jcprepct.c - third_party/libjpeg-turbo - Git at Google

 /*
  * jcprepct.c
  *
  * This file is part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1996, Thomas G. Lane.
  * It was modified by The libjpeg-turbo Project to include only code relevant
  * to libjpeg-turbo.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
  * This file contains the compression preprocessing controller.
  * This controller manages the color conversion, downsampling,
  * and edge expansion steps.
  *
  * Most of the complexity here is associated with buffering input rows
  * as required by the downsampler.  See the comments at the head of
  * jcsample.c for the downsampler's needs.
  */

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


 /* At present, jcsample.c can request context rows only for smoothing.
  * In the future, we might also need context rows for CCIR601 sampling
  * or other more-complex downsampling procedures.  The code to support
  * context rows should be compiled only if needed.
  */
 #ifdef INPUT_SMOOTHING_SUPPORTED
 #define CONTEXT_ROWS_SUPPORTED
 #endif


 /*
  * 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.
  */


 /* Private buffer controller object */

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

   /* Downsampling input buffer.  This buffer holds color-converted data
    * until we have enough to do a downsample step.
    */
   JSAMPARRAY color_buf[MAX_COMPONENTS];

   JDIMENSION rows_to_go;        /* counts rows remaining in source image */
   int next_buf_row;             /* index of next row to store in color_buf */

 #ifdef CONTEXT_ROWS_SUPPORTED   /* only needed for context case */
   int this_row_group;           /* starting row index of group to process */
   int next_buf_stop;            /* downsample when we reach this index */
 #endif
 } my_prep_controller;

 typedef my_prep_controller * my_prep_ptr;


 /*
  * Initialize for a processing pass.
  */

 METHODDEF(void)
 start_pass_prep (j_compress_ptr cinfo, J_BUF_MODE pass_mode)
 {
   my_prep_ptr prep = (my_prep_ptr) cinfo->prep;

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

   /* Initialize total-height counter for detecting bottom of image */
   prep->rows_to_go = cinfo->image_height;
   /* Mark the conversion buffer empty */
   prep->next_buf_row = 0;
 #ifdef CONTEXT_ROWS_SUPPORTED
   /* Preset additional state variables for context mode.
    * These aren't used in non-context mode, so we needn't test which mode.
    */
   prep->this_row_group = 0;
   /* Set next_buf_stop to stop after two row groups have been read in. */
   prep->next_buf_stop = 2 * cinfo->max_v_samp_factor;
 #endif
 }


 /*
  * Expand an image vertically from height input_rows to height output_rows,
  * by duplicating the bottom row.
  */

 LOCAL(void)
 expand_bottom_edge (JSAMPARRAY image_data, JDIMENSION num_cols,
                     int input_rows, int output_rows)
 {
   register int row;

   for (row = input_rows; row < output_rows; row++) {
     jcopy_sample_rows(image_data, input_rows-1, image_data, row,
                       1, num_cols);
   }
 }


 /*
  * Process some data in the simple no-context case.
  *
  * Preprocessor output data is counted in "row groups".  A row group
  * is defined to be v_samp_factor sample rows of each component.
  * Downsampling will produce this much data from each max_v_samp_factor
  * input rows.
  */

 METHODDEF(void)
 pre_process_data (j_compress_ptr cinfo,
                   JSAMPARRAY input_buf, JDIMENSION *in_row_ctr,
                   JDIMENSION in_rows_avail,
                   JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
                   JDIMENSION out_row_groups_avail)
 {
   my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
   int numrows, ci;
   JDIMENSION inrows;
   jpeg_component_info * compptr;

