[Gdal-dev] featureDataset->RasterIO efficient pixel value reading question

[Andrew Finley]

Hi Frank,
Some time ago, you helped me out with some code to speed up multi-band
image pixel value access.  I was using rasterIO to read one pixel at a
time.  You suggested the code below to grab a larger block (xsize*bands):

  // Make sure that lineBuf holds one whole line of data.
  float *lineBuf;
  int  xsize = featureDataset->GetRasterXSize();
  int  ysize = featureDataset->GetRasterYSize();
  int  bandCount = featureDataset->GetRasterCount();

  lineBuf = (float *) CPLMalloc(xsize * bandCount);

  //every row loop
  for (int row = 0; row < featureDataset->GetRasterYSize(); row++){
      featureDataset->RasterIO( GF_Read, 0, row, xsize, 1,
                                lineBuf, xsize, 1, GDT_Float32,
                                bandCount, NULL,
                                4 * bandCount, 0, 4 );
      // every pixel in row.
      for (int col = 0; col < featureDataset->GetRasterXSize(); col++){
          float *targetBandVec = lineBuf + col;
          ... perform classification ..
      }
  }



I'm having some trouble understanding the parameters that go into
featureDataset rasterIO and how it packs the data into the row lineBuf. 
What are the values of 4 you put in the RasterIO above?  Could you take me
one step further in the above example, so I can see how the pixel bands are
layed out in the lineBuf.  I took a shot below:

 // Make sure that lineBuf holds one whole line of data.
  float *lineBuf;
  int xsize = featureDataset->GetRasterXSize();
  int ysize = featureDataset->GetRasterYSize();
  int bandCount = featureDataset->GetRasterCount();
  lineBuf = (float *) CPLMalloc(sizeof(float)*xsize*bandCount);

  //every row loop
  for (int row = 0; row < featureDataset->GetRasterYSize(); row++){
    featureDataset->RasterIO( GF_Read, 0, row, xsize, 1, lineBuf, xsize, 1,
GDT_Float32, bandCount, NULL, 4 * bandCount, 0, 4 );
    
    //every pixel in row.
    for (int col = 0; col < featureDataset->GetRasterXSize(); col++){
      float *targetBandVec = lineBuf + col;
      
      //every band value in pixel (i.e., each cout row is a pixel and row
elements are specific band values in that pixel)
      //I know this part is wrong.

 	for(int band = 0; band < bandCount; band++)
	  cout << targetBandVec[band] <<" ";
	cout << endl;
    } 
  }


As always thanks for your time, I really appreciate your help.
-andy