# dlib C++ Library - jcsample.c

    /*
     * jcsample.c
     *
     * Copyright (C) 1991-1996, 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 downsampling routines.
     *
     * Downsampling input data is counted in "row groups".  A row group
     * is defined to be max_v_samp_factor pixel rows of each component,
     * from which the downsampler produces v_samp_factor sample rows.
     * A single row group is processed in each call to the downsampler module.
     *
     * The downsampler is responsible for edge-expansion of its output data
     * to fill an integral number of DCT blocks horizontally.  The source buffer
     * may be modified if it is helpful for this purpose (the source buffer is
     * allocated wide enough to correspond to the desired output width).
     * The caller (the prep controller) is responsible for vertical padding.
     *
     * The downsampler may request "context rows" by setting need_context_rows
     * during startup.  In this case, the input arrays will contain at least
     * one row group's worth of pixels above and below the passed-in data;
     * the caller will create dummy rows at image top and bottom by replicating
     * the first or last real pixel row.
     *
     * An excellent reference for image resampling is
     *   Digital Image Warping, George Wolberg, 1990.
     *   Pub. by IEEE Computer Society Press, Los Alamitos, CA. ISBN 0-8186-8944-7.
     *
     * The downsampling algorithm used here is a simple average of the source
     * pixels covered by the output pixel.  The hi-falutin sampling literature
     * refers to this as a "box filter".  In general the characteristics of a box
     * filter are not very good, but for the specific cases we normally use (1:1
     * and 2:1 ratios) the box is equivalent to a "triangle filter" which is not
     * nearly so bad.  If you intend to use other sampling ratios, you'd be well
     * advised to improve this code.
     *
     * A simple input-smoothing capability is provided.  This is mainly intended
     * for cleaning up color-dithered GIF input files (if you find it inadequate,
     * we suggest using an external filtering program such as pnmconvol).  When
     * enabled, each input pixel P is replaced by a weighted sum of itself and its
     * eight neighbors.  P's weight is 1-8*SF and each neighbor's weight is SF,
     * where SF = (smoothing_factor / 1024).
     * Currently, smoothing is only supported for 2h2v sampling factors.
     */

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

    /* Pointer to routine to downsample a single component */
    typedef JMETHOD(void, downsample1_ptr,
    		(j_compress_ptr cinfo, jpeg_component_info * compptr,
    		 JSAMPARRAY input_data, JSAMPARRAY output_data));

    /* Private subobject */

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

      /* Downsampling method pointers, one per component */
      downsample1_ptr methods[MAX_COMPONENTS];

      /* Height of an output row group for each component. */
      int rowgroup_height[MAX_COMPONENTS];

      /* These arrays save pixel expansion factors so that int_downsample need not
       * recompute them each time.  They are unused for other downsampling methods.
       */
      UINT8 h_expand[MAX_COMPONENTS];
      UINT8 v_expand[MAX_COMPONENTS];
    } my_downsampler;

    typedef my_downsampler * my_downsample_ptr;

    /*
     * Initialize for a downsampling pass.
     */

    METHODDEF(void)
    start_pass_downsample (j_compress_ptr cinfo)
    {
      /* no work for now */
    }

    /*
     * Expand a component horizontally from width input_cols to width output_cols,
     * by duplicating the rightmost samples.
     */

    LOCAL(void)
    expand_right_edge (JSAMPARRAY image_data, int num_rows,
    		   JDIMENSION input_cols, JDIMENSION output_cols)
    {
      register JSAMPROW ptr;
      register JSAMPLE pixval;
      register int count;
      int row;
      int numcols = (int) (output_cols - input_cols);

      if (numcols > 0) {
        for (row = 0; row < num_rows; row++) {
          ptr = image_data[row] + input_cols;
          pixval = ptr[-1];		/* don't need GETJSAMPLE() here */
          for (count = numcols; count > 0; count--)
    	*ptr++ = pixval;
        }
      }
    }

