wrgif.c

这是JPEG解码、编码的源代码

      if (i < 0)
	i += HSIZE;
      if (dinfo->hash_code[i] == 0)
	break;			/* hit empty slot */
      if (dinfo->hash_value[i] == probe_value) {
	dinfo->waiting_code = dinfo->hash_code[i];
	return;
      }
    }
  }

  /* here when hashtable[i] is an empty slot; desired symbol not in table */
  output(dinfo, dinfo->waiting_code);
  if (dinfo->free_code < LZW_TABLE_SIZE) {
    dinfo->hash_code[i] = dinfo->free_code++; /* add symbol to hashtable */
    dinfo->hash_value[i] = probe_value;
  } else
    clear_block(dinfo);
  dinfo->waiting_code = c;
}


LOCAL(void)
compress_term (gif_dest_ptr dinfo)
/* Clean up at end */
{
  /* Flush out the buffered code */
  if (! dinfo->first_byte)
    output(dinfo, dinfo->waiting_code);
  /* Send an EOF code */
  output(dinfo, dinfo->EOFCode);
  /* Flush the bit-packing buffer */
  if (dinfo->cur_bits > 0) {
    CHAR_OUT(dinfo, dinfo->cur_accum & 0xFF);
  }
  /* Flush the packet buffer */
  flush_packet(dinfo);
}


/* GIF header construction */


LOCAL(void)
put_word (gif_dest_ptr dinfo, unsigned int w)
/* Emit a 16-bit word, LSB first */
{
  putc(w & 0xFF, dinfo->pub.output_file);
  putc((w >> 8) & 0xFF, dinfo->pub.output_file);
}


LOCAL(void)
put_3bytes (gif_dest_ptr dinfo, int val)
/* Emit 3 copies of same byte value --- handy subr for colormap construction */
{
  putc(val, dinfo->pub.output_file);
  putc(val, dinfo->pub.output_file);
  putc(val, dinfo->pub.output_file);
}


LOCAL(void)
emit_header (gif_dest_ptr dinfo, int num_colors, JSAMPARRAY colormap)
/* Output the GIF file header, including color map */
/* If colormap==NULL, synthesize a gray-scale colormap */
{
  int BitsPerPixel, ColorMapSize, InitCodeSize, FlagByte;
  int cshift = dinfo->cinfo->data_precision - 8;
  int i;

  if (num_colors > 256)
    ERREXIT1(dinfo->cinfo, JERR_TOO_MANY_COLORS, num_colors);
  /* Compute bits/pixel and related values */
  BitsPerPixel = 1;
  while (num_colors > (1 << BitsPerPixel))
    BitsPerPixel++;
  ColorMapSize = 1 << BitsPerPixel;
  if (BitsPerPixel <= 1)
    InitCodeSize = 2;
  else
    InitCodeSize = BitsPerPixel;
  /*
   * Write the GIF header.
   * Note that we generate a plain GIF87 header for maximum compatibility.
   */
  putc('G', dinfo->pub.output_file);
  putc('I', dinfo->pub.output_file);
  putc('F', dinfo->pub.output_file);
  putc('8', dinfo->pub.output_file);
  putc('7', dinfo->pub.output_file);
  putc('a', dinfo->pub.output_file);
  /* Write the Logical Screen Descriptor */
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_width);
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_height);
  FlagByte = 0x80;		/* Yes, there is a global color table */
  FlagByte |= (BitsPerPixel-1) << 4; /* color resolution */
  FlagByte |= (BitsPerPixel-1);	/* size of global color table */
  putc(FlagByte, dinfo->pub.output_file);
  putc(0, dinfo->pub.output_file); /* Background color index */
  putc(0, dinfo->pub.output_file); /* Reserved (aspect ratio in GIF89) */
  /* Write the Global Color Map */
  /* If the color map is more than 8 bits precision, */
  /* we reduce it to 8 bits by shifting */
  for (i=0; i < ColorMapSize; i++) {
    if (i < num_colors) {
      if (colormap != NULL) {
	if (dinfo->cinfo->out_color_space == JCS_RGB) {
	  /* Normal case: RGB color map */
	  putc(GETJSAMPLE(colormap[0][i]) >> cshift, dinfo->pub.output_file);
	  putc(GETJSAMPLE(colormap[1][i]) >> cshift, dinfo->pub.output_file);
	  putc(GETJSAMPLE(colormap[2][i]) >> cshift, dinfo->pub.output_file);
	} else {
	  /* Grayscale "color map": possible if quantizing grayscale image */
	  put_3bytes(dinfo, GETJSAMPLE(colormap[0][i]) >> cshift);
	}
      } else {
	/* Create a gray-scale map of num_colors values, range 0..255 */
	put_3bytes(dinfo, (i * 255 + (num_colors-1)/2) / (num_colors-1));
      }
    } else {
      /* fill out the map to a power of 2 */
      put_3bytes(dinfo, 0);
    }
  }
  /* Write image separator and Image Descriptor */
  putc(',', dinfo->pub.output_file); /* separator */
  put_word(dinfo, 0);		/* left/top offset */
  put_word(dinfo, 0);
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_width); /* image size */
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_height);
  /* flag byte: not interlaced, no local color map */
  putc(0x00, dinfo->pub.output_file);
  /* Write Initial Code Size byte */
  putc(InitCodeSize, dinfo->pub.output_file);

