On slow computer (with imposed antivirus checking reading and writing) run latex can be a slow process. Sometimes I only need to see how an equation or a small table will look. I do not means a real time viewer but a fast viewer of the selected text. The implementation is easy: save the selected test in a temp file, add a custom preamble (even only the \documentaclass{article}\begin{document}) and a closing matter (\end{document}), compile the temp file and show the result on a dvi or on a pdf viewer. In my opinion is not a break in the philosofy of separated composition and typesetting phases when you are tryng to improve the looking of an equation or of an image.
Hope you like this suggestion
Bie
Gaetano
P.S. I tried to implement using custom feature but was out of my skills (if possible)
P.P.S. I hate to say that, but winedit has this feature.
