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The PICT file format consists essentially of serialized QuickDraw opcodes. The original version, PICT 1, was designed to be as compact as possible while describing vector graphics. To this end, it featured single byte opcodes, many of which embodied operations such as "do the previous operation again". As such it was quite memory efficient, but not very expandable. With the introduction of the Macintosh II and Color QuickDraw, PICT was revised to version 2. This version featured 16-bit opcodes and numerous changes which enhanced its utility. PICT 1 opcodes were supported as a subset for backward compatibility.
Within a Mac application, any sequence of drawing operations could be simply recorded/encoded to the PICT format by opening a "Picture", then closing it after issuing the required commands. By saving the resulting byte stream as a resource, a PICT resource resulted, which could be loaded and played back at any time. The same stream could be saved to a data file on disk (with 512 bytes of unused header space added) as a PICT file.
With the change to OS X and deprecation of QuickDraw, PICT was dropped in favor of Portable Document Format (PDF) as the native metafile format, though PICT support is retained by many applications as it was so widely supported on Classic Mac OS.