Support paletted and multi-band nodata values in PNG file creation

[hannah.valbonesi@…]

The following is an excerpt from my email correspondence with Frank about nodata value support in the PNG CreateCopy routine:


I see that the PNG driver supports conventional nodata values in
CreateCopy() for one band greyscale files.  For RGB files it only supports the "NODATA_VALUES" metadata item which is used when all the bands have to match a particular value for the pixel to be
considered nodata.   The code looks like this:

/* -------------------------------------------------------------------- */
/*      Try to handle nodata values as a tRNS block (note for           */
/*      paletted images, we save the effect to apply as part of         */
/*      palette).                                                       */
/* -------------------------------------------------------------------- */
     int		bHaveNoData = FALSE;
     double	dfNoDataValue = -1;
     png_color_16 sTRNSColor;

     dfNoDataValue = poSrcDS->GetRasterBand(1)->GetNoDataValue( &bHaveNoData );

     if( (nColorType == PNG_COLOR_TYPE_GRAY )
         && dfNoDataValue > 0 && dfNoDataValue < 65536 )
     {
         sTRNSColor.gray = (png_uint_16) dfNoDataValue;
         png_set_tRNS( hPNG, psPNGInfo, NULL, 0, &sTRNSColor );
     }

     // RGB case.
     if( nColorType == PNG_COLOR_TYPE_RGB
         && poSrcDS->GetMetadataItem( "NODATA_VALUES" ) != NULL )
     {
         char **papszValues = CSLTokenizeString(
             poSrcDS->GetMetadataItem( "NODATA_VALUES" ) );

         if( CSLCount(papszValues) >= 3 )
         {
             sTRNSColor.red   = (png_uint_16) atoi(papszValues[0]);
             sTRNSColor.green = (png_uint_16) atoi(papszValues[1]);
             sTRNSColor.blue  = (png_uint_16) atoi(papszValues[2]);
             png_set_tRNS( hPNG, psPNGInfo, NULL, 0, &sTRNSColor );
         }

         CSLDestroy( papszValues );
     }

It seems to me that at the very least we the above code should be upgrade to support nodata for paletted images as well.  I also think that if NODATA_VALUES does not exist,we should consider using the regular nodata values for RGB images.

[hannah.valbonesi@…]

Submitted the fix for this.