    /*
     * Do downsampling for a whole row group (all components).
     *
     * In this version we simply downsample each component independently.
     */

    METHODDEF(void)
    sep_downsample (j_compress_ptr cinfo,
    		JSAMPIMAGE input_buf, JDIMENSION in_row_index,
    		JSAMPIMAGE output_buf, JDIMENSION out_row_group_index)
    {
      my_downsample_ptr downsample = (my_downsample_ptr) cinfo->downsample;
      int ci;
      jpeg_component_info * compptr;
      JSAMPARRAY in_ptr, out_ptr;

      for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
           ci++, compptr++) {
        in_ptr = input_buf[ci] + in_row_index;
        out_ptr = output_buf[ci] +
    	      (out_row_group_index * downsample->rowgroup_height[ci]);
        (*downsample->methods[ci]) (cinfo, compptr, in_ptr, out_ptr);
      }
    }

    /*
     * Downsample pixel values of a single component.
     * One row group is processed per call.
     * This version handles arbitrary integral sampling ratios, without smoothing.
     * Note that this version is not actually used for customary sampling ratios.
     */

    METHODDEF(void)
    int_downsample (j_compress_ptr cinfo, jpeg_component_info * compptr,
    		JSAMPARRAY input_data, JSAMPARRAY output_data)
    {
      my_downsample_ptr downsample = (my_downsample_ptr) cinfo->downsample;
      int inrow, outrow, h_expand, v_expand, numpix, numpix2, h, v;
      JDIMENSION outcol, outcol_h;	/* outcol_h == outcol*h_expand */
      JDIMENSION output_cols = compptr->width_in_blocks * compptr->DCT_h_scaled_size;
      JSAMPROW inptr, outptr;
      INT32 outvalue;

      h_expand = downsample->h_expand[compptr->component_index];
      v_expand = downsample->v_expand[compptr->component_index];
      numpix = h_expand * v_expand;
      numpix2 = numpix/2;

      /* Expand input data enough to let all the output samples be generated
       * by the standard loop.  Special-casing padded output would be more
       * efficient.
       */
      expand_right_edge(input_data, cinfo->max_v_samp_factor,
    		    cinfo->image_width, output_cols * h_expand);

      inrow = outrow = 0;
      while (inrow < cinfo->max_v_samp_factor) {
        outptr = output_data[outrow];
        for (outcol = 0, outcol_h = 0; outcol < output_cols;
    	 outcol++, outcol_h += h_expand) {
          outvalue = 0;
          for (v = 0; v < v_expand; v++) {
    	inptr = input_data[inrow+v] + outcol_h;
    	for (h = 0; h < h_expand; h++) {
    	  outvalue += (INT32) GETJSAMPLE(*inptr++);
    	}
          }
          *outptr++ = (JSAMPLE) ((outvalue + numpix2) / numpix);
        }
        inrow += v_expand;
        outrow++;
      }
    }

    /*
     * Downsample pixel values of a single component.
     * This version handles the special case of a full-size component,
     * without smoothing.
     */

    METHODDEF(void)
    fullsize_downsample (j_compress_ptr cinfo, jpeg_component_info * compptr,
    		     JSAMPARRAY input_data, JSAMPARRAY output_data)
    {
      /* Copy the data */
      jcopy_sample_rows(input_data, 0, output_data, 0,
    		    cinfo->max_v_samp_factor, cinfo->image_width);
      /* Edge-expand */
      expand_right_edge(output_data, cinfo->max_v_samp_factor, cinfo->image_width,
    		    compptr->width_in_blocks * compptr->DCT_h_scaled_size);
    }

    /*
     * Downsample pixel values of a single component.
     * This version handles the common case of 2:1 horizontal and 1:1 vertical,
     * without smoothing.
     *
     * A note about the "bias" calculations: when rounding fractional values to
     * integer, we do not want to always round 0.5 up to the next integer.
     * If we did that, we'd introduce a noticeable bias towards larger values.
     * Instead, this code is arranged so that 0.5 will be rounded up or down at
     * alternate pixel locations (a simple ordered dither pattern).
     */

