[Gdal-dev] GDAL in delphi .. reading pixel values (Esri grids)

[jk nz]

Hi,

Thanks thats a start at least :) .. why does the code have GetRasterBand 
repeated twice in a row, or is that a typo?

    for (int layer = 1; layer <= poDataset->GetRasterCount(); layer++){
      poBand = poDataset->GetRasterBand( layer );

      poBand = poDataset->GetRasterBand( layer );
      poBand->RasterIO( GF_Read, xPixel, yLine, 1, 1, pafScanline, 1, 1, 
GDT_Float32, 0, 0 );

And what is the point of the transform?  I thought I would be able to do 
RasterIO and that would give me one line of the grid, and I would then just 
read that line one element at a time .. or is it not that simple?  I dont 
really know too much about GIS stuff..  I have an application that works 
fine with tab delimited ascii grids, but thats not going to be useful in the 
real world :)

Regards

JK.



>From: Andrew Finley <afinley at gis.umn.edu>
>Reply-To: gdal-dev at remotesensing.org
>To: gdal-dev at remotesensing.org
>Subject: Re: [Gdal-dev] GDAL in delphi .. reading pixel values (Esri grids)
>Date: 26 Apr 2004 08:42:50 -0500
>
>Hi JK,
>I think there is a similar chunk of code in the list archives, but here
>it is again.  Attached are two files, getPixelValueGdal.cpp and
>parameterfile.  The parameterfile describes the image location and the
>coordinate file (i.e., point xy locations).  This is set up for ERDAS
>images, I think you will only have to change the driver for it to read
>grids.  Hope this helps.  Let me know if you have any questions.
>-Andy
>
>
>On Mon, 2004-04-26 at 00:48, jk nz wrote:
> > Has anyone tried using GDAL from the Borland Delphi language?
> >
> > I have been mostly sucessful in doing so, but having problems reading 
>pixel
> > values from an esri grid..
> >
> > Does anyone have sample code of reading pixels? C is ok .. I have seen
> > sample of reading blocks but that wasnt quite what I wanted ...
> >
> > I do not know how to extract the pixel values as I dont know how they 
>are
> > laid out in memory .. is it a 2d grid, or a 1d grid?
> >
> > eg is it laid out in memory as
> >
> > row1 [px1][px2][px3][...]
> > row2 (possible some memory gap in between rows) [px10][px11][...]
> >
> > or as row1 [px1][px2][...] (row2, no gap in memory, next pixel is at 
>address
> > pxN+ sizeof(px)) [px10][...]
> >
> > It appears I have to use pointer arithmetic to index the block, ie 
>pixelN :=
> > block[index] works fine if it is a 1d grid in memory, but its not so 
>easy if
> > its a 2d grid, as the operator block[x,y] is not valid in delphi
> >
> > So I'm trying to figure out what happens at the end of a row? is the 
>first
> > element of the next row located in the next memory address or ...?
> >
> > Any help appreciated
> >
> > Regards
> >
> > JK.
> >
> >
> > Here is my code: - everything appears to work, apart from reading 
>individual
> > pixel values.
> >
> >             for i := 0 to nYBlocks - 1 do // i = Y block index
> >               for j := 0 to nYBlocks - 1 do // j = X block index
> >               begin
> >                 GDALReadBlock(RasterBand, j, i, pafScanline);
> >
> >                 // compute which part of the block is valid for partial
> > blocks
> >                 if(i * nBlockYSize > nY) then
> >                   nYValid := nY - i * nBlockYSize
> >                 else
> >                   nYValid := nBlockYSize;
> >
> >                 if(j * nBlockXSize > nX) then
> >                   nXValid := nX - j * nBlockXSize
> >                 else
> >                   nXValid := nBlockXSize;
> >
> >                 doubleptr := @pafScanline;
> >
> >                 for k := 0 to nYValid - 1 do // k = Y cell index
> >                   for l := 0 to nXValid - 1 do // l = X cell index
> >                   begin
> >                     inc(doubleptr);
> >                     cell := doubleptr^;
> >                     // debugging stuff, did we get an ok value or not?
> >                     if cell > 0 then
> >                       ShowMessage(FloatToStr(cell));
> >                   end; // for l, for k
> >               end; // for j, for i
> >
> > _________________________________________________________________
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> >
> > _______________________________________________
> > Gdal-dev mailing list
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>--
>Andrew Finley, Research Fellow
>Department of Forest Resources
>College of Natural Resources
>University of Minnesota
>305 Green Hall
>1530 Cleveland Avenue N.
>St. Paul, MN 55108
>
>Ph 612-624-1714 office
>www.cnr.umn.edu/FR/people/facstaff/finley/index.html
><< getPixelValueGdal.cpp >>
><< parameterfile >>

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