows for command line access (via, say, Tcl's [exec]) so it can be used by just about any language with shell access and I/O.  This is, as far as I know, the only way of using GD from Tcl.
  
http://martin.gleeson.com/fly/installation.html

[FW]'s knowledge is incomplete; refer to the [GD] page for other options.

As of 2012, there is also [TclGD] from flightaware, which provides GD as an extension.

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