start_input_tga(j_compress_ptr cinfo, cjpeg_source_ptr sinfo)
{
  tga_source_ptr source = (tga_source_ptr)sinfo;
  U_CHAR targaheader[18];
  int idlen, cmaptype, subtype, flags, interlace_type, components;
  unsigned int width, height, maplen;
  boolean is_bottom_up;

#define GET_2B(offset)  ((unsigned int)UCH(targaheader[offset]) + \
                         (((unsigned int)UCH(targaheader[offset + 1])) << 8))

  if (!ReadOK(source->pub.input_file, targaheader, 18))
    ERREXIT(cinfo, JERR_INPUT_EOF);

  /* Pretend "15-bit" pixels are 16-bit --- we ignore attribute bit anyway */
  if (targaheader[16] == 15)
    targaheader[16] = 16;

  idlen = UCH(targaheader[0]);
  cmaptype = UCH(targaheader[1]);
  subtype = UCH(targaheader[2]);
  maplen = GET_2B(5);
  width = GET_2B(12);
  height = GET_2B(14);
  source->pixel_size = UCH(targaheader[16]) >> 3;
  flags = UCH(targaheader[17]); /* Image Descriptor byte */

  is_bottom_up = ((flags & 0x20) == 0); /* bit 5 set => top-down */
  interlace_type = flags >> 6;  /* bits 6/7 are interlace code */

  if (cmaptype > 1 ||           /* cmaptype must be 0 or 1 */
      source->pixel_size < 1 || source->pixel_size > 4 ||
      (UCH(targaheader[16]) & 7) != 0 || /* bits/pixel must be multiple of 8 */
      interlace_type != 0 ||      /* currently don't allow interlaced image */
      width == 0 || height == 0)  /* image width/height must be non-zero */
    ERREXIT(cinfo, JERR_TGA_BADPARMS);

  if (subtype > 8) {
    /* It's an RLE-coded file */
    source->read_pixel = read_rle_pixel;
    source->block_count = source->dup_pixel_count = 0;
    subtype -= 8;
  } else {
    /* Non-RLE file */
    source->read_pixel = read_non_rle_pixel;
  }

  /* Now should have subtype 1, 2, or 3 */
  components = 3;               /* until proven different */
  cinfo->in_color_space = JCS_RGB;

  switch (subtype) {
  case 1:                       /* Colormapped image */
    if (source->pixel_size == 1 && cmaptype == 1)
      source->get_pixel_rows = get_8bit_row;
    else
      ERREXIT(cinfo, JERR_TGA_BADPARMS);
    TRACEMS2(cinfo, 1, JTRC_TGA_MAPPED, width, height);
    break;
  case 2:                       /* RGB image */
    switch (source->pixel_size) {
    case 2:
      source->get_pixel_rows = get_16bit_row;
      break;
    case 3:
      source->get_pixel_rows = get_24bit_row;
      break;
    case 4:
      source->get_pixel_rows = get_32bit_row;
      break;
    default:
      ERREXIT(cinfo, JERR_TGA_BADPARMS);
      break;
    }
    TRACEMS2(cinfo, 1, JTRC_TGA, width, height);
    break;
  case 3:                       /* Grayscale image */
    components = 1;
    cinfo->in_color_space = JCS_GRAYSCALE;
    if (source->pixel_size == 1)
      source->get_pixel_rows = get_8bit_gray_row;
    else
      ERREXIT(cinfo, JERR_TGA_BADPARMS);
    TRACEMS2(cinfo, 1, JTRC_TGA_GRAY, width, height);
    break;
  default:
    ERREXIT(cinfo, JERR_TGA_BADPARMS);
    break;
  }

  if (is_bottom_up) {
    /* Create a virtual array to buffer the upside-down image. */
    source->whole_image = (*cinfo->mem->request_virt_sarray)
      ((j_common_ptr)cinfo, JPOOL_IMAGE, FALSE,
       (JDIMENSION)width * components, (JDIMENSION)height, (JDIMENSION)1);
    if (cinfo->progress != NULL) {
      cd_progress_ptr progress = (cd_progress_ptr)cinfo->progress;
      progress->total_extra_passes++; /* count file input as separate pass */
    }
    /* source->pub.buffer will point to the virtual array. */
    source->pub.buffer_height = 1; /* in case anyone looks at it */
    source->pub.get_pixel_rows = preload_image;
  } else {
    /* Don't need a virtual array, but do need a one-row input buffer. */
    source->whole_image = NULL;
    source->pub.buffer = (*cinfo->mem->alloc_sarray)
      ((j_common_ptr)cinfo, JPOOL_IMAGE,
       (JDIMENSION)width * components, (JDIMENSION)1);
    source->pub.buffer_height = 1;
    source->pub.get_pixel_rows = source->get_pixel_rows;
  }

  while (idlen--)               /* Throw away ID field */
    (void)read_byte(source);

  if (maplen > 0) {
    if (maplen > 256 || GET_2B(3) != 0)
      ERREXIT(cinfo, JERR_TGA_BADCMAP);
    /* Allocate space to store the colormap */
    source->colormap = (*cinfo->mem->alloc_sarray)
      ((j_common_ptr)cinfo, JPOOL_IMAGE, (JDIMENSION)maplen, (JDIMENSION)3);
    /* and read it from the file */
    read_colormap(source, (int)maplen, UCH(targaheader[7]));
  } else {
    if (cmaptype)               /* but you promised a cmap! */
      ERREXIT(cinfo, JERR_TGA_BADPARMS);
    source->colormap = NULL;
  }

  cinfo->input_components = components;
  cinfo->data_precision = 8;
  cinfo->image_width = width;
  cinfo->image_height = height;
}