    METHODDEF(void)
    h2v1_downsample (j_compress_ptr cinfo, jpeg_component_info * compptr,
    		 JSAMPARRAY input_data, JSAMPARRAY output_data)
    {
      int inrow;
      JDIMENSION outcol;
      JDIMENSION output_cols = compptr->width_in_blocks * compptr->DCT_h_scaled_size;
      register JSAMPROW inptr, outptr;
      register int bias;

      /* Expand input data enough to let all the output samples be generated
       * by the standard loop.  Special-casing padded output would be more
       * efficient.
       */
      expand_right_edge(input_data, cinfo->max_v_samp_factor,
    		    cinfo->image_width, output_cols * 2);

      for (inrow = 0; inrow < cinfo->max_v_samp_factor; inrow++) {
        outptr = output_data[inrow];
        inptr = input_data[inrow];
        bias = 0;			/* bias = 0,1,0,1,... for successive samples */
        for (outcol = 0; outcol < output_cols; outcol++) {
          *outptr++ = (JSAMPLE) ((GETJSAMPLE(*inptr) + GETJSAMPLE(inptr[1])
    			      + bias) >> 1);
          bias ^= 1;		/* 0=>1, 1=>0 */
          inptr += 2;
        }
      }
    }

    /*
     * Downsample pixel values of a single component.
     * This version handles the standard case of 2:1 horizontal and 2:1 vertical,
     * without smoothing.
     */

    METHODDEF(void)
    h2v2_downsample (j_compress_ptr cinfo, jpeg_component_info * compptr,
    		 JSAMPARRAY input_data, JSAMPARRAY output_data)
    {
      int inrow, outrow;
      JDIMENSION outcol;
      JDIMENSION output_cols = compptr->width_in_blocks * compptr->DCT_h_scaled_size;
      register JSAMPROW inptr0, inptr1, outptr;
      register int bias;

      /* Expand input data enough to let all the output samples be generated
       * by the standard loop.  Special-casing padded output would be more
       * efficient.
       */
      expand_right_edge(input_data, cinfo->max_v_samp_factor,
    		    cinfo->image_width, output_cols * 2);

      inrow = outrow = 0;
      while (inrow < cinfo->max_v_samp_factor) {
        outptr = output_data[outrow];
        inptr0 = input_data[inrow];
        inptr1 = input_data[inrow+1];
        bias = 1;			/* bias = 1,2,1,2,... for successive samples */
        for (outcol = 0; outcol < output_cols; outcol++) {
          *outptr++ = (JSAMPLE) ((GETJSAMPLE(*inptr0) + GETJSAMPLE(inptr0[1]) +
    			      GETJSAMPLE(*inptr1) + GETJSAMPLE(inptr1[1])
    			      + bias) >> 2);
          bias ^= 3;		/* 1=>2, 2=>1 */
          inptr0 += 2; inptr1 += 2;
        }
        inrow += 2;
        outrow++;
      }
    }

    #ifdef INPUT_SMOOTHING_SUPPORTED

    /*
     * Downsample pixel values of a single component.
     * This version handles the standard case of 2:1 horizontal and 2:1 vertical,
     * with smoothing.  One row of context is required.
     */

    METHODDEF(void)
    h2v2_smooth_downsample (j_compress_ptr cinfo, jpeg_component_info * compptr,
    			JSAMPARRAY input_data, JSAMPARRAY output_data)
    {
      int inrow, outrow;
      JDIMENSION colctr;
      JDIMENSION output_cols = compptr->width_in_blocks * compptr->DCT_h_scaled_size;
      register JSAMPROW inptr0, inptr1, above_ptr, below_ptr, outptr;
      INT32 membersum, neighsum, memberscale, neighscale;

      /* Expand input data enough to let all the output samples be generated
       * by the standard loop.  Special-casing padded output would be more
       * efficient.
       */
      expand_right_edge(input_data - 1, cinfo->max_v_samp_factor + 2,
    		    cinfo->image_width, output_cols * 2);

      /* We don't bother to form the individual "smoothed" input pixel values;
       * we can directly compute the output which is the average of the four
       * smoothed values.  Each of the four member pixels contributes a fraction
       * (1-8*SF) to its own smoothed image and a fraction SF to each of the three
       * other smoothed pixels, therefore a total fraction (1-5*SF)/4 to the final
       * output.  The four corner-adjacent neighbor pixels contribute a fraction
       * SF to just one smoothed pixel, or SF/4 to the final output; while the
       * eight edge-adjacent neighbors contribute SF to each of two smoothed
       * pixels, or SF/2 overall.  In order to use integer arithmetic, these
       * factors are scaled by 2^16 = 65536.
       * Also recall that SF = smoothing_factor / 1024.
       */

      memberscale = 16384 - cinfo->smoothing_factor * 80; /* scaled (1-5*SF)/4 */
      neighscale = cinfo->smoothing_factor * 16; /* scaled SF/4 */

      inrow = outrow = 0;
      while (inrow < cinfo->max_v_samp_factor) {
        outptr = output_data[outrow];
        inptr0 = input_data[inrow];
        inptr1 = input_data[inrow+1];
        above_ptr = input_data[inrow-1];
        below_ptr = input_data[inrow+2];

        /* Special case for first column: pretend column -1 is same as column 0 */
        membersum = GETJSAMPLE(*inptr0) + GETJSAMPLE(inptr0[1]) +
    		GETJSAMPLE(*inptr1) + GETJSAMPLE(inptr1[1]);
        neighsum = GETJSAMPLE(*above_ptr) + GETJSAMPLE(above_ptr[1]) +
    	       GETJSAMPLE(*below_ptr) + GETJSAMPLE(below_ptr[1]) +
    	       GETJSAMPLE(*inptr0) + GETJSAMPLE(inptr0[2]) +
    	       GETJSAMPLE(*inptr1) + GETJSAMPLE(inptr1[2]);
        neighsum += neighsum;
        neighsum += GETJSAMPLE(*above_ptr) + GETJSAMPLE(above_ptr[2]) +
    		GETJSAMPLE(*below_ptr) + GETJSAMPLE(below_ptr[2]);
        membersum = membersum * memberscale + neighsum * neighscale;
        *outptr++ = (JSAMPLE) ((membersum + 32768) >> 16);
        inptr0 += 2; inptr1 += 2; above_ptr += 2; below_ptr += 2;

        for (colctr = output_cols - 2; colctr > 0; colctr--) {
          /* sum of pixels directly mapped to this output element */
          membersum = GETJSAMPLE(*inptr0) + GETJSAMPLE(inptr0[1]) +
    		  GETJSAMPLE(*inptr1) + GETJSAMPLE(inptr1[1]);
          /* sum of edge-neighbor pixels */
          neighsum = GETJSAMPLE(*above_ptr) + GETJSAMPLE(above_ptr[1]) +
    		 GETJSAMPLE(*below_ptr) + GETJSAMPLE(below_ptr[1]) +
    		 GETJSAMPLE(inptr0[-1]) + GETJSAMPLE(inptr0[2]) +
    		 GETJSAMPLE(inptr1[-1]) + GETJSAMPLE(inptr1[2]);
          /* The edge-neighbors count twice as much as corner-neighbors */
          neighsum += neighsum;
          /* Add in the corner-neighbors */
          neighsum += GETJSAMPLE(above_ptr[-1]) + GETJSAMPLE(above_ptr[2]) +
    		  GETJSAMPLE(below_ptr[-1]) + GETJSAMPLE(below_ptr[2]);
          /* form final output scaled up by 2^16 */
          membersum = membersum * memberscale + neighsum * neighscale;
          /* round, descale and output it */
          *outptr++ = (JSAMPLE) ((membersum + 32768) >> 16);
          inptr0 += 2; inptr1 += 2; above_ptr += 2; below_ptr += 2;
        }