   while (*in_row_ctr < in_rows_avail &&
          *out_row_group_ctr < out_row_groups_avail) {
     /* Do color conversion to fill the conversion buffer. */
     inrows = in_rows_avail - *in_row_ctr;
     numrows = cinfo->max_v_samp_factor - prep->next_buf_row;
     numrows = (int) MIN((JDIMENSION) numrows, inrows);
     (*cinfo->cconvert->color_convert) (cinfo, input_buf + *in_row_ctr,
                                        prep->color_buf,
                                        (JDIMENSION) prep->next_buf_row,
                                        numrows);
     *in_row_ctr += numrows;
     prep->next_buf_row += numrows;
     prep->rows_to_go -= numrows;
     /* If at bottom of image, pad to fill the conversion buffer. */
     if (prep->rows_to_go == 0 &&
         prep->next_buf_row < cinfo->max_v_samp_factor) {
       for (ci = 0; ci < cinfo->num_components; ci++) {
         expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
                            prep->next_buf_row, cinfo->max_v_samp_factor);
       }
       prep->next_buf_row = cinfo->max_v_samp_factor;
     }
     /* If we've filled the conversion buffer, empty it. */
     if (prep->next_buf_row == cinfo->max_v_samp_factor) {
       (*cinfo->downsample->downsample) (cinfo,
                                         prep->color_buf, (JDIMENSION) 0,
                                         output_buf, *out_row_group_ctr);
       prep->next_buf_row = 0;
       (*out_row_group_ctr)++;
     }
     /* If at bottom of image, pad the output to a full iMCU height.
      * Note we assume the caller is providing a one-iMCU-height output buffer!
      */
     if (prep->rows_to_go == 0 &&
         *out_row_group_ctr < out_row_groups_avail) {
       for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
            ci++, compptr++) {
         expand_bottom_edge(output_buf[ci],
                            compptr->width_in_blocks * DCTSIZE,
                            (int) (*out_row_group_ctr * compptr->v_samp_factor),
                            (int) (out_row_groups_avail * compptr->v_samp_factor));
       }
       *out_row_group_ctr = out_row_groups_avail;
       break;                    /* can exit outer loop without test */
     }
   }
 }


 #ifdef CONTEXT_ROWS_SUPPORTED

 /*
  * Process some data in the context case.
  */

 METHODDEF(void)
 pre_process_context (j_compress_ptr cinfo,
                      JSAMPARRAY input_buf, JDIMENSION *in_row_ctr,
                      JDIMENSION in_rows_avail,
                      JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
                      JDIMENSION out_row_groups_avail)
 {
   my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
   int numrows, ci;
   int buf_height = cinfo->max_v_samp_factor * 3;
   JDIMENSION inrows;

   while (*out_row_group_ctr < out_row_groups_avail) {
     if (*in_row_ctr < in_rows_avail) {
       /* Do color conversion to fill the conversion buffer. */
       inrows = in_rows_avail - *in_row_ctr;
       numrows = prep->next_buf_stop - prep->next_buf_row;
       numrows = (int) MIN((JDIMENSION) numrows, inrows);
       (*cinfo->cconvert->color_convert) (cinfo, input_buf + *in_row_ctr,
                                          prep->color_buf,
                                          (JDIMENSION) prep->next_buf_row,
                                          numrows);
       /* Pad at top of image, if first time through */
       if (prep->rows_to_go == cinfo->image_height) {
         for (ci = 0; ci < cinfo->num_components; ci++) {
           int row;
           for (row = 1; row <= cinfo->max_v_samp_factor; row++) {
             jcopy_sample_rows(prep->color_buf[ci], 0,
                               prep->color_buf[ci], -row,
                               1, cinfo->image_width);
           }
         }
       }
       *in_row_ctr += numrows;
       prep->next_buf_row += numrows;
       prep->rows_to_go -= numrows;
     } else {
       /* Return for more data, unless we are at the bottom of the image. */
       if (prep->rows_to_go != 0)
         break;
       /* When at bottom of image, pad to fill the conversion buffer. */
       if (prep->next_buf_row < prep->next_buf_stop) {
         for (ci = 0; ci < cinfo->num_components; ci++) {
           expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
                              prep->next_buf_row, prep->next_buf_stop);
         }
         prep->next_buf_row = prep->next_buf_stop;
       }
     }
     /* If we've gotten enough data, downsample a row group. */
     if (prep->next_buf_row == prep->next_buf_stop) {
       (*cinfo->downsample->downsample) (cinfo,
                                         prep->color_buf,
                                         (JDIMENSION) prep->this_row_group,
                                         output_buf, *out_row_group_ctr);
       (*out_row_group_ctr)++;
       /* Advance pointers with wraparound as necessary. */
       prep->this_row_group += cinfo->max_v_samp_factor;
       if (prep->this_row_group >= buf_height)
         prep->this_row_group = 0;
       if (prep->next_buf_row >= buf_height)
         prep->next_buf_row = 0;
       prep->next_buf_stop = prep->next_buf_row + cinfo->max_v_samp_factor;
     }
   }
 }


