jcmainct.c

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Hash : e8b40f3c
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Date : 2022-11-01T21:45:39

Vastly improve 12-bit JPEG integration

The Gordian knot that 7fec5074f962b20ed00b4f5da4533e1e8d4ed8ac attempted
to unravel was caused by the fact that there are several
data-precision-dependent (JSAMPLE-dependent) fields and methods in the
exposed libjpeg API structures, and if you change the exposed libjpeg
API structures, then you have to change the whole API.  If you change
the whole API, then you have to provide a whole new library to support
the new API, and that makes it difficult to support multiple data
precisions in the same application.  (It is not impossible, as example.c
demonstrated, but using data-precision-dependent libjpeg API structures
would have made the cjpeg, djpeg, and jpegtran source code hard to read,
so it made more sense to build, install, and package 12-bit-specific
versions of those applications.)

Unfortunately, the result of that initial integration effort was an
unreadable and unmaintainable mess, which is a problem for a library
that is an ISO/ITU-T reference implementation.  Also, as I dug into the
problem of lossless JPEG support, I realized that 16-bit lossless JPEG
images are a thing, and supporting yet another version of the libjpeg
API just for those images is untenable.

In fact, however, the touch points for JSAMPLE in the exposed libjpeg
API structures are minimal:

  - The colormap and sample_range_limit fields in jpeg_decompress_struct
  - The alloc_sarray() and access_virt_sarray() methods in
    jpeg_memory_mgr
  - jpeg_write_scanlines() and jpeg_write_raw_data()
  - jpeg_read_scanlines() and jpeg_read_raw_data()
  - jpeg_skip_scanlines() and jpeg_crop_scanline()
    (This is subtle, but both of those functions use JSAMPLE-dependent
    opaque structures behind the scenes.)

It is much more readable and maintainable to provide 12-bit-specific
versions of those six top-level API functions and to document that the
aforementioned methods and fields must be type-cast when using 12-bit
samples.  Since that eliminates the need to provide a 12-bit-specific
version of the exposed libjpeg API structures, we can:

  - Compile only the precision-dependent libjpeg modules (the
    coefficient buffer controllers, the colorspace converters, the
    DCT/IDCT managers, the main buffer controllers, the preprocessing
    and postprocessing controller, the downsampler and upsamplers, the
    quantizers, the integer DCT methods, and the IDCT methods) for
    multiple data precisions.
  - Introduce 12-bit-specific methods into the various internal
    structures defined in jpegint.h.
  - Create precision-independent data type, macro, method, field, and
    function names that are prefixed by an underscore, and use an
    internal header to convert those into precision-dependent data
    type, macro, method, field, and function names, based on the value
    of BITS_IN_JSAMPLE, when compiling the precision-dependent libjpeg
    modules.
  - Expose precision-dependent jinit*() functions for each of the
    precision-dependent libjpeg modules.
  - Abstract the precision-dependent libjpeg modules by calling the
    appropriate precision-dependent jinit*() function, based on the
    value of cinfo->data_precision, from top-level libjpeg API
    functions.

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jcmainct.c

/*
 * jcmainct.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Copyright (C) 1994-1996, Thomas G. Lane.
 * libjpeg-turbo Modifications:
 * Copyright (C) 2022, D. R. Commander.
 * For conditions of distribution and use, see the accompanying README.ijg
 * file.
 *
 * This file contains the main buffer controller for compression.
 * The main buffer lies between the pre-processor and the JPEG
 * compressor proper; it holds downsampled data in the JPEG colorspace.
 */

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


/* Private buffer controller object */

typedef struct {
  struct jpeg_c_main_controller pub; /* public fields */

  JDIMENSION cur_iMCU_row;      /* number of current iMCU row */
  JDIMENSION rowgroup_ctr;      /* counts row groups received in iMCU row */
  boolean suspended;            /* remember if we suspended output */
  J_BUF_MODE pass_mode;         /* current operating mode */

  /* If using just a strip buffer, this points to the entire set of buffers
   * (we allocate one for each component).  In the full-image case, this
   * points to the currently accessible strips of the virtual arrays.
   */
  _JSAMPARRAY buffer[MAX_COMPONENTS];
} my_main_controller;

typedef my_main_controller *my_main_ptr;