        /* Special case for last column */
        membersum = GETJSAMPLE(*inptr0) + GETJSAMPLE(inptr0[1]) +
    		GETJSAMPLE(*inptr1) + GETJSAMPLE(inptr1[1]);
        neighsum = GETJSAMPLE(*above_ptr) + GETJSAMPLE(above_ptr[1]) +
    	       GETJSAMPLE(*below_ptr) + GETJSAMPLE(below_ptr[1]) +
    	       GETJSAMPLE(inptr0[-1]) + GETJSAMPLE(inptr0[1]) +
    	       GETJSAMPLE(inptr1[-1]) + GETJSAMPLE(inptr1[1]);
        neighsum += neighsum;
        neighsum += GETJSAMPLE(above_ptr[-1]) + GETJSAMPLE(above_ptr[1]) +
    		GETJSAMPLE(below_ptr[-1]) + GETJSAMPLE(below_ptr[1]);
        membersum = membersum * memberscale + neighsum * neighscale;
        *outptr = (JSAMPLE) ((membersum + 32768) >> 16);

        inrow += 2;
        outrow++;
      }
    }

    /*
     * Downsample pixel values of a single component.
     * This version handles the special case of a full-size component,
     * with smoothing.  One row of context is required.
     */

    METHODDEF(void)
    fullsize_smooth_downsample (j_compress_ptr cinfo, jpeg_component_info *compptr,
    			    JSAMPARRAY input_data, JSAMPARRAY output_data)
    {
      int inrow;
      JDIMENSION colctr;
      JDIMENSION output_cols = compptr->width_in_blocks * compptr->DCT_h_scaled_size;
      register JSAMPROW inptr, above_ptr, below_ptr, outptr;
      INT32 membersum, neighsum, memberscale, neighscale;
      int colsum, lastcolsum, nextcolsum;

      /* Expand input data enough to let all the output samples be generated
       * by the standard loop.  Special-casing padded output would be more
       * efficient.
       */
      expand_right_edge(input_data - 1, cinfo->max_v_samp_factor + 2,
    		    cinfo->image_width, output_cols);

      /* Each of the eight neighbor pixels contributes a fraction SF to the
       * smoothed pixel, while the main pixel contributes (1-8*SF).  In order
       * to use integer arithmetic, these factors are multiplied by 2^16 = 65536.
       * Also recall that SF = smoothing_factor / 1024.
       */

      memberscale = 65536L - cinfo->smoothing_factor * 512L; /* scaled 1-8*SF */
      neighscale = cinfo->smoothing_factor * 64; /* scaled SF */

      for (inrow = 0; inrow < cinfo->max_v_samp_factor; inrow++) {
        outptr = output_data[inrow];
        inptr = input_data[inrow];
        above_ptr = input_data[inrow-1];
        below_ptr = input_data[inrow+1];

        /* Special case for first column */
        colsum = GETJSAMPLE(*above_ptr++) + GETJSAMPLE(*below_ptr++) +
    	     GETJSAMPLE(*inptr);
        membersum = GETJSAMPLE(*inptr++);
        nextcolsum = GETJSAMPLE(*above_ptr) + GETJSAMPLE(*below_ptr) +
    		 GETJSAMPLE(*inptr);
        neighsum = colsum + (colsum - membersum) + nextcolsum;
        membersum = membersum * memberscale + neighsum * neighscale;
        *outptr++ = (JSAMPLE) ((membersum + 32768) >> 16);
        lastcolsum = colsum; colsum = nextcolsum;

        for (colctr = output_cols - 2; colctr > 0; colctr--) {
          membersum = GETJSAMPLE(*inptr++);
          above_ptr++; below_ptr++;
          nextcolsum = GETJSAMPLE(*above_ptr) + GETJSAMPLE(*below_ptr) +
    		   GETJSAMPLE(*inptr);
          neighsum = lastcolsum + (colsum - membersum) + nextcolsum;
          membersum = membersum * memberscale + neighsum * neighscale;
          *outptr++ = (JSAMPLE) ((membersum + 32768) >> 16);
          lastcolsum = colsum; colsum = nextcolsum;
        }

