2006.04.07 07:34 "[Tiff] LibTiff Suggestion, Need help to support application supplied JPEGTables.", by Kelvin Zhong

Hi, All
  Currently, whether in "tif_ojpeg.c" or "tif_jpeg.c" we can a comment like this:

"  /* We do not support application-supplied JPEG tables, so mark the field<BR>          "not present".<BR>       */</DIV>

<DIV>"

I don't know why this was not implemented in the very first time.

and after i go through the code that related to this area (support application supplied JPEGTables).

It seems not so complicated, for IJG in TIFF can perform application-supplied JPEG tables easily.

for example, like cjpeg.c in IJG does, we just use function jpeg_add_quant_table to do this.

and in LibTIFF side ,we can define some pseudo tags along with(like) JPEGTABLESMODE_QUANT for specified different TIFFTAG_JPEGTABLESMODE(that's application-supplied), for i see some the code in libtiff like this:

 /* Create a JPEGTables field if appropriate */
 if (sp->jpegtablesmode & (JPEGTABLESMODE_QUANT|JPEGTABLESMODE_HUFF)) {
  if (!prepare_JPEGTables(tif))
   return (0);
  /* Mark the field present */
  /* Can't use TIFFSetField since BEENWRITING is already set! */
  TIFFSetFieldBit(tif, FIELD_JPEGTABLES);
  tif->tif_flags |= TIFF_DIRTYDIRECT;
 } else {
  /* We do not support application-supplied JPEGTables, */
  /* so mark the field not present */
  TIFFClrFieldBit(tif, FIELD_JPEGTABLES);
 }

It's quit useful if the use can specified it's own quantization tables for JPEG compression.

and I would like to propose the addition of the capablity of supporting application supplied JPEGTables. this would make the control of image quality of JPEG compress more powerful. currently, the only way to control the jpeg quality is TIFFSetField(tif, TIFFTAG_JPEGQUALITY, nQuality); and if we supply a way like:

TIFFSedField(tif,TIFFTAG_JPEGQUALITYTABLE,nTableType,nTableData,nTableDataSize);

or something else.

I think it's useful.

anyone, who have simliar ideas pls share with me.

and I am curiously to know why this feature was not implemented until these days,

Do i neglect some difficult problems?

anyone who have abundance knowledge in this field,It's great appreciated that you can post your comments here.

thanks ,all.

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