/* Forward declarations */
METHODDEF(void) process_data_simple_main(j_compress_ptr cinfo,
                                         _JSAMPARRAY input_buf,
                                         JDIMENSION *in_row_ctr,
                                         JDIMENSION in_rows_avail);


/*
 * Initialize for a processing pass.
 */

METHODDEF(void)
start_pass_main(j_compress_ptr cinfo, J_BUF_MODE pass_mode)
{
  my_main_ptr main_ptr = (my_main_ptr)cinfo->main;

  /* Do nothing in raw-data mode. */
  if (cinfo->raw_data_in)
    return;

  if (pass_mode != JBUF_PASS_THRU)
    ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);

  main_ptr->cur_iMCU_row = 0;   /* initialize counters */
  main_ptr->rowgroup_ctr = 0;
  main_ptr->suspended = FALSE;
  main_ptr->pass_mode = pass_mode;      /* save mode for use by process_data */
  main_ptr->pub._process_data = process_data_simple_main;
}


/*
 * Process some data.
 * This routine handles the simple pass-through mode,
 * where we have only a strip buffer.
 */

METHODDEF(void)
process_data_simple_main(j_compress_ptr cinfo, _JSAMPARRAY input_buf,
                         JDIMENSION *in_row_ctr, JDIMENSION in_rows_avail)
{
  my_main_ptr main_ptr = (my_main_ptr)cinfo->main;

  while (main_ptr->cur_iMCU_row < cinfo->total_iMCU_rows) {
    /* Read input data if we haven't filled the main buffer yet */
    if (main_ptr->rowgroup_ctr < DCTSIZE)
      (*cinfo->prep->_pre_process_data) (cinfo, input_buf, in_row_ctr,
                                         in_rows_avail, main_ptr->buffer,
                                         &main_ptr->rowgroup_ctr,
                                         (JDIMENSION)DCTSIZE);

    /* If we don't have a full iMCU row buffered, return to application for
     * more data.  Note that preprocessor will always pad to fill the iMCU row
     * at the bottom of the image.
     */
    if (main_ptr->rowgroup_ctr != DCTSIZE)
      return;

    /* Send the completed row to the compressor */
    if (!(*cinfo->coef->_compress_data) (cinfo, main_ptr->buffer)) {
      /* If compressor did not consume the whole row, then we must need to
       * suspend processing and return to the application.  In this situation
       * we pretend we didn't yet consume the last input row; otherwise, if
       * it happened to be the last row of the image, the application would
       * think we were done.
       */
      if (!main_ptr->suspended) {
        (*in_row_ctr)--;
        main_ptr->suspended = TRUE;
      }
      return;
    }
    /* We did finish the row.  Undo our little suspension hack if a previous
     * call suspended; then mark the main buffer empty.
     */
    if (main_ptr->suspended) {
      (*in_row_ctr)++;
      main_ptr->suspended = FALSE;
    }
    main_ptr->rowgroup_ctr = 0;
    main_ptr->cur_iMCU_row++;
  }
}


/*
 * Initialize main buffer controller.
 */

GLOBAL(void)
_jinit_c_main_controller(j_compress_ptr cinfo, boolean need_full_buffer)
{
  my_main_ptr main_ptr;
  int ci;
  jpeg_component_info *compptr;

  if (cinfo->data_precision != BITS_IN_JSAMPLE)
    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

  main_ptr = (my_main_ptr)
    (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                sizeof(my_main_controller));
  cinfo->main = (struct jpeg_c_main_controller *)main_ptr;
  main_ptr->pub.start_pass = start_pass_main;

  /* We don't need to create a buffer in raw-data mode. */
  if (cinfo->raw_data_in)
    return;

  /* Create the buffer.  It holds downsampled data, so each component
   * may be of a different size.
   */
  if (need_full_buffer) {
    ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);
  } else {
    /* Allocate a strip buffer for each component */
    for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
         ci++, compptr++) {
      main_ptr->buffer[ci] = (_JSAMPARRAY)(*cinfo->mem->alloc_sarray)
        ((j_common_ptr)cinfo, JPOOL_IMAGE,
         compptr->width_in_blocks * DCTSIZE,
         (JDIMENSION)(compptr->v_samp_factor * DCTSIZE));
    }
  }
}

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jcmainct.c