[gdal-dev] Dynamically displaying the Overviews

[HariPrasad]

Iam writing a custom GDAL driver (RBD format). I was able to display the base
image (overview no = -1) but for a big image(5 GB) it takes huge amount of
time. 
So i was displaying its overview (ex; image has 3 overviews and i always
displays the last overview). In this case ArcGIS 10 displays image fastly
since its a overview. 

But i require help on dynamically displaying the overviews (i.e 0,1,2 or 3)
when the overview image is Zoomed in/out. 

IReadBlock() gets invoked only for the first time when displaying the
overview image. 
Here is the code written in IReadBlock() 

CPLErr RBDRasterBand::IReadBlock( int nBlockXOff, int nBlockYOff, void *
pImage ) 
{ 
        RBDDataset *poGDS = (RBDDataset *) poDS; 
        long nBand = 0, nOVNumber = 0; 
        unsigned long nMinX = 0, nMaxX = 0, nMinY = 0, nMaxY = 0; 
        long nTileSizeX = 0, nTileSizeY = 0; 
        long nBandFlag; 
        long nBufferSize; 
        ICTM *pICTM; 

        pICTM = poGDS->pziiFile->pICTM; 
        nOVNumber = poGDS ->pziiFile ->nNumberOfOVs - 1;  // if it has
overviews 0,1,2 it dispalys 2 Ov. 
        nBand = this->nBand; 

        if(-1 == nOVNumber) 
        { 
                nTileSizeX = poGDS->pziiFile->nXTileSize; 
                nTileSizeY = poGDS->pziiFile->nYTileSize; 
        } 
        else 
        { 
                nTileSizeX =
poGDS->pziiFile->pzii_LayerOVInfo[nOVNumber]->nXTileSize; 
                nTileSizeY =
poGDS->pziiFile->pzii_LayerOVInfo[nOVNumber]->nYTileSize; 
        } 

        //Get the RowMin, RowMax, ColMin and ColMax values 
        nMinX = nBlockXOff * nTileSizeX; 
        nMinY = nBlockYOff * nTileSizeY; 
        nMaxX = nMinX + nTileSizeX-1;//Zero Index 
        nMaxY = nMinY + nTileSizeY-1;//Zero Index 

        //Set the band flag 
        if(nBand == 1) 
                nBandFlag = RED_FLAG; 
        else if(nBand == 2) 
                nBandFlag = GREEN_FLAG; 
        else if(nBand == 3) 
                nBandFlag = BLUE_FLAG; 

        //Calculate buffer size 
        nBufferSize = (nMaxX + 1 - nMinX) * (nMaxY + 1 - nMinY) *
poGDS->pziiFile->nNumberOfBytesPerBand; 

        ////Call RenderRequestByOV on CTM object - it is our 3 part method
which fills the image buffer. 
       
pICTM->RenderRequestByOV(nOVNumber,nBandFlag,nMinX,nMinY,nMaxX,nMaxY,&nBufferSize,(unsigned
char*)pImage,FALSE,0); 
        return CE_None ; 



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