potrace-1.16/src/greymap.c

/* Copyright (C) 2001-2019 Peter Selinger.
   This file is part of Potrace. It is free software and it is covered
   by the GNU General Public License. See the file COPYING for details. */


/* Routines for manipulating greymaps, including reading pgm files. We
   only deal with greymaps of depth 8 bits. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <errno.h>
#include <stddef.h>
#ifdef HAVE_INTTYPES_H
#include <inttypes.h>
#endif

#include "greymap.h"
#include "bitops.h"

#define INTBITS (8*sizeof(int))

#define mod(a,n) ((a)>=(n) ? (a)%(n) : (a)>=0 ? (a) : (n)-1-(-1-(a))%(n))

static int gm_readbody_pnm(FILE *f, greymap_t **gmp, int magic);
static int gm_readbody_bmp(FILE *f, greymap_t **gmp);

#define TRY(x) if (x) goto try_error
#define TRY_EOF(x) if (x) goto eof
#define TRY_STD(x) if (x) goto std_error

/* ---------------------------------------------------------------------- */
/* basic greymap routines */

/* calculate the size, in bytes, required for the data area of a
   greymap of the given dy and h. Assume h >= 0. Return -1 if the size
   does not fit into the ptrdiff_t type. */
static inline ptrdiff_t getsize(int dy, int h) {
  ptrdiff_t size;

  if (dy < 0) {
    dy = -dy;
  }

  size = (ptrdiff_t)dy * (ptrdiff_t)h * (ptrdiff_t)GM_SAMPLESIZE;

  /* check for overflow error */
  if (size < 0 || (h != 0 && dy != 0 && size / h / dy != GM_SAMPLESIZE)) {
    return -1;
  }

  return size;
}

/* return the size, in bytes, of the data area of the greymap. Return
   -1 if the size does not fit into the ptrdiff_t type; however, this
   cannot happen if the bitmap is well-formed, i.e., if created with
   gm_new or gm_dup. */
static inline ptrdiff_t gm_size(const greymap_t *gm) {
  return getsize(gm->dy, gm->h);
}


/* return new greymap initialized to 0. NULL with errno on error.
   Assumes w, h >= 0. */
greymap_t *gm_new(int w, int h) {
  greymap_t *gm;
  int dy = w;
  ptrdiff_t size;

  size = getsize(dy, h);
  if (size < 0) {
    errno = ENOMEM;
    return NULL;
  }
  if (size == 0) {
    size = GM_SAMPLESIZE; /* make sure calloc() doesn't return NULL */
  }

  gm = (greymap_t *) malloc(sizeof(greymap_t));
  if (!gm) {
    return NULL;
  }
  gm->w = w;
  gm->h = h;
  gm->dy = dy;
  gm->base = (gm_sample_t *) calloc(1, size);
  if (!gm->base) {
    free(gm);
    return NULL;
  }
  gm->map = gm->base;
  return gm;
}

/* free the given greymap */
void gm_free(greymap_t *gm) {
  if (gm) {
    free(gm->base);
  }
  free(gm);
}

/* duplicate the given greymap. Return NULL on error with errno set. */
greymap_t *gm_dup(greymap_t *gm) {
  greymap_t *gm1 = gm_new(gm->w, gm->h);
  int y;

  if (!gm1) {
    return NULL;
  }
  for (y=0; y<gm->h; y++) {
    memcpy(gm_scanline(gm1, y), gm_scanline(gm, y), (size_t)gm1->dy * GM_SAMPLESIZE);
  }
  return gm1;
}

/* clear the given greymap to color b. */
void gm_clear(greymap_t *gm, int b) {
  ptrdiff_t size = gm_size(gm);
  int x, y;

  if (b==0) {
    memset(gm->base, 0, size);
  } else {
    for (y=0; y<gm->h; y++) {
      for (x=0; x<gm->w; x++) {
        GM_UPUT(gm, x, y, b);
      }
    }
  }
}

/* turn the given greymap upside down. This does not move the pixel
   data or change the base address. */
static inline void gm_flip(greymap_t *gm) {
  int dy = gm->dy;

  if (gm->h == 0 || gm->h == 1) {
    return;
  }

  gm->map = gm_scanline(gm, gm->h - 1);
  gm->dy = -dy;
}

/* resize the greymap to the given new height. The pixel data remains
   bottom-aligned (truncated at the top) when dy >= 0 and top-aligned
   (truncated at the bottom) when dy < 0. Return 0 on success, or 1 on
   error with errno set. If the new height is <= the old one, no error
   should occur. If the new height is larger, the additional pixel
   data is *not* initialized. */
static inline int gm_resize(greymap_t *gm, int h) {
  int dy = gm->dy;
  ptrdiff_t newsize;
  gm_sample_t *newbase;

  if (dy < 0) {
    gm_flip(gm);
  }

  newsize = getsize(dy, h);
  if (newsize < 0) {
    errno = ENOMEM;
    goto error;
  }
  if (newsize == 0) {
    newsize = GM_SAMPLESIZE; /* make sure realloc() doesn't return NULL */
  }

  newbase = (gm_sample_t *)realloc(gm->base, newsize);
  if (newbase == NULL) {
    goto error;
  }
  gm->base = newbase;
  gm->map = newbase;
  gm->h = h;

  if (dy < 0) {
    gm_flip(gm);
  }
  return 0;

 error:
  if (dy < 0) {
    gm_flip(gm);
  }
  return 1;
}


/* ---------------------------------------------------------------------- */
/* routines for reading pnm streams */

/* read next character after whitespace and comments. Return EOF on
   end of file or error. */
static int fgetc_ws(FILE *f) {
  int c;

  while (1) {
    c = fgetc(f);
    if (c=='#') {
      while (1) {
	c = fgetc(f);
	if (c=='\n' || c==EOF) {
	  break;
	}
      }
    }
    /* space, tab, line feed, carriage return, form-feed */
    if (c!=' ' && c!='\t' && c!='\r' && c!='\n' && c!=12) {
      return c;
    }
  }
}

/* skip whitespace and comments, then read a non-negative decimal
   number from a stream. Return -1 on EOF. Tolerate other errors (skip
   bad characters). Do not the read any characters following the
   number (put next character back into the stream) */

static int readnum(FILE *f) {
  int c;
  uint64_t acc;

  /* skip whitespace and comments */
  while (1) {
    c = fgetc_ws(f);
    if (c==EOF) {
      return -1;
    }
    if (c>='0' && c<='9') {
      break;
    }
  }

  /* first digit is already in c */
  acc = c-'0';
  while (1) {
    c = fgetc(f);
    if (c==EOF) {
      break;
    }
    if (c<'0' || c>'9') {
      ungetc(c, f);
      break;
    }
    acc *= 10;
    acc += c-'0';
    if (acc > 0x7fffffff) {
      return -1;
    }
  }
  return acc;
}

/* similar to readnum, but read only a single 0 or 1, and do not read
   any characters after it. */

static int readbit(FILE *f) {
  int c;

  /* skip whitespace and comments */
  while (1) {
    c = fgetc_ws(f);
    if (c==EOF) {
      return -1;
    }
    if (c>='0' && c<='1') {
      break;
    }
  }

  return c-'0';
}

/* ---------------------------------------------------------------------- */

/* read a PNM stream: P1-P6 format (see pnm(5)), or a BMP stream, and
   convert the output to a greymap. Return greymap in *gmp. Return 0
   on success, -1 on error with errno set, -2 on bad file format (with
   error message in gm_read_error), and 1 on premature end of file, -3
   on empty file (including files with only whitespace and comments),
   -4 if wrong magic number. If the return value is >=0, *gmp is
   valid. */

const char *gm_read_error = NULL;

int gm_read(FILE *f, greymap_t **gmp) {
  int magic[2];

  /* read magic number. We ignore whitespace and comments before the
     magic, for the benefit of concatenated files in P1-P3 format.
     Multiple P1-P3 images in a single file are not formally allowed
     by the PNM standard, but there is no harm in being lenient. */

  magic[0] = fgetc_ws(f);
  if (magic[0] == EOF) {
    /* files which contain only comments and whitespace count as "empty" */
    return -3;
  }
  magic[1] = fgetc(f);
  if (magic[0] == 'P' && magic[1] >= '1' && magic[1] <= '6') {
    return gm_readbody_pnm(f, gmp, magic[1]);
  }
  if (magic[0] == 'B' && magic[1] == 'M') {
    return gm_readbody_bmp(f, gmp);
  }
  return -4;
}

/* ---------------------------------------------------------------------- */
/* read PNM format */

/* read PNM stream after magic number. Return values as for gm_read */
static int gm_readbody_pnm(FILE *f, greymap_t **gmp, int magic) {
  greymap_t *gm;
  int x, y, i, j, b, b1, sum;
  int bpr; /* bytes per row (as opposed to 4*gm->c) */
  int w, h, max;
  int realheight;  /* in case of incomplete file, keeps track of how
                      many scan lines actually contain data */

  gm = NULL;

  w = readnum(f);
  if (w<0) {
    goto format_error;
  }

  h = readnum(f);
  if (h<0) {
    goto format_error;
  }

  /* allocate greymap */
  gm = gm_new(w, h);
  if (!gm) {
    goto std_error;
  }

  realheight = 0;

  switch (magic) {
  default:
    /* not reached */
    goto format_error;

  case '1':
    /* read P1 format: PBM ascii */

    for (y=0; y<h; y++) {
      realheight = y+1;
      for (x=0; x<w; x++) {
	b = readbit(f);
	if (b<0) {
	  goto eof;
	}
	GM_UPUT(gm, x, y, b ? 0 : 255);
      }
    }
    break;

  case '2':
    /* read P2 format: PGM ascii */

    max = readnum(f);
    if (max<1) {
      goto format_error;
    }

    for (y=0; y<h; y++) {
      realheight = y+1;
      for (x=0; x<w; x++) {
        b = readnum(f);
        if (b<0) {
          goto eof;
        }
        GM_UPUT(gm, x, y, b*255/max);
      }
    }
    break;

  case '3':
    /* read P3 format: PPM ascii */

    max = readnum(f);
    if (max<1) {
      goto format_error;
    }

    for (y=0; y<h; y++) {
      realheight = y+1;
      for (x=0; x<w; x++) {
	sum = 0;
	for (i=0; i<3; i++) {
	  b = readnum(f);
	  if (b<0) {
	    goto eof;
	  }
	  sum += b;
	}
        GM_UPUT(gm, x, y, sum*(255/3)/max);
      }
    }
    break;

  case '4':
    /* read P4 format: PBM raw */

    b = fgetc(f);  /* read single white-space character after height */
    if (b==EOF) {
      goto format_error;
    }

    bpr = (w+7)/8;

    for (y=0; y<h; y++) {
      realheight = y+1;
      for (i=0; i<bpr; i++) {
	b = fgetc(f);
	if (b==EOF) {
	  goto eof;
	}
	for (j=0; j<8; j++) {
	  GM_PUT(gm, i*8+j, y, b & (0x80 >> j) ? 0 : 255);
	}
      }
    }
    break;

  case '5':
    /* read P5 format: PGM raw */

    max = readnum(f);
    if (max<1) {
      goto format_error;
    }

    b = fgetc(f);  /* read single white-space character after max */
    if (b==EOF) {
      goto format_error;
    }

    for (y=0; y<h; y++) {
      realheight = y+1;
      for (x=0; x<w; x++) {
        b = fgetc(f);
        if (b==EOF)
          goto eof;
        if (max>=256) {
          b <<= 8;
          b1 = fgetc(f);
          if (b1==EOF)
            goto eof;
          b |= b1;
        }
        GM_UPUT(gm, x, y, b*255/max);
      }
    }
    break;

  case '6':
    /* read P6 format: PPM raw */

    max = readnum(f);
    if (max<1) {
      goto format_error;
    }

    b = fgetc(f);  /* read single white-space character after max */
    if (b==EOF) {
      goto format_error;
    }

    for (y=0; y<h; y++) {
      realheight = y+1;
      for (x=0; x<w; x++) {
        sum = 0;
        for (i=0; i<3; i++) {
          b = fgetc(f);
          if (b==EOF) {
            goto eof;
	  }
          if (max>=256) {
            b <<= 8;
            b1 = fgetc(f);
            if (b1==EOF)
              goto eof;
            b |= b1;
          }
          sum += b;
        }
        GM_UPUT(gm, x, y, sum*(255/3)/max);
      }
    }
    break;
  }

  gm_flip(gm);
  *gmp = gm;
  return 0;

 eof:
  TRY_STD(gm_resize(gm, realheight));
  gm_flip(gm);
  *gmp = gm;
  return 1;

 format_error:
  gm_free(gm);
  if (magic == '1' || magic == '4') {
    gm_read_error = "invalid pbm file";
  } else if (magic == '2' || magic == '5') {
    gm_read_error = "invalid pgm file";
  } else {
    gm_read_error = "invalid ppm file";
  }
  return -2;

 std_error:
  gm_free(gm);
  return -1;
}

/* ---------------------------------------------------------------------- */
/* read BMP format */

struct bmp_info_s {
  unsigned int FileSize;
  unsigned int reserved;
  unsigned int DataOffset;
  unsigned int InfoSize;
  unsigned int w;              /* width */
  unsigned int h;              /* height */
  unsigned int Planes;
  unsigned int bits;           /* bits per sample */
  unsigned int comp;           /* compression mode */
  unsigned int ImageSize;
  unsigned int XpixelsPerM;
  unsigned int YpixelsPerM;
  unsigned int ncolors;        /* number of colors in palette */
  unsigned int ColorsImportant;
  unsigned int RedMask;
  unsigned int GreenMask;
  unsigned int BlueMask;
  unsigned int AlphaMask;
  unsigned int ctbits;         /* sample size for color table */
  int topdown;                 /* top-down mode? */
};
typedef struct bmp_info_s bmp_info_t;

/* auxiliary */

static int bmp_count = 0; /* counter for byte padding */
static int bmp_pos = 0;   /* counter from start of BMP data */

/* read n-byte little-endian integer. Return 1 on EOF or error, else
   0. Assume n<=4. */
static int bmp_readint(FILE *f, int n, unsigned int *p) {
  int i;
  unsigned int sum = 0;
  int b;

  for (i=0; i<n; i++) {
    b = fgetc(f);
    if (b==EOF) {
      return 1;
    }
    sum += (unsigned)b << (8*i);
  }
  bmp_count += n;
  bmp_pos += n;
  *p = sum;
  return 0;
}

/* reset padding boundary */
static void bmp_pad_reset(void) {
  bmp_count = 0;
}

/* read padding bytes to 4-byte boundary. Return 1 on EOF or error,
   else 0. */
static int bmp_pad(FILE *f) {
  int c, i, b;

  c = (-bmp_count) & 3;
  for (i=0; i<c; i++) {
    b = fgetc(f);
    if (b==EOF) {
      return 1;
    }
  }
  bmp_pos += c;
  bmp_count = 0;
  return 0;
}

/* forward to the new file position. Return 1 on EOF or error, else 0 */
static int bmp_forward(FILE *f, int pos) {
  int b;

  while (bmp_pos < pos) {
    b = fgetc(f);
    if (b==EOF) {
      return 1;
    }
    bmp_pos++;
    bmp_count++;
  }
  return 0;
}

/* safe colortable access */
#define COLTABLE(c) ((c) < bmpinfo.ncolors ? coltable[(c)] : 0)

/* read BMP stream after magic number. Return values as for gm_read.
   We choose to be as permissive as possible, since there are many
   programs out there which produce BMP. For instance, ppmtobmp can
   produce codings with anywhere from 1-8 or 24 bits per sample,
   although most specifications only allow 1,4,8,24,32. We can also
   read both the old and new OS/2 BMP formats in addition to the
   Windows BMP format. */
static int gm_readbody_bmp(FILE *f, greymap_t **gmp) {
  bmp_info_t bmpinfo;
  int *coltable;
  unsigned int b, c;
  unsigned int i, j;
  greymap_t *gm;
  unsigned int x, y;
  int col[2];
  unsigned int bitbuf;
  unsigned int n;
  unsigned int redshift, greenshift, blueshift;
  int realheight;  /* in case of incomplete file, keeps track of how
                      many scan lines actually contain data */

  gm_read_error = NULL;
  gm = NULL;
  coltable = NULL;

  bmp_pos = 2;  /* set file position */

  /* file header (minus magic number) */
  TRY(bmp_readint(f, 4, &bmpinfo.FileSize));
  TRY(bmp_readint(f, 4, &bmpinfo.reserved));
  TRY(bmp_readint(f, 4, &bmpinfo.DataOffset));

  /* info header */
  TRY(bmp_readint(f, 4, &bmpinfo.InfoSize));
  if (bmpinfo.InfoSize == 40 || bmpinfo.InfoSize == 64
      || bmpinfo.InfoSize == 108 || bmpinfo.InfoSize == 124) {
    /* Windows or new OS/2 format */
    bmpinfo.ctbits = 32; /* sample size in color table */
    TRY(bmp_readint(f, 4, &bmpinfo.w));
    TRY(bmp_readint(f, 4, &bmpinfo.h));
    TRY(bmp_readint(f, 2, &bmpinfo.Planes));
    TRY(bmp_readint(f, 2, &bmpinfo.bits));
    TRY(bmp_readint(f, 4, &bmpinfo.comp));
    TRY(bmp_readint(f, 4, &bmpinfo.ImageSize));
    TRY(bmp_readint(f, 4, &bmpinfo.XpixelsPerM));
    TRY(bmp_readint(f, 4, &bmpinfo.YpixelsPerM));
    TRY(bmp_readint(f, 4, &bmpinfo.ncolors));
    TRY(bmp_readint(f, 4, &bmpinfo.ColorsImportant));
    if (bmpinfo.InfoSize >= 108) { /* V4 and V5 bitmaps */
      TRY(bmp_readint(f, 4, &bmpinfo.RedMask));
      TRY(bmp_readint(f, 4, &bmpinfo.GreenMask));
      TRY(bmp_readint(f, 4, &bmpinfo.BlueMask));
      TRY(bmp_readint(f, 4, &bmpinfo.AlphaMask));
    }
    if (bmpinfo.w > 0x7fffffff) {
      goto format_error;
    }
    if (bmpinfo.h > 0x7fffffff) {
      bmpinfo.h = (-bmpinfo.h) & 0xffffffff;
      bmpinfo.topdown = 1;
    } else {
      bmpinfo.topdown = 0;
    }
    if (bmpinfo.h > 0x7fffffff) {
      goto format_error;
    }
  } else if (bmpinfo.InfoSize == 12) {
    /* old OS/2 format */
    bmpinfo.ctbits = 24; /* sample size in color table */
    TRY(bmp_readint(f, 2, &bmpinfo.w));
    TRY(bmp_readint(f, 2, &bmpinfo.h));
    TRY(bmp_readint(f, 2, &bmpinfo.Planes));
    TRY(bmp_readint(f, 2, &bmpinfo.bits));
    bmpinfo.comp = 0;
    bmpinfo.ncolors = 0;
    bmpinfo.topdown = 0;
  } else {
    goto format_error;
  }

  if (bmpinfo.comp == 3 && bmpinfo.InfoSize < 108) {
    /* bitfield feature is only understood with V4 and V5 format */
    goto format_error;
  }

  if (bmpinfo.comp > 3 || bmpinfo.bits > 32) {
    goto format_error;
  }

  /* forward to color table (e.g., if bmpinfo.InfoSize == 64) */
  TRY(bmp_forward(f, 14+bmpinfo.InfoSize));

  if (bmpinfo.Planes != 1) {
    gm_read_error = "cannot handle bmp planes";
    goto format_error;  /* can't handle planes */
  }

  if (bmpinfo.ncolors == 0 && bmpinfo.bits <= 8) {
    bmpinfo.ncolors = 1 << bmpinfo.bits;
  }

  /* color table, present only if bmpinfo.bits <= 8. */
  if (bmpinfo.bits <= 8) {
    coltable = (int *) calloc(bmpinfo.ncolors, sizeof(int));
    if (!coltable) {
      goto std_error;
    }
    /* NOTE: since we are reading a greymap, we can immediately convert
       the color table entries to grey values. */
    for (i=0; i<bmpinfo.ncolors; i++) {
      TRY(bmp_readint(f, bmpinfo.ctbits/8, &c));
      c = ((c>>16) & 0xff) + ((c>>8) & 0xff) + (c & 0xff);
      coltable[i] = c/3;
    }
  }

  /* forward to data */
  if (bmpinfo.InfoSize != 12) { /* not old OS/2 format */
    TRY(bmp_forward(f, bmpinfo.DataOffset));
  }

  /* allocate greymap */
  gm = gm_new(bmpinfo.w, bmpinfo.h);
  if (!gm) {
    goto std_error;
  }

  realheight = 0;

  switch (bmpinfo.bits + 0x100*bmpinfo.comp) {

  default:
    goto format_error;
    break;

  case 0x001:  /* monochrome palette */

    /* raster data */
    for (y=0; y<bmpinfo.h; y++) {
      realheight = y+1;
      bmp_pad_reset();
      for (i=0; 8*i<bmpinfo.w; i++) {
	TRY_EOF(bmp_readint(f, 1, &b));
	for (j=0; j<8; j++) {
	  GM_PUT(gm, i*8+j, y, b & (0x80 >> j) ? COLTABLE(1) : COLTABLE(0));
	}
      }
      TRY(bmp_pad(f));
    }
    break;

  case 0x002:  /* 2-bit to 8-bit palettes */
  case 0x003:
  case 0x004:
  case 0x005:
  case 0x006:
  case 0x007:
  case 0x008:
    for (y=0; y<bmpinfo.h; y++) {
      realheight = y+1;
      bmp_pad_reset();
      bitbuf = 0;  /* bit buffer: bits in buffer are high-aligned */
      n = 0;       /* number of bits currently in bitbuffer */
      for (x=0; x<bmpinfo.w; x++) {
	if (n < bmpinfo.bits) {
	  TRY_EOF(bmp_readint(f, 1, &b));
	  bitbuf |= b << (INTBITS - 8 - n);
	  n += 8;
	}
	b = bitbuf >> (INTBITS - bmpinfo.bits);
	bitbuf <<= bmpinfo.bits;
	n -= bmpinfo.bits;
	GM_UPUT(gm, x, y, COLTABLE(b));
      }
      TRY(bmp_pad(f));
    }
    break;

  case 0x010:  /* 16-bit encoding */
    /* can't do this format because it is not well-documented and I
       don't have any samples */
    gm_read_error = "cannot handle bmp 16-bit coding";
    goto format_error;
    break;

  case 0x018:  /* 24-bit encoding */
  case 0x020:  /* 32-bit encoding */
    for (y=0; y<bmpinfo.h; y++) {
      realheight = y+1;
      bmp_pad_reset();
      for (x=0; x<bmpinfo.w; x++) {
        TRY_EOF(bmp_readint(f, bmpinfo.bits/8, &c));
	c = ((c>>16) & 0xff) + ((c>>8) & 0xff) + (c & 0xff);
        GM_UPUT(gm, x, y, c/3);
      }
      TRY(bmp_pad(f));
    }
    break;

  case 0x320:  /* 32-bit encoding with bitfields */
    redshift = lobit(bmpinfo.RedMask);
    greenshift = lobit(bmpinfo.GreenMask);
    blueshift = lobit(bmpinfo.BlueMask);

    for (y=0; y<bmpinfo.h; y++) {
      realheight = y+1;
      bmp_pad_reset();
      for (x=0; x<bmpinfo.w; x++) {
        TRY_EOF(bmp_readint(f, bmpinfo.bits/8, &c));
	c = ((c & bmpinfo.RedMask) >> redshift) + ((c & bmpinfo.GreenMask) >> greenshift) + ((c & bmpinfo.BlueMask) >> blueshift);
        GM_UPUT(gm, x, y, c/3);
      }
      TRY(bmp_pad(f));
    }
    break;

  case 0x204:  /* 4-bit runlength compressed encoding (RLE4) */
    x = 0;
    y = 0;
    while (1) {
      TRY_EOF(bmp_readint(f, 1, &b)); /* opcode */
      TRY_EOF(bmp_readint(f, 1, &c)); /* argument */
      if (b>0) {
	/* repeat count */
	col[0] = COLTABLE((c>>4) & 0xf);
	col[1] = COLTABLE(c & 0xf);
	for (i=0; i<b && x<bmpinfo.w; i++) {
	  if (x>=bmpinfo.w) {
	    x=0;
	    y++;
	  }
	  if (x>=bmpinfo.w || y>=bmpinfo.h) {
	    break;
	  }
          realheight = y+1;
	  GM_PUT(gm, x, y, col[i&1]);
	  x++;
	}
      } else if (c == 0) {
	/* end of line */
	y++;
	x = 0;
      } else if (c == 1) {
	/* end of greymap */
	break;
      } else if (c == 2) {
	/* "delta": skip pixels in x and y directions */
	TRY_EOF(bmp_readint(f, 1, &b)); /* x offset */
	TRY_EOF(bmp_readint(f, 1, &c)); /* y offset */
	x += b;
	y += c;
      } else {
	/* verbatim segment */
	for (i=0; i<c; i++) {
	  if ((i&1)==0) {
	    TRY_EOF(bmp_readint(f, 1, &b));
	  }
	  if (x>=bmpinfo.w) {
	    x=0;
	    y++;
	  }
	  if (x>=bmpinfo.w || y>=bmpinfo.h) {
	    break;
	  }
          realheight = y+1;
	  GM_PUT(gm, x, y, COLTABLE((b>>(4-4*(i&1))) & 0xf));
	  x++;
	}
	if ((c+1) & 2) {
	  /* pad to 16-bit boundary */
	  TRY_EOF(bmp_readint(f, 1, &b));
	}
      }
    }
    break;

  case 0x108:  /* 8-bit runlength compressed encoding (RLE8) */
    x = 0;
    y = 0;
    while (1) {
      TRY_EOF(bmp_readint(f, 1, &b)); /* opcode */
      TRY_EOF(bmp_readint(f, 1, &c)); /* argument */
      if (b>0) {
	/* repeat count */
	for (i=0; i<b; i++) {
	  if (x>=bmpinfo.w) {
	    x=0;
	    y++;
	  }
	  if (x>=bmpinfo.w || y>=bmpinfo.h) {
	    break;
	  }
          realheight = y+1;
	  GM_PUT(gm, x, y, COLTABLE(c));
	  x++;
	}
      } else if (c == 0) {
	/* end of line */
	y++;
	x = 0;
      } else if (c == 1) {
	/* end of greymap */
	break;
      } else if (c == 2) {
	/* "delta": skip pixels in x and y directions */
	TRY_EOF(bmp_readint(f, 1, &b)); /* x offset */
	TRY_EOF(bmp_readint(f, 1, &c)); /* y offset */
	x += b;
	y += c;
      } else {
	/* verbatim segment */
	for (i=0; i<c; i++) {
	  TRY_EOF(bmp_readint(f, 1, &b));
          if (x>=bmpinfo.w) {
            x=0;
            y++;
          }
	  if (x>=bmpinfo.w || y>=bmpinfo.h) {
            break;
          }
          realheight = y+1;
	  GM_PUT(gm, x, y, COLTABLE(b));
	  x++;
	}
	if (c & 1) {
	  /* pad input to 16-bit boundary */
	  TRY_EOF(bmp_readint(f, 1, &b));
	}
      }
    }
    break;

  } /* switch */

  /* skip any potential junk after the data section, but don't
     complain in case EOF is encountered */
  bmp_forward(f, bmpinfo.FileSize);

  free(coltable);
  if (bmpinfo.topdown) {
    gm_flip(gm);
  }
  *gmp = gm;
  return 0;

 eof:
  TRY_STD(gm_resize(gm, realheight));
  free(coltable);
  if (bmpinfo.topdown) {
    gm_flip(gm);
  }
  *gmp = gm;
  return 1;

 format_error:
 try_error:
  free(coltable);
  gm_free(gm);
  if (!gm_read_error) {
    gm_read_error = "invalid bmp file";
  }
  return -2;

 std_error:
  free(coltable);
  gm_free(gm);
  return -1;
}

/* ---------------------------------------------------------------------- */

/* write a pgm stream, either P2 or (if raw != 0) P5 format. Include
   one-line comment if non-NULL. Mode determines how out-of-range
   color values are converted. Gamma is the desired gamma correction,
   if any (set to 2.2 if the image is to look optimal on a CRT monitor,
   2.8 for LCD). Set to 1.0 for no gamma correction */

int gm_writepgm(FILE *f, greymap_t *gm, const char *comment, int raw, int mode, double gamma) {
  int x, y, v;
  int gammatable[256];

  /* prepare gamma correction lookup table */
  if (gamma != 1.0) {
    gammatable[0] = 0;
    for (v=1; v<256; v++) {
      gammatable[v] = (int)(255 * exp(log(v/255.0)/gamma) + 0.5);
    }
  } else {
    for (v=0; v<256; v++) {
      gammatable[v] = v;
    }
  }

  fprintf(f, raw ? "P5\n" : "P2\n");
  if (comment && *comment) {
    fprintf(f, "# %s\n", comment);
  }
  fprintf(f, "%d %d 255\n", gm->w, gm->h);
  for (y=gm->h-1; y>=0; y--) {
    for (x=0; x<gm->w; x++) {
      v = GM_UGET(gm, x, y);
      if (mode == GM_MODE_NONZERO) {
	if (v > 255) {
	  v = 510 - v;
	}
	if (v < 0) {
	  v = 0;
	}
      } else if (mode == GM_MODE_ODD) {
	v = mod(v, 510);
	if (v > 255) {
	  v = 510 - v;
	}
      } else if (mode == GM_MODE_POSITIVE) {
	if (v < 0) {
	  v = 0;
	} else if (v > 255) {
	  v = 255;
	}
      } else if (mode == GM_MODE_NEGATIVE) {
	v = 510 - v;
	if (v < 0) {
	  v = 0;
	} else if (v > 255) {
	  v = 255;
	}
      }
      v = gammatable[v];

      if (raw) {
	fputc(v, f);
      } else {
	fprintf(f, x == gm->w-1 ? "%d\n" : "%d ", v);
      }
    }
  }
  return 0;
}

/* ---------------------------------------------------------------------- */
/* output - for primitive debugging purposes only! */

/* print greymap to screen */
int gm_print(FILE *f, greymap_t *gm) {
  int x, y;
  int xx, yy;
  int d, t;
  int sw, sh;

  sw = gm->w < 79 ? gm->w : 79;
  sh = gm->w < 79 ? gm->h : gm->h*sw*44/(79*gm->w);

  for (yy=sh-1; yy>=0; yy--) {
    for (xx=0; xx<sw; xx++) {
      d=0;
      t=0;
      for (x=xx*gm->w/sw; x<(xx+1)*gm->w/sw; x++) {
	for (y=yy*gm->h/sh; y<(yy+1)*gm->h/sh; y++) {
	  d += GM_GET(gm, x, y);
	  t += 256;
	}
      }
      fputc("*#=- "[5*d/t], f);  /* what a cute trick :) */
    }
    fputc('\n', f);
  }
  return 0;
}