jdmainct.c

00001 /*
00002  * jdmainct.c
00003  *
00004  * Copyright (C) 1994-1996, Thomas G. Lane.
00005  * Modified 2002-2012 by Guido Vollbeding.
00006  * This file is part of the Independent JPEG Group's software.
00007  * For conditions of distribution and use, see the accompanying README file.
00008  *
00009  * This file contains the main buffer controller for decompression.
00010  * The main buffer lies between the JPEG decompressor proper and the
00011  * post-processor; it holds downsampled data in the JPEG colorspace.
00012  *
00013  * Note that this code is bypassed in raw-data mode, since the application
00014  * supplies the equivalent of the main buffer in that case.
00015  */
00016 
00017 #define JPEG_INTERNALS
00018 #include "jinclude.h"
00019 #include "jpeglib.h"
00020 
00021 
00022 /*
00023  * In the current system design, the main buffer need never be a full-image
00024  * buffer; any full-height buffers will be found inside the coefficient or
00025  * postprocessing controllers.  Nonetheless, the main controller is not
00026  * trivial.  Its responsibility is to provide context rows for upsampling/
00027  * rescaling, and doing this in an efficient fashion is a bit tricky.
00028  *
00029  * Postprocessor input data is counted in "row groups".  A row group
00030  * is defined to be (v_samp_factor * DCT_scaled_size / min_DCT_scaled_size)
00031  * sample rows of each component.  (We require DCT_scaled_size values to be
00032  * chosen such that these numbers are integers.  In practice DCT_scaled_size
00033  * values will likely be powers of two, so we actually have the stronger
00034  * condition that DCT_scaled_size / min_DCT_scaled_size is an integer.)
00035  * Upsampling will typically produce max_v_samp_factor pixel rows from each
00036  * row group (times any additional scale factor that the upsampler is
00037  * applying).
00038  *
00039  * The coefficient controller will deliver data to us one iMCU row at a time;
00040  * each iMCU row contains v_samp_factor * DCT_scaled_size sample rows, or
00041  * exactly min_DCT_scaled_size row groups.  (This amount of data corresponds
00042  * to one row of MCUs when the image is fully interleaved.)  Note that the
00043  * number of sample rows varies across components, but the number of row
00044  * groups does not.  Some garbage sample rows may be included in the last iMCU
00045  * row at the bottom of the image.
00046  *
00047  * Depending on the vertical scaling algorithm used, the upsampler may need
00048  * access to the sample row(s) above and below its current input row group.
00049  * The upsampler is required to set need_context_rows TRUE at global selection
00050  * time if so.  When need_context_rows is FALSE, this controller can simply
00051  * obtain one iMCU row at a time from the coefficient controller and dole it
00052  * out as row groups to the postprocessor.
00053  *
00054  * When need_context_rows is TRUE, this controller guarantees that the buffer
00055  * passed to postprocessing contains at least one row group's worth of samples
00056  * above and below the row group(s) being processed.  Note that the context
00057  * rows "above" the first passed row group appear at negative row offsets in
00058  * the passed buffer.  At the top and bottom of the image, the required
00059  * context rows are manufactured by duplicating the first or last real sample
00060  * row; this avoids having special cases in the upsampling inner loops.
00061  *
00062  * The amount of context is fixed at one row group just because that's a
00063  * convenient number for this controller to work with.  The existing
00064  * upsamplers really only need one sample row of context.  An upsampler
00065  * supporting arbitrary output rescaling might wish for more than one row
00066  * group of context when shrinking the image; tough, we don't handle that.
00067  * (This is justified by the assumption that downsizing will be handled mostly
00068  * by adjusting the DCT_scaled_size values, so that the actual scale factor at
00069  * the upsample step needn't be much less than one.)
00070  *
00071  * To provide the desired context, we have to retain the last two row groups
00072  * of one iMCU row while reading in the next iMCU row.  (The last row group
00073  * can't be processed until we have another row group for its below-context,
00074  * and so we have to save the next-to-last group too for its above-context.)