 /*
  * Create the wrapped-around downsampling input buffer needed for context mode.
  */

 LOCAL(void)
 create_context_buffer (j_compress_ptr cinfo)
 {
   my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
   int rgroup_height = cinfo->max_v_samp_factor;
   int ci, i;
   jpeg_component_info * compptr;
   JSAMPARRAY true_buffer, fake_buffer;

   /* Grab enough space for fake row pointers for all the components;
    * we need five row groups' worth of pointers for each component.
    */
   fake_buffer = (JSAMPARRAY)
     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
                                 (cinfo->num_components * 5 * rgroup_height) *
                                 sizeof(JSAMPROW));

   for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
        ci++, compptr++) {
     /* Allocate the actual buffer space (3 row groups) for this component.
      * We make the buffer wide enough to allow the downsampler to edge-expand
      * horizontally within the buffer, if it so chooses.
      */
     true_buffer = (*cinfo->mem->alloc_sarray)
       ((j_common_ptr) cinfo, JPOOL_IMAGE,
        (JDIMENSION) (((long) compptr->width_in_blocks * DCTSIZE *
                       cinfo->max_h_samp_factor) / compptr->h_samp_factor),
        (JDIMENSION) (3 * rgroup_height));
     /* Copy true buffer row pointers into the middle of the fake row array */
     MEMCOPY(fake_buffer + rgroup_height, true_buffer,
             3 * rgroup_height * sizeof(JSAMPROW));
     /* Fill in the above and below wraparound pointers */
     for (i = 0; i < rgroup_height; i++) {
       fake_buffer[i] = true_buffer[2 * rgroup_height + i];
       fake_buffer[4 * rgroup_height + i] = true_buffer[i];
     }
     prep->color_buf[ci] = fake_buffer + rgroup_height;
     fake_buffer += 5 * rgroup_height; /* point to space for next component */
   }
 }

 #endif /* CONTEXT_ROWS_SUPPORTED */


 /*
  * Initialize preprocessing controller.
  */

 GLOBAL(void)
 jinit_c_prep_controller (j_compress_ptr cinfo, boolean need_full_buffer)
 {
   my_prep_ptr prep;
   int ci;
   jpeg_component_info * compptr;

   if (need_full_buffer)         /* safety check */
     ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);

   prep = (my_prep_ptr)
     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
                                 sizeof(my_prep_controller));
   cinfo->prep = (struct jpeg_c_prep_controller *) prep;
   prep->pub.start_pass = start_pass_prep;

   /* Allocate the color conversion buffer.
    * We make the buffer wide enough to allow the downsampler to edge-expand
    * horizontally within the buffer, if it so chooses.
    */
   if (cinfo->downsample->need_context_rows) {
     /* Set up to provide context rows */
 #ifdef CONTEXT_ROWS_SUPPORTED
     prep->pub.pre_process_data = pre_process_context;
     create_context_buffer(cinfo);
 #else
     ERREXIT(cinfo, JERR_NOT_COMPILED);
 #endif
   } else {
     /* No context, just make it tall enough for one row group */
     prep->pub.pre_process_data = pre_process_data;
     for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
          ci++, compptr++) {
       prep->color_buf[ci] = (*cinfo->mem->alloc_sarray)
         ((j_common_ptr) cinfo, JPOOL_IMAGE,
          (JDIMENSION) (((long) compptr->width_in_blocks * DCTSIZE *
                         cinfo->max_h_samp_factor) / compptr->h_samp_factor),
          (JDIMENSION) cinfo->max_v_samp_factor);
     }
   }
 }
	/*
	* jcprepct.c
	*
	* This file is part of the Independent JPEG Group's software:
	* Copyright (C) 1994-1996, Thomas G. Lane.
	* It was modified by The libjpeg-turbo Project to include only code relevant
	* to libjpeg-turbo.
	* For conditions of distribution and use, see the accompanying README.ijg
	* file.
	*
	* This file contains the compression preprocessing controller.
	* This controller manages the color conversion, downsampling,
	* and edge expansion steps.
	*
	* Most of the complexity here is associated with buffering input rows
	* as required by the downsampler. See the comments at the head of
	* jcsample.c for the downsampler's needs.
	*/

