Folks,
I am trying to understand the relationships between some of the LZW
parameters found in ImageMagick's coders/gif.c, so that I can alter
them and still produce a valid LZW output stream e.g. GIF image for an
embedded app. If I want my string table to be no larger than 256, then
my max code word must be less than 2^8. TIFF uncompressed data, for
example, is an index into a 256 3-tuple (RGB) color palette, meaning 8
bits worth of index.
ImageMagick's coders/gif.c lists function EncodeImage, which performs
the LZW compression. Typically the function is invoked with data to be
converted, and the output encoded in the second arg, Image *image. The
third arg contains is set to value of 9 is the data size for typical
input data.
The MaxHashTable is set to 5003, which appears and works, but I don't
understand why? If the MaxGIFBits size is 12-bits, then that implies
that the string 2^12 = 4096. I want to set MaxGIFBits to 9, 10, and 11
bits, respectively and set MaxHashTable to a corresponding size value
that works.
Any hints?
Thanks,
O. Usifer
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% %
% %
% %
% 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.
%
%
*/
#define MaxCode(number_bits) ((1UL << (number_bits))-1)
#define MaxHashTable 5003
#define MaxGIFBits 12UL
#define MaxGIFTable (1UL << MaxGIFBits)
...
static MagickBooleanType EncodeImage(const ImageInfo *image_info,Image *image,
const unsigned long data_size)
{
...
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=indexes[x] & 0xff;
p++;
k=(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,
&bits, &datum, packet, &length, image, free_code,
&max_code, &number_bits);
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,
&bits, &datum, packet, &length, image, free_code, &max_code,
&number_bits);
number_bits=data_size;
max_code=MaxCode(number_bits);
}
waiting_code=(short) index;
}
...
}
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