C# Class BitMiracle.LibJpeg.Classic.Internal.jpeg_c_prep_controller

Compression preprocessing (downsampling input buffer control). For the simple (no-context-row) case, we just need to buffer one row group's worth of pixels for the downsampling step. At the bottom of the image, we pad to a full row group by replicating the last pixel row. The downsampler's last output row is then replicated if needed to pad out to a full iMCU row. When providing context rows, we must buffer three row groups' worth of pixels. Three row groups are physically allocated, but the row pointer arrays are made five row groups high, with the extra pointers above and below "wrapping around" to point to the last and first real row groups. This allows the downsampler to access the proper context rows. At the top and bottom of the image, we create dummy context rows by copying the first or last real pixel row. This copying could be avoided by pointer hacking as is done in jdmainct.c, but it doesn't seem worth the trouble on the compression side.