  /* Initialize for LZW compression of image data */
  compress_init(dinfo, InitCodeSize+1);
}


/*
 * Startup: write the file header.
 */

METHODDEF(void)
start_output_gif (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;

  if (cinfo->quantize_colors)
    emit_header(dest, cinfo->actual_number_of_colors, cinfo->colormap);
  else
    emit_header(dest, 256, (JSAMPARRAY) NULL);
}


/*
 * Write some pixel data.
 * In this module rows_supplied will always be 1.
 */

METHODDEF(void)
put_pixel_rows (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo,
		JDIMENSION rows_supplied)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;
  register JSAMPROW ptr;
  register JDIMENSION col;

  ptr = dest->pub.buffer[0];
  for (col = cinfo->output_width; col > 0; col--) {
    compress_byte(dest, GETJSAMPLE(*ptr++));
  }
}


/*
 * Finish up at the end of the file.
 */

METHODDEF(void)
finish_output_gif (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;

  /* Flush LZW mechanism */
  compress_term(dest);
  /* Write a zero-length data block to end the series */
  putc(0, dest->pub.output_file);
  /* Write the GIF terminator mark */
  putc(';', dest->pub.output_file);
  /* Make sure we wrote the output file OK */
  fflush(dest->pub.output_file);
  if (ferror(dest->pub.output_file))
    ERREXIT(cinfo, JERR_FILE_WRITE);
}


/*
 * The module selection routine for GIF format output.
 */

GLOBAL(djpeg_dest_ptr)
jinit_write_gif (j_decompress_ptr cinfo)
{
  gif_dest_ptr dest;

  /* Create module interface object, fill in method pointers */
  dest = (gif_dest_ptr)
      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
				  SIZEOF(gif_dest_struct));
  dest->cinfo = cinfo;		/* make back link for subroutines */
  dest->pub.start_output = start_output_gif;
  dest->pub.put_pixel_rows = put_pixel_rows;
  dest->pub.finish_output = finish_output_gif;

  if (cinfo->out_color_space != JCS_GRAYSCALE &&
      cinfo->out_color_space != JCS_RGB)
    ERREXIT(cinfo, JERR_GIF_COLORSPACE);

  /* Force quantization if color or if > 8 bits input */
  if (cinfo->out_color_space != JCS_GRAYSCALE || cinfo->data_precision > 8) {
    /* Force quantization to at most 256 colors */
    cinfo->quantize_colors = TRUE;
    if (cinfo->desired_number_of_colors > 256)
      cinfo->desired_number_of_colors = 256;
  }

  /* Calculate output image dimensions so we can allocate space */
  jpeg_calc_output_dimensions(cinfo);

  if (cinfo->output_components != 1) /* safety check: just one component? */
    ERREXIT(cinfo, JERR_GIF_BUG);

  /* Create decompressor output buffer. */
  dest->pub.buffer = (*cinfo->mem->alloc_sarray)
    ((j_common_ptr) cinfo, JPOOL_IMAGE, cinfo->output_width, (JDIMENSION) 1);
  dest->pub.buffer_height = 1;

  /* Allocate space for hash table */
  dest->hash_code = (code_int *)
    (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
				HSIZE * SIZEOF(code_int));
  dest->hash_value = (hash_entry FAR *)
    (*cinfo->mem->alloc_large) ((j_common_ptr) cinfo, JPOOL_IMAGE,
				HSIZE * SIZEOF(hash_entry));

  return (djpeg_dest_ptr) dest;
}

#endif /* GIF_SUPPORTED */