I see that BeyondCompare can be extended to include additional file formats, as in Additional File Viewer Rules for Beyond Compare 2 and also Additional file format downloads for version 3 but after a quick initial search I don't see how user's develop these special viewers. Is that documented anywhere?
I downloaded a few additional viewers which are handily imported via the BCFormats.bcpkg file
C:\Program Files (x86)\Beyond Compare 3\Helpers>dir /b /s
C:\Program Files (x86)\Beyond Compare 3\Helpers\HtmlTidy
C:\Program Files (x86)\Beyond Compare 3\Helpers\PdfToText.exe
C:\Program Files (x86)\Beyond Compare 3\Helpers\XLS_to_TAB_Single.vbs
C:\Program Files (x86)\Beyond Compare 3\Helpers\HtmlTidy\HtmlTidy.exe
C:\Program Files (x86)\Beyond Compare 3\Helpers\HtmlTidy\XML_tidied_sorted.bat
C:\Program Files (x86)\Beyond Compare 3\Helpers\HtmlTidy\XML_tidied_sorted_config.txt
What's the design to these things, are they something to the effect of a command line tool that reads in a text file as the first argument and outputs the converted file to standard output?
They are command line tools that preprocess a file before it is loaded for comparison. The first argument is an input file and the second argument is an output filename. As an example, the pdftotext.exe tool extracts a .pdf file to a plain text .txt file, then displays the temp file in Beyond Compare's Text Compare.
See Beyond Compare's help file topic Text Format Conversion Settings for details.