00075  * We could do this most simply by copying data around in our buffer, but
00076  * that'd be very slow.  We can avoid copying any data by creating a rather
00077  * strange pointer structure.  Here's how it works.  We allocate a workspace
00078  * consisting of M+2 row groups (where M = min_DCT_scaled_size is the number
00079  * of row groups per iMCU row).  We create two sets of redundant pointers to
00080  * the workspace.  Labeling the physical row groups 0 to M+1, the synthesized
00081  * pointer lists look like this:
00082  *                   M+1                          M-1
00083  * master pointer --> 0         master pointer --> 0
00084  *                    1                            1
00085  *                   ...                          ...
00086  *                   M-3                          M-3
00087  *                   M-2                           M
00088  *                   M-1                          M+1
00089  *                    M                           M-2
00090  *                   M+1                          M-1
00091  *                    0                            0
00092  * We read alternate iMCU rows using each master pointer; thus the last two
00093  * row groups of the previous iMCU row remain un-overwritten in the workspace.
00094  * The pointer lists are set up so that the required context rows appear to
00095  * be adjacent to the proper places when we pass the pointer lists to the
00096  * upsampler.
00097  *
00098  * The above pictures describe the normal state of the pointer lists.
00099  * At top and bottom of the image, we diddle the pointer lists to duplicate
00100  * the first or last sample row as necessary (this is cheaper than copying
00101  * sample rows around).
00102  *
00103  * This scheme breaks down if M < 2, ie, min_DCT_scaled_size is 1.  In that
00104  * situation each iMCU row provides only one row group so the buffering logic
00105  * must be different (eg, we must read two iMCU rows before we can emit the
00106  * first row group).  For now, we simply do not support providing context
00107  * rows when min_DCT_scaled_size is 1.  That combination seems unlikely to
00108  * be worth providing --- if someone wants a 1/8th-size preview, they probably
00109  * want it quick and dirty, so a context-free upsampler is sufficient.
00110  */
00111 
00112 
00113 /* Private buffer controller object */
00114 
00115 typedef struct {
00116   struct jpeg_d_main_controller pub; /* public fields */
00117 
00118   /* Pointer to allocated workspace (M or M+2 row groups). */
00119   JSAMPARRAY buffer[MAX_COMPONENTS];
00120 
00121   boolean buffer_full;      /* Have we gotten an iMCU row from decoder? */
00122   JDIMENSION rowgroup_ctr;  /* counts row groups output to postprocessor */
00123 
00124   /* Remaining fields are only used in the context case. */
00125 
00126   /* These are the master pointers to the funny-order pointer lists. */
00127   JSAMPIMAGE xbuffer[2];    /* pointers to weird pointer lists */
00128 
00129   int whichptr;         /* indicates which pointer set is now in use */
00130   int context_state;        /* process_data state machine status */
00131   JDIMENSION rowgroups_avail;   /* row groups available to postprocessor */
00132   JDIMENSION iMCU_row_ctr;  /* counts iMCU rows to detect image top/bot */
00133 } my_main_controller;
00134 
00135 typedef my_main_controller * my_main_ptr;
00136 
00137 /* context_state values: */
00138 #define CTX_PREPARE_FOR_IMCU    0   /* need to prepare for MCU row */
00139 #define CTX_PROCESS_IMCU    1   /* feeding iMCU to postprocessor */
00140 #define CTX_POSTPONED_ROW   2   /* feeding postponed row group */
00141 
00142 
00143 /* Forward declarations */
00144 METHODDEF(void) process_data_simple_main
00145     JPP((j_decompress_ptr cinfo, JSAMPARRAY output_buf,
00146          JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail));
00147 METHODDEF(void) process_data_context_main
00148     JPP((j_decompress_ptr cinfo, JSAMPARRAY output_buf,
00149          JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail));
00150 #ifdef QUANT_2PASS_SUPPORTED
00151 METHODDEF(void) process_data_crank_post
00152     JPP((j_decompress_ptr cinfo, JSAMPARRAY output_buf,
00153          JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail));
00154 #endif
00155 
00156 
00157 LOCAL(void)
00158 alloc_funny_pointers (j_decompress_ptr cinfo)
00159 /* Allocate space for the funny pointer lists.
00160  * This is done only once, not once per pass.
