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2006.04.07 07:34 "application supplied JPEGTables", by <yourpattern@126.com>
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 not present". */
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.
yourpattern@126.com