The GD graphics drawing library is a software library that programmers use to draw, paint, create graphs and tables, resize images, draw text and add watermarks, and so forth.
tcl.gd aka tclgd is a C-coded Tcl extension that brings nearly 100% of GD's capabilities to Tcl programmers. Although there are a couple of other Tcl interfaces to GD around, they haven't been updated in recent years and are missing many of GD's more recent abilities such as True Type font rendering and alpha channel support, among others.
Tcl coders can use GD to read and write GIFs, JPEGs, PNGs, and native GD images. From Tcl you can draw pixels, lines, polygons, rectangles, arcs, and ellipses. You can do fills, create text, resize, resample, rotate, sharpen, compare and merge images, and create animated GIFs, all following a familiar Tk-style object model.
tclgd is one component of flightaware-tcltools.
Source repository is at https://github.com/flightaware/tcl.gd .