Optimising a video editor plugin

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During the past few weeks, I have been writing a C++ plugin to grade C41 digital intermediates in Cinelerra, an open-source Linux video editor. C41 is the most common chemical process for negatives, resulting in films that look like this — you probably know it if you’ve ever shot film cameras. Of course, after scanning those negatives, you have to process (“grade”) them to turn them back to positive. And it’s not as simple as merely inverting the values of each channel for each pixel; C41 has a very pronounced orange shift that you have to take into account.