00161  */
00162 {
00163   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00164   int ci, rgroup;
00165   int M = cinfo->min_DCT_v_scaled_size;
00166   jpeg_component_info *compptr;
00167   JSAMPARRAY xbuf;
00168 
00169   /* Get top-level space for component array pointers.
00170    * We alloc both arrays with one call to save a few cycles.
00171    */
00172   mainp->xbuffer[0] = (JSAMPIMAGE)
00173     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00174                 cinfo->num_components * 2 * SIZEOF(JSAMPARRAY));
00175   mainp->xbuffer[1] = mainp->xbuffer[0] + cinfo->num_components;
00176 
00177   for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
00178        ci++, compptr++) {
00179     rgroup = (compptr->v_samp_factor * compptr->DCT_v_scaled_size) /
00180       cinfo->min_DCT_v_scaled_size; /* height of a row group of component */
00181     /* Get space for pointer lists --- M+4 row groups in each list.
00182      * We alloc both pointer lists with one call to save a few cycles.
00183      */
00184     xbuf = (JSAMPARRAY)
00185       (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00186                   2 * (rgroup * (M + 4)) * SIZEOF(JSAMPROW));
00187     xbuf += rgroup;     /* want one row group at negative offsets */
00188     mainp->xbuffer[0][ci] = xbuf;
00189     xbuf += rgroup * (M + 4);
00190     mainp->xbuffer[1][ci] = xbuf;
00191   }
00192 }
00193 
00194 
00195 LOCAL(void)
00196 make_funny_pointers (j_decompress_ptr cinfo)
00197 /* Create the funny pointer lists discussed in the comments above.
00198  * The actual workspace is already allocated (in main->buffer),
00199  * and the space for the pointer lists is allocated too.
00200  * This routine just fills in the curiously ordered lists.
00201  * This will be repeated at the beginning of each pass.
00202  */
00203 {
00204   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00205   int ci, i, rgroup;
00206   int M = cinfo->min_DCT_v_scaled_size;
00207   jpeg_component_info *compptr;
00208   JSAMPARRAY buf, xbuf0, xbuf1;
00209 
00210   for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
00211        ci++, compptr++) {
00212     rgroup = (compptr->v_samp_factor * compptr->DCT_v_scaled_size) /
00213       cinfo->min_DCT_v_scaled_size; /* height of a row group of component */
00214     xbuf0 = mainp->xbuffer[0][ci];
00215     xbuf1 = mainp->xbuffer[1][ci];
00216     /* First copy the workspace pointers as-is */
00217     buf = mainp->buffer[ci];
00218     for (i = 0; i < rgroup * (M + 2); i++) {
00219       xbuf0[i] = xbuf1[i] = buf[i];
00220     }
00221     /* In the second list, put the last four row groups in swapped order */
00222     for (i = 0; i < rgroup * 2; i++) {
00223       xbuf1[rgroup*(M-2) + i] = buf[rgroup*M + i];
00224       xbuf1[rgroup*M + i] = buf[rgroup*(M-2) + i];
00225     }
00226     /* The wraparound pointers at top and bottom will be filled later
00227      * (see set_wraparound_pointers, below).  Initially we want the "above"
00228      * pointers to duplicate the first actual data line.  This only needs
00229      * to happen in xbuffer[0].
00230      */
00231     for (i = 0; i < rgroup; i++) {
00232       xbuf0[i - rgroup] = xbuf0[0];
00233     }
00234   }
00235 }
00236 
00237 
00238 LOCAL(void)
00239 set_wraparound_pointers (j_decompress_ptr cinfo)
00240 /* Set up the "wraparound" pointers at top and bottom of the pointer lists.
00241  * This changes the pointer list state from top-of-image to the normal state.
