root / ImageMagick / trunk / coders / gif.c

/*
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                             %
%                                                                             %
%                                                                             %
%                             GGGG  IIIII  FFFFF                              %
%                            G        I    F                                  %
%                            G  GG    I    FFF                                %
%                            G   G    I    F                                  %
%                             GGG   IIIII  F                                  %
%                                                                             %
%                                                                             %
%            Read/Write Compuserv Graphics Interchange Format.                %
%                                                                             %
%                              Software Design                                %
%                                John Cristy                                  %
%                                 July 1992                                   %
%                                                                             %
%                                                                             %
%  Copyright 1999-2008 ImageMagick Studio LLC, a non-profit organization      %
%  dedicated to making software imaging solutions freely available.           %
%                                                                             %
%  You may not use this file except in compliance with the License.  You may  %
%  obtain a copy of the License at                                            %
%                                                                             %
%    http://www.imagemagick.org/script/license.php                            %
%                                                                             %
%  Unless required by applicable law or agreed to in writing, software        %
%  distributed under the License is distributed on an "AS IS" BASIS,          %
%  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.   %
%  See the License for the specific language governing permissions and        %
%  limitations under the License.                                             %
%                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%
*/

/*
  Include declarations.
*/
#include "magick/studio.h"
#include "magick/blob.h"
#include "magick/blob-private.h"
#include "magick/color.h"
#include "magick/color-private.h"
#include "magick/colorspace.h"
#include "magick/exception.h"
#include "magick/exception-private.h"
#include "magick/image.h"
#include "magick/image-private.h"
#include "magick/list.h"
#include "magick/profile.h"
#include "magick/magick.h"
#include "magick/memory_.h"
#include "magick/monitor.h"
#include "magick/monitor-private.h"
#include "magick/option.h"
#include "magick/property.h"
#include "magick/quantize.h"
#include "magick/quantum-private.h"
#include "magick/static.h"
#include "magick/string_.h"
#include "magick/module.h"

/*
  Define declarations.
*/
#define MaximumLZWBits  12
#define MaximumLZWCode  (1UL << MaximumLZWBits)

/*
  Typdef declarations.
*/
typedef struct _LZWCodeInfo
{
  unsigned char
    buffer[280];

  unsigned long
    count,
    bit;

  MagickBooleanType
    eof;
} LZWCodeInfo;

typedef struct _LZWStack
{
  unsigned long
    *codes,
    *index,
    *top;
} LZWStack;

typedef struct _LZWInfo
{
  Image
    *image;

  LZWStack
    *stack;

  MagickBooleanType
    genesis;

  unsigned long
    data_size,
    maximum_data_value,
    clear_code,
    end_code,
    bits,
    first_code,
    last_code,
    maximum_code,
    slot,
    *table[2];

  LZWCodeInfo
    code_info;
} LZWInfo;

/*
  Forward declarations.
*/
static inline int
  GetNextLZWCode(LZWInfo *,const unsigned long);

static MagickBooleanType
  WriteGIFImage(const ImageInfo *,Image *);

static ssize_t
  ReadBlobBlock(Image *,unsigned char *);

/*
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                             %
%                                                                             %
%                                                                             %
%   D e c o d e I m a g e                                                     %
%                                                                             %
%                                                                             %
%                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  DecodeImage uncompresses an image via GIF-coding.
%
%  The format of the DecodeImage method is:
%
%      MagickBooleanType DecodeImage(Image *image,const long opacity)
%
%  A description of each parameter follows:
%
%    o image: the address of a structure of type Image.
%
%    o opacity:  The colormap index associated with the transparent color.
%
*/