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


	/* At present, jcsample.c can request context rows only for smoothing.
	* In the future, we might also need context rows for CCIR601 sampling
	* or other more-complex downsampling procedures. The code to support
	* context rows should be compiled only if needed.
	*/
	#ifdef INPUT_SMOOTHING_SUPPORTED
	#define CONTEXT_ROWS_SUPPORTED
	#endif


	/*
	* 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.
	*/


	/* Private buffer controller object */

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

	/* Downsampling input buffer. This buffer holds color-converted data
	* until we have enough to do a downsample step.
	*/
	JSAMPARRAY color_buf[MAX_COMPONENTS];

	JDIMENSION rows_to_go; /* counts rows remaining in source image */
	int next_buf_row; /* index of next row to store in color_buf */

	#ifdef CONTEXT_ROWS_SUPPORTED /* only needed for context case */
	int this_row_group; /* starting row index of group to process */
	int next_buf_stop; /* downsample when we reach this index */
	#endif
	} my_prep_controller;

	typedef my_prep_controller * my_prep_ptr;


	/*
	* Initialize for a processing pass.
	*/

	METHODDEF(void)
	start_pass_prep (j_compress_ptr cinfo, J_BUF_MODE pass_mode)
	{
	my_prep_ptr prep = (my_prep_ptr) cinfo->prep;

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

	/* Initialize total-height counter for detecting bottom of image */
	prep->rows_to_go = cinfo->image_height;
	/* Mark the conversion buffer empty */
	prep->next_buf_row = 0;
	#ifdef CONTEXT_ROWS_SUPPORTED
	/* Preset additional state variables for context mode.
	* These aren't used in non-context mode, so we needn't test which mode.
	*/
	prep->this_row_group = 0;
	/* Set next_buf_stop to stop after two row groups have been read in. */
	prep->next_buf_stop = 2 * cinfo->max_v_samp_factor;
	#endif
	}


	/*
	* Expand an image vertically from height input_rows to height output_rows,
	* by duplicating the bottom row.
	*/

	LOCAL(void)
	expand_bottom_edge (JSAMPARRAY image_data, JDIMENSION num_cols,
	int input_rows, int output_rows)
	{
	register int row;

	for (row = input_rows; row < output_rows; row++) {
	jcopy_sample_rows(image_data, input_rows-1, image_data, row,
	1, num_cols);
	}
	}


	/*
	* Process some data in the simple no-context case.
	*
	* Preprocessor output data is counted in "row groups". A row group
	* is defined to be v_samp_factor sample rows of each component.
	* Downsampling will produce this much data from each max_v_samp_factor
	* input rows.
	*/

	METHODDEF(void)
	pre_process_data (j_compress_ptr cinfo,
	JSAMPARRAY input_buf, JDIMENSION *in_row_ctr,
	JDIMENSION in_rows_avail,
	JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
	JDIMENSION out_row_groups_avail)
	{
	my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
	int numrows, ci;
	JDIMENSION inrows;
	jpeg_component_info * compptr;

