JPEGView/Independent JPEG Group/jdmainct.c

/*
 * jdmainct.c
 *
 * Copyright (C) 1994, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * This file contains the main buffer controller for decompression.
 * The main buffer lies between the JPEG decompressor proper and the
 * post-processor; it holds downsampled data in the JPEG colorspace.
 */

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"


/*
 * In the current system design, the main buffer need never be a full-image
 * buffer; any full-height buffers will be found inside the coefficient or
 * postprocessing controllers.  Nonetheless, the main controller is not
 * trivial.  Its responsibility is to provide context rows for upsampling/
 * rescaling, and doing this in an efficient fashion is a bit tricky.
 *
 * Postprocessor input data is counted in "row groups".  A row group
 * is defined to be (v_samp_factor * DCT_scaled_size / min_DCT_scaled_size)
 * sample rows of each component.  (We require DCT_scaled_size values to be
 * chosen such that these numbers are integers.  In practice DCT_scaled_size
 * values will likely be powers of two, so we actually have the stronger
 * condition that DCT_scaled_size / min_DCT_scaled_size is an integer.)
 * Upsampling will typically produce max_v_samp_factor pixel rows from each
 * row group (times any additional scale factor that the upsampler is
 * applying).
 *
 * The coefficient controller will deliver data to us one iMCU row at a time;
 * each iMCU row contains v_samp_factor * DCT_scaled_size sample rows, or
 * exactly min_DCT_scaled_size row groups.  (This amount of data corresponds
 * to one row of MCUs when the image is fully interleaved.)  Note that the
 * number of sample rows varies across components, but the number of row
 * groups does not.  Some garbage sample rows may be included in the last iMCU
 * row at the bottom of the image.
 *
 * Depending on the vertical scaling algorithm used, the upsampler may need
 * access to the sample row(s) above and below its current input row group.
 * The upsampler is required to set need_context_rows TRUE at global selection
 * time if so.  When need_context_rows is FALSE, this controller can simply
 * obtain one iMCU row at a time from the coefficient controller and dole it
 * out as row groups to the postprocessor.
 *
 * When need_context_rows is TRUE, this controller guarantees that the buffer
 * passed to postprocessing contains at least one row group's worth of samples
 * above and below the row group(s) being processed.  Note that the context
 * rows "above" the first passed row group appear at negative row offsets in
 * the passed buffer.  At the top and bottom of the image, the required
 * context rows are manufactured by duplicating the first or last real sample
 * row; this avoids having special cases in the upsampling inner loops.
 *
 * The amount of context is fixed at one row group just because that's a
 * convenient number for this controller to work with.  The existing
 * upsamplers really only need one sample row of context.  An upsampler
 * supporting arbitrary output rescaling might wish for more than one row
 * group of context when shrinking the image; tough, we don't handle that.
 * (This is justified by the assumption that downsizing will be handled mostly
 * by adjusting the DCT_scaled_size values, so that the actual scale factor at
 * the upsample step needn't be much less than one.)
 *
 * To provide the desired context, we have to retain the last two row groups
 * of one iMCU row while reading in the next iMCU row.  (The last row group
 * can't be processed until we have another row group for its below-context,
 * and so we have to save the next-to-last group too for its above-context.)
 * We could do this most simply by copying data around in our buffer, but
 * that'd be very slow.  We can avoid copying any data by creating a rather
 * strange pointer structure.  Here's how it works.  We allocate a workspace
 * consisting of M+2 row groups (where M = min_DCT_scaled_size i