static LZWInfo *RelinquishLZWInfo(LZWInfo *lzw_info)
{
  if (lzw_info->table[0] != (unsigned long *) NULL)
    lzw_info->table[0]=(unsigned long *) RelinquishMagickMemory(
      lzw_info->table[0]);
  if (lzw_info->table[1] != (unsigned long *) NULL)
    lzw_info->table[1]=(unsigned long *) RelinquishMagickMemory(
      lzw_info->table[1]);
  if (lzw_info->stack != (LZWStack *) NULL)
    {
      if (lzw_info->stack->codes != (unsigned long *) NULL)
        lzw_info->stack->codes=(unsigned long *) RelinquishMagickMemory(
          lzw_info->stack->codes);
      lzw_info->stack=(LZWStack *) RelinquishMagickMemory(lzw_info->stack);
    }
  lzw_info=(LZWInfo *) RelinquishMagickMemory(lzw_info);
  return((LZWInfo *) NULL);
}

static inline void ResetLZWInfo(LZWInfo *lzw_info)
{
  lzw_info->bits=lzw_info->data_size+1;
  lzw_info->maximum_code=1UL << lzw_info->bits;
  lzw_info->slot=lzw_info->maximum_data_value+3;
  lzw_info->genesis=MagickTrue;
}

static LZWInfo *AcquireLZWInfo(Image *image,const unsigned long data_size)
{
  LZWInfo
    *lzw_info;

  register long
    i;

  lzw_info=(LZWInfo *) AcquireMagickMemory(sizeof(*lzw_info));
  if (lzw_info == (LZWInfo *) NULL)
    return((LZWInfo *) NULL);
  (void) ResetMagickMemory(lzw_info,0,sizeof(*lzw_info));
  lzw_info->image=image;
  lzw_info->data_size=data_size;
  lzw_info->maximum_data_value=(1UL << data_size)-1;
  lzw_info->clear_code=lzw_info->maximum_data_value+1;
  lzw_info->end_code=lzw_info->maximum_data_value+2;
  lzw_info->table[0]=(unsigned long *) AcquireQuantumMemory(MaximumLZWCode,
    sizeof(*lzw_info->table));
  lzw_info->table[1]=(unsigned long *) AcquireQuantumMemory(MaximumLZWCode,
    sizeof(*lzw_info->table));
  if ((lzw_info->table[0] == (unsigned long *) NULL) ||
      (lzw_info->table[1] == (unsigned long *) NULL))
    {
      lzw_info=RelinquishLZWInfo(lzw_info);
      return((LZWInfo *) NULL);
    }
  for (i=0; i <= (long) lzw_info->maximum_data_value; i++)
  {
    lzw_info->table[0][i]=0;
    lzw_info->table[1][i]=(unsigned long) i;
  }
  ResetLZWInfo(lzw_info);
  lzw_info->code_info.buffer[0]='\0';
  lzw_info->code_info.buffer[1]='\0';
  lzw_info->code_info.count=2;
  lzw_info->code_info.bit=8*lzw_info->code_info.count;
  lzw_info->code_info.eof=MagickFalse;
  lzw_info->genesis=MagickTrue;
  lzw_info->stack=(LZWStack *) AcquireMagickMemory(sizeof(*lzw_info->stack));
  if (lzw_info->stack == (LZWStack *) NULL)
    {
      lzw_info=RelinquishLZWInfo(lzw_info);
      return((LZWInfo *) NULL);
    }
  lzw_info->stack->codes=(unsigned long *) AcquireQuantumMemory(2UL*
    MaximumLZWCode,sizeof(*lzw_info->stack->codes));
  if (lzw_info->stack->codes == (unsigned long *) NULL)
    {
      lzw_info=RelinquishLZWInfo(lzw_info);
      return((LZWInfo *) NULL);
    }
  lzw_info->stack->index=lzw_info->stack->codes;
  lzw_info->stack->top=lzw_info->stack->codes+2*MaximumLZWCode;
  return(lzw_info);
}

static inline int GetNextLZWCode(LZWInfo *lzw_info,const unsigned long bits)
{
  int
    code;

  register long
    i;

  while (((lzw_info->code_info.bit+bits) > (8*lzw_info->code_info.count)) &&
         (lzw_info->code_info.eof == MagickFalse))
  {
    ssize_t
      count;

