[gdal-dev] The use of GDALColorTable

[Frank Warmerdam]

Belaid MOA wrote:
> Greetings everyone,
>   I noticed that some of the GTiff images I am working with are single 
> bands and have a ColorInterp=Palette (they are colored when viewed in 
> tiff viewer). So, I assume they are using a Color Table to achieve that. 
> Am I right? Where can I find some tutorials or C/C++ examples using 
> GDALColorTable to associate a color table with a single band image?

Belaid,


The following is an example of creating a color table:

             // Allocate memory for colour table and read it
             if ( poDS->sInfoHeader.iClrUsed )
                 poDS->nColorTableSize = poDS->sInfoHeader.iClrUsed;
             else
                 poDS->nColorTableSize = 1 << poDS->sInfoHeader.iBitCount;
             poDS->pabyColorTable =
                 (GByte *)VSIMalloc2( poDS->nColorElems, poDS->nColorTableSize );
             if (poDS->pabyColorTable == NULL)
             {
                 CPLError(CE_Failure, CPLE_OutOfMemory, "Color palette will be 
ignored");
                 poDS->nColorTableSize = 0;
                 break;
             }

             VSIFSeekL( poDS->fp, BFH_SIZE + poDS->sInfoHeader.iSize, SEEK_SET );
             VSIFReadL( poDS->pabyColorTable, poDS->nColorElems,
                       poDS->nColorTableSize, poDS->fp );

             GDALColorEntry oEntry;
             poDS->poColorTable = new GDALColorTable();
             for( i = 0; i < poDS->nColorTableSize; i++ )
             {
                 oEntry.c1 = poDS->pabyColorTable[i * poDS->nColorElems + 2]; 
// Red
                 oEntry.c2 = poDS->pabyColorTable[i * poDS->nColorElems + 1]; 
// Green
                 oEntry.c3 = poDS->pabyColorTable[i * poDS->nColorElems]; 
// Blue
                 oEntry.c4 = 255;

                 poDS->poColorTable->SetColorEntry( i, &oEntry );
             }

The color table can be associated with a band using:

   GDALSetColorTable(GDALRasterBandH,GDALColorTableH);

In the case of creating a GeoTIFF file, you may need to
pass the creation option PHOTOMETRIC=PALETTE to ensure the
created file has a palette.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent