start_input_bmp(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
{
  bmp_source_ptr source = (bmp_source_ptr)sinfo;
  U_CHAR bmpfileheader[14];
  U_CHAR bmpinfoheader[64];

#define GET_2B(array, offset) \
  ((unsigned short)UCH(array[offset]) + \
   (((unsigned short)UCH(array[offset + 1])) << 8))
#define GET_4B(array, offset) \
  ((unsigned int)UCH(array[offset]) + \
   (((unsigned int)UCH(array[offset + 1])) << 8) + \
   (((unsigned int)UCH(array[offset + 2])) << 16) + \
   (((unsigned int)UCH(array[offset + 3])) << 24))

  unsigned int bfOffBits;
  unsigned int headerSize;
  int biWidth;
  int biHeight;
  unsigned short biPlanes;
  unsigned int biCompression;
  int biXPelsPerMeter, biYPelsPerMeter;
  unsigned int biClrUsed = 0;
  int mapentrysize = 0;         /* 0 indicates no colormap */
  int bPad;
  JDIMENSION row_width = 0;

  /* Read and verify the bitmap file header */
  if (!ReadOK(source->pub.input_file, bmpfileheader, 14))
    ERREXIT(cinfo, JERR_INPUT_EOF);
  if (GET_2B(bmpfileheader, 0) != 0x4D42) /* 'BM' */
    ERREXIT(cinfo, JERR_BMP_NOT);
  bfOffBits = GET_4B(bmpfileheader, 10);
  /* We ignore the remaining fileheader fields */

  /* The infoheader might be 12 bytes (OS/2 1.x), 40 bytes (Windows),
   * or 64 bytes (OS/2 2.x).  Check the first 4 bytes to find out which.
   */
  if (!ReadOK(source->pub.input_file, bmpinfoheader, 4))
    ERREXIT(cinfo, JERR_INPUT_EOF);
  headerSize = GET_4B(bmpinfoheader, 0);
  if (headerSize < 12 || headerSize > 64)
    ERREXIT(cinfo, JERR_BMP_BADHEADER);
  if (!ReadOK(source->pub.input_file, bmpinfoheader + 4, headerSize - 4))
    ERREXIT(cinfo, JERR_INPUT_EOF);

  switch (headerSize) {
  case 12:
    /* Decode OS/2 1.x header (Microsoft calls this a BITMAPCOREHEADER) */
    biWidth = (int)GET_2B(bmpinfoheader, 4);
    biHeight = (int)GET_2B(bmpinfoheader, 6);
    biPlanes = GET_2B(bmpinfoheader, 8);
    source->bits_per_pixel = (int)GET_2B(bmpinfoheader, 10);

    switch (source->bits_per_pixel) {
    case 8:                     /* colormapped image */
      mapentrysize = 3;         /* OS/2 uses RGBTRIPLE colormap */
      TRACEMS2(cinfo, 1, JTRC_BMP_OS2_MAPPED, biWidth, biHeight);
      break;
    case 24:                    /* RGB image */
      TRACEMS2(cinfo, 1, JTRC_BMP_OS2, biWidth, biHeight);
      break;
    default:
      ERREXIT(cinfo, JERR_BMP_BADDEPTH);
      break;
    }
    break;
  case 40:
  case 64:
    /* Decode Windows 3.x header (Microsoft calls this a BITMAPINFOHEADER) */
    /* or OS/2 2.x header, which has additional fields that we ignore */
    biWidth = (int)GET_4B(bmpinfoheader, 4);
    biHeight = (int)GET_4B(bmpinfoheader, 8);
    biPlanes = GET_2B(bmpinfoheader, 12);
    source->bits_per_pixel = (int)GET_2B(bmpinfoheader, 14);
    biCompression = GET_4B(bmpinfoheader, 16);
    biXPelsPerMeter = (int)GET_4B(bmpinfoheader, 24);
    biYPelsPerMeter = (int)GET_4B(bmpinfoheader, 28);
    biClrUsed = GET_4B(bmpinfoheader, 32);
    /* biSizeImage, biClrImportant fields are ignored */

    switch (source->bits_per_pixel) {
    case 8:                     /* colormapped image */
      mapentrysize = 4;         /* Windows uses RGBQUAD colormap */
      TRACEMS2(cinfo, 1, JTRC_BMP_MAPPED, biWidth, biHeight);
      break;
    case 24:                    /* RGB image */
      TRACEMS2(cinfo, 1, JTRC_BMP, biWidth, biHeight);
      break;
    case 32:                    /* RGB image + Alpha channel */
      TRACEMS2(cinfo, 1, JTRC_BMP, biWidth, biHeight);
      break;
    default:
      ERREXIT(cinfo, JERR_BMP_BADDEPTH);
      break;
    }
    if (biCompression != 0)
      ERREXIT(cinfo, JERR_BMP_COMPRESSED);

    if (biXPelsPerMeter > 0 && biYPelsPerMeter > 0) {
      /* Set JFIF density parameters from the BMP data */
      cinfo->X_density = (UINT16)(biXPelsPerMeter / 100); /* 100 cm per meter */
      cinfo->Y_density = (UINT16)(biYPelsPerMeter / 100);
      cinfo->density_unit = 2;  /* dots/cm */
    }
    break;
  default:
    ERREXIT(cinfo, JERR_BMP_BADHEADER);
    return;
  }

  if (biWidth <= 0 || biHeight <= 0)
    ERREXIT(cinfo, JERR_BMP_EMPTY);
  if (biPlanes != 1)
    ERREXIT(cinfo, JERR_BMP_BADPLANES);

  /* Compute distance to bitmap data --- will adjust for colormap below */
  bPad = bfOffBits - (headerSize + 14);

  /* Read the colormap, if any */
  if (mapentrysize > 0) {
    if (biClrUsed <= 0)
      biClrUsed = 256;          /* assume it's 256 */
    else if (biClrUsed > 256)
      ERREXIT(cinfo, JERR_BMP_BADCMAP);
    /* Allocate space to store the colormap */
    source->colormap = (*cinfo->mem->alloc_sarray)
      ((j_common_ptr)cinfo, JPOOL_IMAGE, (JDIMENSION)biClrUsed, (JDIMENSION)3);
    /* and read it from the file */
    read_colormap(source, (int)biClrUsed, mapentrysize);
    /* account for size of colormap */
    bPad -= biClrUsed * mapentrysize;
  }

  /* Skip any remaining pad bytes */
  if (bPad < 0)                 /* incorrect bfOffBits value? */
    ERREXIT(cinfo, JERR_BMP_BADHEADER);
  while (--bPad >= 0) {
    (void)read_byte(source);
  }

  /* Compute row width in file, including padding to 4-byte boundary */
  switch (source->bits_per_pixel) {
  case 8:
    if (cinfo->in_color_space == JCS_UNKNOWN)
      cinfo->in_color_space = JCS_EXT_RGB;
    if (IsExtRGB(cinfo->in_color_space))
      cinfo->input_components = rgb_pixelsize[cinfo->in_color_space];
    else if (cinfo->in_color_space == JCS_GRAYSCALE)
      cinfo->input_components = 1;
    else if (cinfo->in_color_space == JCS_CMYK)
      cinfo->input_components = 4;
    else
      ERREXIT(cinfo, JERR_BAD_IN_COLORSPACE);
    row_width = (JDIMENSION)biWidth;
    break;
  case 24:
    if (cinfo->in_color_space == JCS_UNKNOWN)
      cinfo->in_color_space = JCS_EXT_BGR;
    if (IsExtRGB(cinfo->in_color_space))
      cinfo->input_components = rgb_pixelsize[cinfo->in_color_space];
    else if (cinfo->in_color_space == JCS_CMYK)
      cinfo->input_components = 4;
    else
      ERREXIT(cinfo, JERR_BAD_IN_COLORSPACE);
    row_width = (JDIMENSION)(biWidth * 3);
    break;
  case 32:
    if (cinfo->in_color_space == JCS_UNKNOWN)
      cinfo->in_color_space = JCS_EXT_BGRA;
    if (IsExtRGB(cinfo->in_color_space))
      cinfo->input_components = rgb_pixelsize[cinfo->in_color_space];
    else if (cinfo->in_color_space == JCS_CMYK)
      cinfo->input_components = 4;
    else
      ERREXIT(cinfo, JERR_BAD_IN_COLORSPACE);
    row_width = (JDIMENSION)(biWidth * 4);
    break;
  default:
    ERREXIT(cinfo, JERR_BMP_BADDEPTH);
  }
  while ((row_width & 3) != 0) row_width++;
  source->row_width = row_width;

  if (source->use_inversion_array) {
    /* Allocate space for inversion array, prepare for preload pass */
    source->whole_image = (*cinfo->mem->request_virt_sarray)
      ((j_common_ptr)cinfo, JPOOL_IMAGE, FALSE,
       row_width, (JDIMENSION)biHeight, (JDIMENSION)1);
    source->pub.get_pixel_rows = preload_image;
    if (cinfo->progress != NULL) {
      cd_progress_ptr progress = (cd_progress_ptr)cinfo->progress;
      progress->total_extra_passes++; /* count file input as separate pass */
    }
  } else {
    source->iobuffer = (U_CHAR *)
      (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE, row_width);
    switch (source->bits_per_pixel) {
    case 8:
      source->pub.get_pixel_rows = get_8bit_row;
      break;
    case 24:
      source->pub.get_pixel_rows = get_24bit_row;
      break;
    case 32:
      source->pub.get_pixel_rows = get_32bit_row;
      break;
    default:
      ERREXIT(cinfo, JERR_BMP_BADDEPTH);
    }
  }

  /* Allocate one-row buffer for returned data */
  source->pub.buffer = (*cinfo->mem->alloc_sarray)
    ((j_common_ptr)cinfo, JPOOL_IMAGE,
     (JDIMENSION)(biWidth * cinfo->input_components), (JDIMENSION)1);
  source->pub.buffer_height = 1;

  cinfo->data_precision = 8;
  cinfo->image_width = (JDIMENSION)biWidth;
  cinfo->image_height = (JDIMENSION)biHeight;
}