    lzw_info->code_info.buffer[0]=lzw_info->code_info.buffer[
      lzw_info->code_info.count-2];
    lzw_info->code_info.buffer[1]=lzw_info->code_info.buffer[
      lzw_info->code_info.count-1];
    lzw_info->code_info.bit-=8*(lzw_info->code_info.count-2);
    lzw_info->code_info.count=2;
    count=ReadBlobBlock(lzw_info->image,&lzw_info->code_info.buffer[
      lzw_info->code_info.count]);
    if (count > 0)
      lzw_info->code_info.count+=count;
    else
      lzw_info->code_info.eof=MagickTrue;
  }
  if ((lzw_info->code_info.bit+bits) > (8*lzw_info->code_info.count))
    return(-1);
  code=0;
  for (i=0; i < (long) bits; i++)
  {
    code|=((lzw_info->code_info.buffer[lzw_info->code_info.bit/8] &
      (1UL << (lzw_info->code_info.bit % 8))) != 0) << i;
    lzw_info->code_info.bit++;
  }
  return(code);
}

static inline long PopLZWStack(LZWStack *stack_info)
{
  if (stack_info->index <= stack_info->codes)
    return(-1);
  stack_info->index--;
  return((long) *stack_info->index);
}

static inline void PushLZWStack(LZWStack *stack_info,const unsigned long value)
{
  if (stack_info->index >= stack_info->top)
    return;
  *stack_info->index=value;
  stack_info->index++;
}

static int ReadBlobLZWByte(LZWInfo *lzw_info)
{
  int
    code;

  ssize_t
    count;

  unsigned long
    value;

  if (lzw_info->stack->index != lzw_info->stack->codes)
    return(PopLZWStack(lzw_info->stack));
  if (lzw_info->genesis != MagickFalse)
    {
      lzw_info->genesis=MagickFalse;
      do
      {
        lzw_info->first_code=(unsigned long) GetNextLZWCode(lzw_info,
          lzw_info->bits);
        lzw_info->last_code=lzw_info->first_code;
      } while (lzw_info->first_code == lzw_info->clear_code);
      return((int) lzw_info->first_code);
    }
  code=GetNextLZWCode(lzw_info,lzw_info->bits);
  if (code < 0)
    return(code);
  if ((unsigned long) code == lzw_info->clear_code)
    {
      ResetLZWInfo(lzw_info);
      return(ReadBlobLZWByte(lzw_info));
    }
  if ((unsigned long) code == lzw_info->end_code)
    return(-1);
  if ((unsigned long) code < lzw_info->slot)
    value=(unsigned long) code;
  else
    {
      PushLZWStack(lzw_info->stack,lzw_info->first_code);
      value=lzw_info->last_code;
    }
  count=0;
  while (value > lzw_info->maximum_data_value)
  {
    if ((size_t) count > MaximumLZWCode)
      return(-1);
    count++;
    if ((size_t) value > MaximumLZWCode)
      return(-1);
    PushLZWStack(lzw_info->stack,lzw_info->table[1][value]);
    value=lzw_info->table[0][value];
  }
  lzw_info->first_code=lzw_info->table[1][value];
  PushLZWStack(lzw_info->stack,lzw_info->first_code);
  if (lzw_info->slot < MaximumLZWCode)
    {
      lzw_info->table[0][lzw_info->slot]=lzw_info->last_code;
      lzw_info->table[1][lzw_info->slot]=lzw_info->first_code;
      lzw_info->slot++;
      if ((lzw_info->slot >= lzw_info->maximum_code) &&
          (lzw_info->bits < MaximumLZWBits))
        {
          lzw_info->bits++;
          lzw_info->maximum_code=1UL << lzw_info->bits;
        }
    }
  lzw_info->last_code=(unsigned long) code;
  return(PopLZWStack(lzw_info->stack));
}

static MagickBooleanType DecodeImage(Image *image,const Quantum opacity)
{
  IndexPacket
    index;

  int
    c;

  long
    offset,
    y;

  register IndexPacket
    *indexes;

  register long
    x;

  register PixelPacket
    *q;

  LZWInfo
    *lzw_info;

  unsigned char
    data_size;

  unsigned long
    pass;

  /*
    Allocate decoder tables.
  */
  assert(image != (Image *) NULL);
  assert(image->signature == MagickSignature);
  if (image->debug != MagickFalse)
    (void) LogMagickEvent(TraceEvent,GetMagickModule(),"%s",image->filename);
  data_size=(unsigned char) ReadBlobByte(image);
  if (data_size > MaximumLZWBits)
    ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
  lzw_info=AcquireLZWInfo(image,data_size);
  if (lzw_info == (LZWInfo *) NULL)
    ThrowBinaryException(ResourceLimitError,"MemoryAllocationFailed",
      image->filename);
  pass=0;
  offset=0;
  for (y=0; y < (long) image->rows; y++)
  {
    q=GetImagePixels(image,0,offset,image->columns,1);
    if (q == (PixelPacket *) NULL)
      break;
    indexes=GetIndexes(image);
    for (x=0; x < (long) image->columns; )
    {
      c=ReadBlobLZWByte(lzw_info);
      if (c < 0)
        break;
      index=ConstrainColormapIndex(image,(unsigned long) c);
      q->red=image->colormap[(long) index].red;
      q->green=image->colormap[(long) index].green;
      q->blue=image->colormap[(long) index].blue;
      q->opacity=index == opacity ? (Quantum) TransparentOpacity :
        (Quantum) OpaqueOpacity;
      indexes[x]=index;
      x++;
      q++;
    }
    if (x < (long) image->columns)
      break;
    if (image->interlace == NoInterlace)
      offset++;
    else
      switch (pass)
      {
        case 0:
        default:
        {
          offset+=8;
          if (offset >= (long) image->rows)
            {
              pass++;
              offset=4;
            }
          break;
        }
        case 1:
        {
          offset+=8;
          if (offset >= (long) image->rows)
            {
              pass++;
              offset=2;
            }
          break;
        }
        case 2:
        {
          offset+=4;
          if (offset >= (long) image->rows)
            {
              pass++;
              offset=1;
            }
          break;
        }
        case 3:
        {
          offset+=2;
          break;
        }
      }
    if (SyncImagePixels(image) == MagickFalse)
      break;
  }
  lzw_info=RelinquishLZWInfo(lzw_info);
  if (y < (long) image->rows)
    ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
  return(MagickTrue);
}

/*
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                             %
%                                                                             %
%                                                                             %
%   E n c o d e I m a g e                                                     %
%                                                                             %
%                                                                             %
%                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  EncodeImage compresses an image via GIF-coding.
%
%  The format of the EncodeImage method is:
%
%      MagickBooleanType EncodeImage(const ImageInfo *image_info,Image *image,
%        const unsigned long data_size)
%
%  A description of each parameter follows:
%
%    o image_info: the image info.
%
%    o image: the address of a structure of type Image.
%
%    o data_size:  The number of bits in the compressed packet.
%
*/
static MagickBooleanType EncodeImage(const ImageInfo *image_info,Image *image,
  const unsigned long data_size)
{
#define MaxCode(number_bits)  ((1UL << (number_bits))-1)
#define MaxHashTable  5003
#define MaxGIFBits  12UL
#define MaxGIFTable  (1UL << MaxGIFBits)
#define GIFOutputCode(code) \
{ \
  /*  \
    Emit a code. \
  */ \
  if (bits > 0) \
    datum|=(code) << bits; \
  else \
    datum=code; \
  bits+=number_bits; \
  while (bits >= 8) \
  { \
    /*  \
      Add a character to current packet. \
    */ \
    packet[length++]=(unsigned char) (datum & 0xff); \
    if (length >= 254) \
      { \
        (void) WriteBlobByte(image,(unsigned char) length); \
        (void) WriteBlob(image,length,packet); \
        length=0; \
      } \
    datum>>=8; \
    bits-=8; \
  } \
  if (free_code > max_code)  \
    { \
      number_bits++; \
      if (number_bits == MaxGIFBits) \
        max_code=MaxGIFTable; \
      else \
        max_code=MaxCode(number_bits); \
    } \
}

  IndexPacket
    index;

  long
    displacement,
    offset,
    k,
    y;

  register const PixelPacket
    *p;

  register IndexPacket
    *indexes;

  register long
    i,
    x;

  size_t
    length;

  short
    *hash_code,
    *hash_prefix,
    waiting_code;

  unsigned char
    *packet,
    *hash_suffix;

  unsigned long
    bits,
    clear_code,
    datum,
    end_of_information_code,
    free_code,
    max_code,
    next_pixel,
    number_bits,
    pass;

  /*
    Allocate encoder tables.
  */
  assert(image != (Image *) NULL);
  packet=(unsigned char *) AcquireQuantumMemory(256,sizeof(*packet));
  hash_code=(short *) AcquireQuantumMemory(MaxHashTable,sizeof(*hash_code));
  hash_prefix=(short *) AcquireQuantumMemory(MaxHashTable,sizeof(*hash_prefix));
  hash_suffix=(unsigned char *) AcquireQuantumMemory(MaxHashTable,
    sizeof(*hash_suffix));
  if ((packet == (unsigned char *) NULL) || (hash_code == (short *) NULL) ||
      (hash_prefix == (short *) NULL) ||
      (hash_suffix == (unsigned char *) NULL))
    {
      if (packet != (unsigned char *) NULL)
        packet=(unsigned char *) RelinquishMagickMemory(packet);
      if (hash_code != (short *) NULL)
        hash_code=(short *) RelinquishMagickMemory(hash_code);
      if (hash_prefix != (short *) NULL)
        hash_prefix=(short *) RelinquishMagickMemory(hash_prefix);
      if (hash_suffix != (unsigned char *) NULL)
        hash_suffix=(unsigned char *) RelinquishMagickMemory(hash_suffix);
      return(MagickFalse);
    }
  /*
    Initialize GIF encoder.
  */
  number_bits=data_size;
  max_code=MaxCode(number_bits);
  clear_code=((short) 1UL << (data_size-1));
  end_of_information_code=clear_code+1;
  free_code=clear_code+2;
  length=0;
  datum=0;
  bits=0;
  for (i=0; i < MaxHashTable; i++)
    hash_code[i]=0;
  GIFOutputCode(clear_code);
  /*
    Encode pixels.
  */
  offset=0;
  pass=0;
  waiting_code=0;
  for (y=0; y < (long) image->rows; y++)
  {
    p=AcquireImagePixels(image,0,offset,image->columns,1,&image->exception);
    if (p == (const PixelPacket *) NULL)
      break;
    indexes=GetIndexes(image);
    if (y == 0)
      waiting_code=(short) (*indexes);
    for (x=(y == 0) ? 1 : 0; x < (long) image->columns; x++)
    {
      /*
        Probe hash table.
      */
      index=(IndexPacket) ((unsigned long) indexes[x] & 0xff);
      p++;
      k=(long) (((unsigned long) index << (MaxGIFBits-8))+waiting_code);
      if (k >= MaxHashTable)
        k-=MaxHashTable;
      next_pixel=MagickFalse;
      displacement=1;
      if (hash_code[k] > 0)
        {
          if ((hash_prefix[k] == waiting_code) &&
              (hash_suffix[k] == (unsigned char) index))
            {
              waiting_code=hash_code[k];
              continue;
            }
          if (k != 0)
            displacement=MaxHashTable-k;
          for ( ; ; )
          {
            k-=displacement;
            if (k < 0)
              k+=MaxHashTable;
            if (hash_code[k] == 0)
              break;
            if ((hash_prefix[k] == waiting_code) &&
                (hash_suffix[k] == (unsigned char) index))
              {
                waiting_code=hash_code[k];
                next_pixel=MagickTrue;
                break;
              }
          }
          if (next_pixel == MagickTrue)
            continue;
        }
      GIFOutputCode((unsigned long) waiting_code);
      if (free_code < MaxGIFTable)
        {
          hash_code[k]=(short) free_code++;
          hash_prefix[k]=waiting_code;
          hash_suffix[k]=(unsigned char) index;
        }
      else
        {
          /*
            Fill the hash table with empty entries.
          */
          for (k=0; k < MaxHashTable; k++)
            hash_code[k]=0;
          /*
            Reset compressor and issue a clear code.
          */
          free_code=clear_code+2;
          GIFOutputCode(clear_code);
          number_bits=data_size;
          max_code=MaxCode(number_bits);
        }
      waiting_code=(short) index;
    }
    if (image_info->interlace == NoInterlace)
      offset++;
    else
      switch (pass)
      {
        case 0:
        default:
        {
          offset+=8;
          if (offset >= (long) image->rows)
            {
              pass++;
              offset=4;
            }
          break;
        }
        case 1:
        {
          offset+=8;
          if (offset >= (long) image->rows)
            {
              pass++;
              offset=2;
            }
          break;
        }
        case 2:
        {
          offset+=4;
          if (offset >= (long) image->rows)
            {
              pass++;
              offset=1;
            }
          break;
        }
        case 3:
        {
          offset+=2;
          break;
        }
      }
  }
  /*
    Flush out the buffered code.
  */
  GIFOutputCode((unsigned long) waiting_code);
  GIFOutputCode(end_of_information_code);
  if (bits > 0)
    {
      /*
        Add a character to current packet.
      */
      packet[length++]=(unsigned char) (datum & 0xff);
      if (length >= 254)
        {
          (void) WriteBlobByte(image,(unsigned char) length);
          (void) WriteBlob(image,length,packet);
          length=0;
        }
    }
  /*
    Flush accumulated data.
  */
  if (length > 0)
    {
      (void) WriteBlobByte(image,(unsigned char) length);
      (void) WriteBlob(image,length,packet);
    }
  /*
    Free encoder memory.
  */
  hash_suffix=(unsigned char *) RelinquishMagickMemory(hash_suffix);
  hash_prefix=(short *) RelinquishMagickMemory(hash_prefix);
  hash_code=(short *) RelinquishMagickMemory(hash_code);
  packet=(unsigned char *) RelinquishMagickMemory(packet);
  return(MagickTrue);
}

