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1) half-size interpolation is

1) half-size interpolation is a special (very fast) mode: each 4 bayer pixels (r-g-b-g) are joined into one RGB pixel with two greens averaged.
2) There is no way to do partial RAW decoding (LibRaw::unpack()), all RAW data always decoded.
3) For partial processing you may set imgdata.params.cropbox (to x1,y1,x2,y2) and LibRaw::dcraw_process() will return you part of image. This is not thread safe, so cannot be used in parallel.
4) LibRaw can use OpenMP for parts of processing (e.g. AHD interpolation), just compile it with OpenMP

-- Alex Tutubalin @LibRaw LLC