        /* Special case for last column */
        membersum = GETJSAMPLE(*inptr);
        neighsum = lastcolsum + (colsum - membersum) + colsum;
        membersum = membersum * memberscale + neighsum * neighscale;
        *outptr = (JSAMPLE) ((membersum + 32768) >> 16);

      }
    }

    #endif /* INPUT_SMOOTHING_SUPPORTED */

    /*
     * Module initialization routine for downsampling.
     * Note that we must select a routine for each component.
     */

    GLOBAL(void)
    jinit_downsampler (j_compress_ptr cinfo)
    {
      my_downsample_ptr downsample;
      int ci;
      jpeg_component_info * compptr;
      boolean smoothok = TRUE;
      int h_in_group, v_in_group, h_out_group, v_out_group;

      downsample = (my_downsample_ptr)
        (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
    				SIZEOF(my_downsampler));
      cinfo->downsample = (struct jpeg_downsampler *) downsample;
      downsample->pub.start_pass = start_pass_downsample;
      downsample->pub.downsample = sep_downsample;
      downsample->pub.need_context_rows = FALSE;

      if (cinfo->CCIR601_sampling)
        ERREXIT(cinfo, JERR_CCIR601_NOTIMPL);

      /* Verify we can handle the sampling factors, and set up method pointers */
      for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
           ci++, compptr++) {
        /* Compute size of an "output group" for DCT scaling.  This many samples
         * are to be converted from max_h_samp_factor * max_v_samp_factor pixels.
         */
        h_out_group = (compptr->h_samp_factor * compptr->DCT_h_scaled_size) /
    		  cinfo->min_DCT_h_scaled_size;
        v_out_group = (compptr->v_samp_factor * compptr->DCT_v_scaled_size) /
    		  cinfo->min_DCT_v_scaled_size;
        h_in_group = cinfo->max_h_samp_factor;
        v_in_group = cinfo->max_v_samp_factor;
        downsample->rowgroup_height[ci] = v_out_group; /* save for use later */
        if (h_in_group == h_out_group && v_in_group == v_out_group) {
    #ifdef INPUT_SMOOTHING_SUPPORTED
          if (cinfo->smoothing_factor) {
    	downsample->methods[ci] = fullsize_smooth_downsample;
    	downsample->pub.need_context_rows = TRUE;
          } else
    #endif
    	downsample->methods[ci] = fullsize_downsample;
        } else if (h_in_group == h_out_group * 2 &&
    	       v_in_group == v_out_group) {
          smoothok = FALSE;
          downsample->methods[ci] = h2v1_downsample;
        } else if (h_in_group == h_out_group * 2 &&
    	       v_in_group == v_out_group * 2) {
    #ifdef INPUT_SMOOTHING_SUPPORTED
          if (cinfo->smoothing_factor) {
    	downsample->methods[ci] = h2v2_smooth_downsample;
    	downsample->pub.need_context_rows = TRUE;
          } else
    #endif
    	downsample->methods[ci] = h2v2_downsample;
        } else if ((h_in_group % h_out_group) == 0 &&
    	       (v_in_group % v_out_group) == 0) {
          smoothok = FALSE;
          downsample->methods[ci] = int_downsample;
          downsample->h_expand[ci] = (UINT8) (h_in_group / h_out_group);
          downsample->v_expand[ci] = (UINT8) (v_in_group / v_out_group);
        } else
          ERREXIT(cinfo, JERR_FRACT_SAMPLE_NOTIMPL);
      }

    #ifdef INPUT_SMOOTHING_SUPPORTED
      if (cinfo->smoothing_factor && !smoothok)
        TRACEMS(cinfo, 0, JTRC_SMOOTH_NOTIMPL);
    #endif
    }