/*
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                             %
%                                                                             %
%                                                                             %
%   I s G I F                                                                 %
%                                                                             %
%                                                                             %
%                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  IsGIF() returns MagickTrue if the image format type, identified by the
%  magick string, is GIF.
%
%  The format of the IsGIF method is:
%
%      MagickBooleanType IsGIF(const unsigned char *magick,const size_t length)
%
%  A description of each parameter follows:
%
%    o magick: This string is generally the first few bytes of an image file
%      or blob.
%
%    o length: Specifies the length of the magick string.
%
*/
static MagickBooleanType IsGIF(const unsigned char *magick,const size_t length)
{
  if (length < 4)
    return(MagickFalse);
  if (LocaleNCompare((char *) magick,"GIF8",4) == 0)
    return(MagickTrue);
  return(MagickFalse);
}

/*
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                             %
%                                                                             %
%                                                                             %
+  R e a d B l o b B l o c k                                                  %
%                                                                             %
%                                                                             %
%                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  ReadBlobBlock() reads data from the image file and returns it.  The
%  amount of data is determined by first reading a count byte.  The number
%  of bytes read is returned.
%
%  The format of the ReadBlobBlock method is:
%
%      size_t ReadBlobBlock(Image *image,unsigned char *data)
%
%  A description of each parameter follows:
%
%    o image: the image.
%
%    o data:  Specifies an area to place the information requested from
%      the file.
%
*/
static ssize_t ReadBlobBlock(Image *image,unsigned char *data)
{
  ssize_t
    count;

  unsigned char
    block_count;

  assert(image != (Image *) NULL);
  assert(image->signature == MagickSignature);
  assert(data != (unsigned char *) NULL);
  count=ReadBlob(image,1,&block_count);
  if (count != 1)
    return(0);
  return(ReadBlob(image,(size_t) block_count,data));
}

