2008.12.17 20:54 "[Tiff] Deleting tags from a directory", by Frank Warmerdam

2008.12.17 20:54 "[Tiff] Deleting tags from a directory", by Frank Warmerdam


I have a client who needs to be able to remove a colormap from an existing TIFF file. This reveals a problem that has vexed me a number of times before when trying to update TIFF files "in place". There does not appear to be any mechanism to delete a tag from an existing directory.

I propose, in libtiff4 to add a TIFFUnsetField() function which will remove a tag from the current directory if present. It would be implemented like this:

  * Clear the contents of the field in the internal structure.
TIFFUnsetField(TIFF* tif, uint32 tag)

     const TIFFField *fip =  TIFFFieldWithTag(tif, tag);
     TIFFDirectory* td = &tif->tif_dir;

     if( !fip )
         return 0;

     if( fip->field_bit != FIELD_CUSTOM )
         TIFFClrFieldBit(tif, fip->field_bit);
         TIFFTagValue *tv;
         int i;

         for (i = 0; i < td->td_customValueCount; i++) {

             tv = td->td_customValues + i;
             if( tv->info->field_tag == tag )

         if( i < td->td_customValueCount )
             for( ; i < td->td_customValueCount-1; i++) {
                 td->td_customValues[i] = td->td_customValues[i+1];

     tif->tif_flags |= TIFF_DIRTYDIRECT;

     return (1);

Does anyone know of any problems with this change? Is there already a way for applications to remove tags I'm not aware of? So far, I have only tested the above a wee bit. The following sample program demonstrates turning a paletted file into unpaletted form:

#include "tiffio.h"

int main()

     TIFF *hTIFF;

     hTIFF = TIFFOpen( "cm.tif", "r+" );

         printf( "Set photometric successful.\n" );
         printf( "Set photometric failed.\n" );

     if( TIFFUnsetField( hTIFF, TIFFTAG_COLORMAP ) )
         printf( "clear colormap successful.\n" );
         printf( "clear colormap failed.\n" );

     TIFFClose( hTIFF );

Best regards,

I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam

and watch the world go round - Rush    | Geospatial Programmer for Rent