	while (*in_row_ctr < in_rows_avail &&
	*out_row_group_ctr < out_row_groups_avail) {
	/* Do color conversion to fill the conversion buffer. */
	inrows = in_rows_avail - *in_row_ctr;
	numrows = cinfo->max_v_samp_factor - prep->next_buf_row;
	numrows = (int) MIN((JDIMENSION) numrows, inrows);
	(cinfo->cconvert->color_convert) (cinfo, input_buf + in_row_ctr,
	prep->color_buf,
	(JDIMENSION) prep->next_buf_row,
	numrows);
	*in_row_ctr += numrows;
	prep->next_buf_row += numrows;
	prep->rows_to_go -= numrows;
	/* If at bottom of image, pad to fill the conversion buffer. */
	if (prep->rows_to_go == 0 &&
	prep->next_buf_row < cinfo->max_v_samp_factor) {
	for (ci = 0; ci < cinfo->num_components; ci++) {
	expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
	prep->next_buf_row, cinfo->max_v_samp_factor);
	}
	prep->next_buf_row = cinfo->max_v_samp_factor;
	}
	/* If we've filled the conversion buffer, empty it. */
	if (prep->next_buf_row == cinfo->max_v_samp_factor) {
	(*cinfo->downsample->downsample) (cinfo,
	prep->color_buf, (JDIMENSION) 0,
	output_buf, *out_row_group_ctr);
	prep->next_buf_row = 0;
	(*out_row_group_ctr)++;
	}
	/* If at bottom of image, pad the output to a full iMCU height.
	* Note we assume the caller is providing a one-iMCU-height output buffer!
	*/
	if (prep->rows_to_go == 0 &&
	*out_row_group_ctr < out_row_groups_avail) {
	for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
	ci++, compptr++) {
	expand_bottom_edge(output_buf[ci],
	compptr->width_in_blocks * DCTSIZE,
	(int) (out_row_group_ctr compptr->v_samp_factor),
	(int) (out_row_groups_avail * compptr->v_samp_factor));
	}
	*out_row_group_ctr = out_row_groups_avail;
	break; /* can exit outer loop without test */
	}
	}
	}


	#ifdef CONTEXT_ROWS_SUPPORTED

	/*
	* Process some data in the context case.
	*/

	METHODDEF(void)
	pre_process_context (j_compress_ptr cinfo,
	JSAMPARRAY input_buf, JDIMENSION *in_row_ctr,
	JDIMENSION in_rows_avail,
	JSAMPIMAGE output_buf, JDIMENSION *out_row_group_ctr,
	JDIMENSION out_row_groups_avail)
	{
	my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
	int numrows, ci;
	int buf_height = cinfo->max_v_samp_factor * 3;
	JDIMENSION inrows;

	while (*out_row_group_ctr < out_row_groups_avail) {
	if (*in_row_ctr < in_rows_avail) {
	/* Do color conversion to fill the conversion buffer. */
	inrows = in_rows_avail - *in_row_ctr;
	numrows = prep->next_buf_stop - prep->next_buf_row;
	numrows = (int) MIN((JDIMENSION) numrows, inrows);
	(cinfo->cconvert->color_convert) (cinfo, input_buf + in_row_ctr,
	prep->color_buf,
	(JDIMENSION) prep->next_buf_row,
	numrows);
	/* Pad at top of image, if first time through */
	if (prep->rows_to_go == cinfo->image_height) {
	for (ci = 0; ci < cinfo->num_components; ci++) {
	int row;
	for (row = 1; row <= cinfo->max_v_samp_factor; row++) {
	jcopy_sample_rows(prep->color_buf[ci], 0,
	prep->color_buf[ci], -row,
	1, cinfo->image_width);
	}
	}
	}
	*in_row_ctr += numrows;
	prep->next_buf_row += numrows;
	prep->rows_to_go -= numrows;
	} else {
	/* Return for more data, unless we are at the bottom of the image. */
	if (prep->rows_to_go != 0)
	break;
	/* When at bottom of image, pad to fill the conversion buffer. */
	if (prep->next_buf_row < prep->next_buf_stop) {
	for (ci = 0; ci < cinfo->num_components; ci++) {
	expand_bottom_edge(prep->color_buf[ci], cinfo->image_width,
	prep->next_buf_row, prep->next_buf_stop);
	}
	prep->next_buf_row = prep->next_buf_stop;
	}
	}
	/* If we've gotten enough data, downsample a row group. */
	if (prep->next_buf_row == prep->next_buf_stop) {
	(*cinfo->downsample->downsample) (cinfo,
	prep->color_buf,
	(JDIMENSION) prep->this_row_group,
	output_buf, *out_row_group_ctr);
	(*out_row_group_ctr)++;
	/* Advance pointers with wraparound as necessary. */
	prep->this_row_group += cinfo->max_v_samp_factor;
	if (prep->this_row_group >= buf_height)
	prep->this_row_group = 0;
	if (prep->next_buf_row >= buf_height)
	prep->next_buf_row = 0;
	prep->next_buf_stop = prep->next_buf_row + cinfo->max_v_samp_factor;
	}
	}
	}