/*
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%                                                                             %
%                                                                             %
%                                                                             %
%   R e a d G I F I m a g e                                                   %
%                                                                             %
%                                                                             %
%                                                                             %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  ReadGIFImage() reads a Compuserve Graphics image file and returns it.
%  It allocates the memory necessary for the new Image structure and returns a
%  pointer to the new image.
%
%  The format of the ReadGIFImage method is:
%
%      Image *ReadGIFImage(const ImageInfo *image_info,ExceptionInfo *exception)
%
%  A description of each parameter follows:
%
%    o image_info: the image info.
%
%    o exception: return any errors or warnings in this structure.
%
*/

static inline size_t MagickMax(const size_t x,const size_t y)
{
  if (x > y)
    return(x);
  return(y);
}

static inline size_t MagickMin(const size_t x,const size_t y)
{
  if (x < y)
    return(x);
  return(y);
}

static MagickBooleanType PingGIFImage(Image *image)
{
  unsigned char
    buffer[256],
    length,
    data_size;

  assert(image != (Image *) NULL);
  assert(image->signature == MagickSignature);
  if (image->debug != MagickFalse)
    (void) LogMagickEvent(TraceEvent,GetMagickModule(),"%s",image->filename);
  if (ReadBlob(image,1,&data_size) != 1)
    ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
  if (data_size > MaximumLZWBits)
    ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
  if (ReadBlob(image,1,&length) != 1)
    ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
  while (length != 0)
  {
    if (ReadBlob(image,length,buffer) != (ssize_t) length)
      ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
    if (ReadBlob(image,1,&length) != 1)
      ThrowBinaryException(CorruptImageError,"CorruptImage",image->filename);
  }
  return(MagickTrue);
}

static Image *ReadGIFImage(const ImageInfo *image_info,ExceptionInfo *exception)
{
#define BitSet(byte,bit)  (((byte) & (bit)) == (bit))
#define LSBFirstOrder(x,y)  (((y) << 8) | (x))

  Image
    *image;

  int
    number_extensionss=0;

  long
    opacity;

  MagickBooleanType
    status;

  RectangleInfo
    page;

  register long
    i;

  register unsigned char
    *p;

  ssize_t
    count;

  unsigned char
    background,
    c,
    flag,
    *global_colormap,
    header[MaxTextExtent],
    magick[12];

  unsigned long
    delay,
    dispose,
    global_colors,
    image_count,
    iterations;

  /*
    Open image file.
  */
  assert(image_info != (const ImageInfo *) NULL);
  assert(image_info->signature == MagickSignature);
  if (image_info->debug != MagickFalse)
    (void) LogMagickEvent(TraceEvent,GetMagickModule(),"%s",
      image_info->filename);
  assert(exception != (ExceptionInfo *) NULL);
  assert(exception->signature == MagickSignature);
  image=AcquireImage(image_info);
  status=OpenBlob(image_info,image,ReadBinaryBlobMode,exception);
  if (status == MagickFalse)
    {
      image=DestroyImageList(image);
      return((Image *) NULL);
    }
  /*
    Determine if this a GIF file.
  */
  count=ReadBlob(image,6,magick);
  if ((count != 6) || ((LocaleNCompare((char *) magick,"GIF87",5) != 0) &&
      (LocaleNCompare((char *) magick,"GIF89",5) != 0)))
    ThrowReaderException(CorruptImageError,"ImproperImageHeader");
  page.width=ReadBlobLSBShort(image);
  page.height=ReadBlobLSBShort(image);
  flag=(unsigned char) ReadBlobByte(image);
  background=(unsigned char) ReadBlobByte(image);
  c=(unsigned char) ReadBlobByte(image);  /* reserved */
  global_colors=1UL << (((unsigned long) flag & 0x07)+1);
  global_colormap=(unsigned char *) AcquireQuantumMemory((size_t)
    MagickMax(global_colors,256),3UL*sizeof(*global_colormap));
  if (global_colormap == (unsigned char *) NULL)
    ThrowReaderException(ResourceLimitError,"MemoryAllocationFailed");
  if (BitSet((int) flag,0x80) != 0)
    count=ReadBlob(image,(size_t) (3*global_colors),global_colormap);
  delay=0;
  dispose=0;
  iterations=1;
  opacity=(-1);
  image_count=0;
  for ( ; ; )
  {
    count=ReadBlob(image,1,&c);
    if (count != 1)
      break;
    if (c == (unsigned