"Might sound like a dumb question, but from your description (and ignoring the decimal pixels anomoly for now), it sounds like you ought to be able to process the data line-by-line to produce the output you need. And thereby avoid having to "build a data structure from the input data" at all."

And ... yeah. I started to think this today, too. :^| It all could be a case of needless 'complexifying' on my part.