	/*
	* Create the wrapped-around downsampling input buffer needed for context mode.
	*/

	LOCAL(void)
	create_context_buffer (j_compress_ptr cinfo)
	{
	my_prep_ptr prep = (my_prep_ptr) cinfo->prep;
	int rgroup_height = cinfo->max_v_samp_factor;
	int ci, i;
	jpeg_component_info * compptr;
	JSAMPARRAY true_buffer, fake_buffer;

	/* Grab enough space for fake row pointers for all the components;
	* we need five row groups' worth of pointers for each component.
	*/
	fake_buffer = (JSAMPARRAY)
	(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
	(cinfo->num_components * 5 * rgroup_height) *
	sizeof(JSAMPROW));

	for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
	ci++, compptr++) {
	/* Allocate the actual buffer space (3 row groups) for this component.
	* We make the buffer wide enough to allow the downsampler to edge-expand
	* horizontally within the buffer, if it so chooses.
	*/
	true_buffer = (*cinfo->mem->alloc_sarray)
	((j_common_ptr) cinfo, JPOOL_IMAGE,
	(JDIMENSION) (((long) compptr->width_in_blocks * DCTSIZE *
	cinfo->max_h_samp_factor) / compptr->h_samp_factor),
	(JDIMENSION) (3 * rgroup_height));
	/* Copy true buffer row pointers into the middle of the fake row array */
	MEMCOPY(fake_buffer + rgroup_height, true_buffer,
	3 * rgroup_height * sizeof(JSAMPROW));
	/* Fill in the above and below wraparound pointers */
	for (i = 0; i < rgroup_height; i++) {
	fake_buffer[i] = true_buffer[2 * rgroup_height + i];
	fake_buffer[4 * rgroup_height + i] = true_buffer[i];
	}
	prep->color_buf[ci] = fake_buffer + rgroup_height;
	fake_buffer += 5 * rgroup_height; /* point to space for next component */
	}
	}

	#endif /* CONTEXT_ROWS_SUPPORTED */


	/*
	* Initialize preprocessing controller.
	*/

	GLOBAL(void)
	jinit_c_prep_controller (j_compress_ptr cinfo, boolean need_full_buffer)
	{
	my_prep_ptr prep;
	int ci;
	jpeg_component_info * compptr;

	if (need_full_buffer) /* safety check */
	ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);

	prep = (my_prep_ptr)
	(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
	sizeof(my_prep_controller));
	cinfo->prep = (struct jpeg_c_prep_controller *) prep;
	prep->pub.start_pass = start_pass_prep;

	/* Allocate the color conversion buffer.
	* We make the buffer wide enough to allow the downsampler to edge-expand
	* horizontally within the buffer, if it so chooses.
	*/
	if (cinfo->downsample->need_context_rows) {
	/* Set up to provide context rows */
	#ifdef CONTEXT_ROWS_SUPPORTED
	prep->pub.pre_process_data = pre_process_context;
	create_context_buffer(cinfo);
	#else
	ERREXIT(cinfo, JERR_NOT_COMPILED);
	#endif
	} else {
	/* No context, just make it tall enough for one row group */
	prep->pub.pre_process_data = pre_process_data;
	for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
	ci++, compptr++) {
	prep->color_buf[ci] = (*cinfo->mem->alloc_sarray)
	((j_common_ptr) cinfo, JPOOL_IMAGE,
	(JDIMENSION) (((long) compptr->width_in_blocks * DCTSIZE *
	cinfo->max_h_samp_factor) / compptr->h_samp_factor),
	(JDIMENSION) cinfo->max_v_samp_factor);
	}
	}
	}