00242  */
00243 {
00244   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00245   int ci, i, rgroup;
00246   int M = cinfo->min_DCT_v_scaled_size;
00247   jpeg_component_info *compptr;
00248   JSAMPARRAY xbuf0, xbuf1;
00249 
00250   for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
00251        ci++, compptr++) {
00252     rgroup = (compptr->v_samp_factor * compptr->DCT_v_scaled_size) /
00253       cinfo->min_DCT_v_scaled_size; /* height of a row group of component */
00254     xbuf0 = mainp->xbuffer[0][ci];
00255     xbuf1 = mainp->xbuffer[1][ci];
00256     for (i = 0; i < rgroup; i++) {
00257       xbuf0[i - rgroup] = xbuf0[rgroup*(M+1) + i];
00258       xbuf1[i - rgroup] = xbuf1[rgroup*(M+1) + i];
00259       xbuf0[rgroup*(M+2) + i] = xbuf0[i];
00260       xbuf1[rgroup*(M+2) + i] = xbuf1[i];
00261     }
00262   }
00263 }
00264 
00265 
00266 LOCAL(void)
00267 set_bottom_pointers (j_decompress_ptr cinfo)
00268 /* Change the pointer lists to duplicate the last sample row at the bottom
00269  * of the image.  whichptr indicates which xbuffer holds the final iMCU row.
00270  * Also sets rowgroups_avail to indicate number of nondummy row groups in row.
00271  */
00272 {
00273   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00274   int ci, i, rgroup, iMCUheight, rows_left;
00275   jpeg_component_info *compptr;
00276   JSAMPARRAY xbuf;
00277 
00278   for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
00279        ci++, compptr++) {
00280     /* Count sample rows in one iMCU row and in one row group */
00281     iMCUheight = compptr->v_samp_factor * compptr->DCT_v_scaled_size;
00282     rgroup = iMCUheight / cinfo->min_DCT_v_scaled_size;
00283     /* Count nondummy sample rows remaining for this component */
00284     rows_left = (int) (compptr->downsampled_height % (JDIMENSION) iMCUheight);
00285     if (rows_left == 0) rows_left = iMCUheight;
00286     /* Count nondummy row groups.  Should get same answer for each component,
00287      * so we need only do it once.
00288      */
00289     if (ci == 0) {
00290       mainp->rowgroups_avail = (JDIMENSION) ((rows_left-1) / rgroup + 1);
00291     }
00292     /* Duplicate the last real sample row rgroup*2 times; this pads out the
00293      * last partial rowgroup and ensures at least one full rowgroup of context.
00294      */
00295     xbuf = mainp->xbuffer[mainp->whichptr][ci];
00296     for (i = 0; i < rgroup * 2; i++) {
00297       xbuf[rows_left + i] = xbuf[rows_left-1];
00298     }
00299   }
00300 }
00301 
00302 
00303 /*
00304  * Initialize for a processing pass.
00305  */
00306 
00307 METHODDEF(void)
00308 start_pass_main (j_decompress_ptr cinfo, J_BUF_MODE pass_mode)
00309 {
00310   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00311 
00312   switch (pass_mode) {
00313   case JBUF_PASS_THRU:
00314     if (cinfo->upsample->need_context_rows) {
00315       mainp->pub.process_data = process_data_context_main;
00316       make_funny_pointers(cinfo); /* Create the xbuffer[] lists */
00317       mainp->whichptr = 0;  /* Read first iMCU row into xbuffer[0] */
00318       mainp->context_state = CTX_PREPARE_FOR_IMCU;
00319       mainp->iMCU_row_ctr = 0;
00320     } else {
00321       /* Simple case with no context needed */
00322       mainp->pub.process_data = process_data_simple_main;
00323     }
00324     mainp->buffer_full = FALSE; /* Mark buffer empty */
00325     mainp->rowgroup_ctr = 0;
00326     break;
00327 #ifdef QUANT_2PASS_SUPPORTED
00328   case JBUF_CRANK_DEST:
00329     /* For last pass of 2-pass quantization, just crank the postprocessor */
00330     mainp->pub.process_data = process_data_crank_post;
00331     break;
00332 #endif
00333   default:
00334     ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);
00335     break;
00336   }
00337 }
00338 
00339 
00340 /*
00341  * Process some data.
00342  * This handles the simple case where no context is required.
00343  */
00344 
00345 METHODDEF(void)
00346 process_data_simple_main (j_decompress_ptr cinfo,
00347               JSAMPARRAY output_buf, JDIMENSION *out_row_ctr,
00348               JDIMENSION out_rows_avail)
00349 {
00350   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00351   JDIMENSION rowgroups_avail;
00352 
00353   /* Read input data if we haven't filled the main buffer yet */
00354   if (! mainp->buffer_full) {
00355     if (! (*cinfo->coef->decompress_data) (cinfo, mainp->buffer))
00356       return;           /* suspension forced, can do nothing more */
00357     mainp->buffer_full = TRUE;  /* OK, we have an iMCU row to work with */
00358   }
00359 
00360   /* There are always min_DCT_scaled_size row groups in an iMCU row. */
00361   rowgroups_avail = (JDIMENSION) cinfo->min_DCT_v_scaled_size;
00362   /* Note: at the bottom of the image, we may pass extra garbage row groups
00363    * to the postprocessor.  The postprocessor has to check for bottom
00364    * of image anyway (at row resolution), so no point in us doing it too.
00365    */
00366 
00367   /* Feed the postprocessor */
00368   (*cinfo->post->post_process_data) (cinfo, mainp->buffer,
00369                      &mainp->rowgroup_ctr, rowgroups_avail,
00370                      output_buf, out_row_ctr, out_rows_avail);
00371 
00372   /* Has postprocessor consumed all the data yet? If so, mark buffer empty */
00373   if (mainp->rowgroup_ctr >= rowgroups_avail) {
00374     mainp->buffer_full = FALSE;
00375     mainp->rowgroup_ctr = 0;
00376   }
00377 }
00378 
00379 
00380 /*
00381  * Process some data.
00382  * This handles the case where context rows must be provided.
00383  */
00384 
00385 METHODDEF(void)
00386 process_data_context_main (j_decompress_ptr cinfo,
00387                JSAMPARRAY output_buf, JDIMENSION *out_row_ctr,
00388                JDIMENSION out_rows_avail)
00389 {
00390   my_main_ptr mainp = (my_main_ptr) cinfo->main;
00391 
00392   /* Read input data if we haven't filled the main buffer yet */
00393   if (! mainp->buffer_full) {
00394     if (! (*cinfo->coef->decompress_data) (cinfo,
00395                        mainp->xbuffer[mainp->whichptr]))
00396       return;           /* suspension forced, can do nothing more */
00397     mainp->buffer_full = TRUE;  /* OK, we have an iMCU row to work with */
00398     mainp->iMCU_row_ctr++;  /* count rows received */
00399   }
00400 
00401   /* Postprocessor typically will not swallow all the input data it is handed
00402    * in one call (due to filling the output buffer first).  Must be prepared
00403    * to exit and restart.  This switch lets us keep track of how far we got.
00404    * Note that each case falls through to the next on successful completion.
00405    */
00406   switch (mainp->context_state) {
00407   case CTX_POSTPONED_ROW:
00408     /* Call postprocessor using previously set pointers for postponed row */
00409     (*cinfo->post->post_process_data) (cinfo, mainp->xbuffer[mainp->whichptr],
00410             &mainp->rowgroup_ctr, mainp->rowgroups_avail,
00411             output_buf, out_row_ctr, out_rows_avail);
00412     if (mainp->rowgroup_ctr < mainp->rowgroups_avail)
00413       return;           /* Need to suspend */
00414     mainp->context_state = CTX_PREPARE_FOR_IMCU;
00415     if (*out_row_ctr >= out_rows_avail)
00416       return;           /* Postprocessor exactly filled output buf */
00417     /*FALLTHROUGH*/
00418   case CTX_PREPARE_FOR_IMCU:
00419     /* Prepare to process first M-1 row groups of this iMCU row */
00420     mainp->rowgroup_ctr = 0;
00421     mainp->rowgroups_avail = (JDIMENSION) (cinfo->min_DCT_v_scaled_size - 1);
00422     /* Check for bottom of image: if so, tweak pointers to "duplicate"
00423      * the last sample row, and adjust rowgroups_avail to ignore padding rows.
00424      */
00425     if (mainp->iMCU_row_ctr == cinfo->total_iMCU_rows)
00426       set_bottom_pointers(cinfo);
00427     mainp->context_state = CTX_PROCESS_IMCU;
00428     /*FALLTHROUGH*/
00429   case CTX_PROCESS_IMCU:
00430     /* Call postprocessor using previously set pointers */
00431     (*cinfo->post->post_process_data) (cinfo, mainp->xbuffer[mainp->whichptr],
00432             &mainp->rowgroup_ctr, mainp->rowgroups_avail,
00433             output_buf, out_row_ctr, out_rows_avail);
00434     if (mainp->rowgroup_ctr < mainp->rowgroups_avail)
00435       return;           /* Need to suspend */
00436     /* After the first iMCU, change wraparound pointers to normal state */
00437     if (mainp->iMCU_row_ctr == 1)
00438       set_wraparound_pointers(cinfo);
00439     /* Prepare to load new iMCU row using other xbuffer list */
00440     mainp->whichptr ^= 1;   /* 0=>1 or 1=>0 */
00441     mainp->buffer_full = FALSE;
00442     /* Still need to process last row group of this iMCU row, */
00443     /* which is saved at index M+1 of the other xbuffer */
00444     mainp->rowgroup_ctr = (JDIMENSION) (cinfo->min_DCT_v_scaled_size + 1);
00445     mainp->rowgroups_avail = (JDIMENSION) (cinfo->min_DCT_v_scaled_size + 2);
00446     mainp->context_state = CTX_POSTPONED_ROW;
00447   }
00448 }
00449 
00450 
00451 /*
00452  * Process some data.
00453  * Final pass of two-pass quantization: just call the postprocessor.
00454  * Source data will be the postprocessor controller's internal buffer.
00455  */
00456 
00457 #ifdef QUANT_2PASS_SUPPORTED
00458 
00459 METHODDEF(void)
00460 process_data_crank_post (j_decompress_ptr cinfo,
00461              JSAMPARRAY output_buf, JDIMENSION *out_row_ctr,
00462              JDIMENSION out_rows_avail)
00463 {
00464   (*cinfo->post->post_process_data) (cinfo, (JSAMPIMAGE) NULL,
00465                      (JDIMENSION *) NULL, (JDIMENSION) 0,
00466                      output_buf, out_row_ctr, out_rows_avail);
00467 }
00468 
00469 #endif /* QUANT_2PASS_SUPPORTED */
00470 
00471 
00472 /*
00473  * Initialize main buffer controller.
00474  */
00475 
00476 GLOBAL(void)
00477 jinit_d_main_controller (j_decompress_ptr cinfo, boolean need_full_buffer)
00478 {
00479   my_main_ptr mainp;
00480   int ci, rgroup, ngroups;
00481   jpeg_component_info *compptr;
00482 
00483   mainp = (my_main_ptr)
00484     (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
00485                 SIZEOF(my_main_controller));
00486   cinfo->main = &mainp->pub;
00487   mainp->pub.start_pass = start_pass_main;
00488 
00489   if (need_full_buffer)     /* shouldn't happen */
00490     ERREXIT(cinfo, JERR_BAD_BUFFER_MODE);
00491 
00492   /* Allocate the workspace.
00493    * ngroups is the number of row groups we need.
00494    */
00495   if (cinfo->upsample->need_context_rows) {
00496     if (cinfo->min_DCT_v_scaled_size < 2) /* unsupported, see comments above */
00497       ERREXIT(cinfo, JERR_NOTIMPL);
00498     alloc_funny_pointers(cinfo); /* Alloc space for xbuffer[] lists */
00499     ngroups = cinfo->min_DCT_v_scaled_size + 2;
00500   } else {
00501     ngroups = cinfo->min_DCT_v_scaled_size;
00502   }
00503 
00504   for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
00505        ci++, compptr++) {
00506     rgroup = (compptr->v_samp_factor * compptr->DCT_v_scaled_size) /
00507       cinfo->min_DCT_v_scaled_size; /* height of a row group of component */
00508     mainp->buffer[ci] = (*cinfo->mem->alloc_sarray)
00509       ((j_common_ptr) cinfo, JPOOL_IMAGE,
00510        compptr->width_in_blocks * ((JDIMENSION) compptr->DCT_h_scaled_size),
00511        (JDIMENSION) (rgroup * ngroups));
00512   }
00513 }