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I am currently working on two-scan raster algorithms (euclidean distance transformations, cost surface analysis, etc.). Those algorithms require a line-wise forward raster scan (from top left to bottom right) as well as a backward/reversed raster scan (from bottom right to top left).

I am currently using GDAL to read the raster data using the C++ code provided by the tutorial.

float *pafScanline;
int   nXSize = poBand->GetXSize();
pafScanline = (float *) CPLMalloc(sizeof(float)*nXSize);
poBand->RasterIO( GF_Read, 0, 0, nXSize, 1,
                pafScanline, nXSize, 1, GDT_Float32,
                0, 0 );

The RasterIo method reads the raster as scanlines into dynamically allocated memory (provided by CPLMalloc) and it does so from left to right. Up to now I did not find any GDAL methods to read a raster scanline from right to left (eg. backwards).

My question: Is there any better method to achieve a backwards raster scan than reading the scanline from left to right and then reorder the values (which would mean that the scanline has to be processed twice)?

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  • I'm confused. The RasterIO call just fills the buffer pafScanline. You then need to execute your algorithm on the buffer, presumably by iterating over the contents. When you operate left to right, you iterate with your index running from 0 to nXSize. Why can't you just iterate from nXSize to 0 when you want to operate right to left?
    – Llaves
    Jan 20 at 4:55
  • I understand your confusion. I know that, under normal circumstances I could iterate beginning with the last element, counting down. In my case I am working within QgsNineCellFilter which uses OpenCL. The iteration is controlled by OpenCL and is performed from left to right. Thats why I have been looking for a solution on the GDAL side. I have not yet checked if OpenCL provides a method to scan the buffer backwards.
    – root676
    Jan 20 at 8:39
  • 2
    Thanks. I actually wondered for a moment if the issue had something to do with GPU calls.
    – Llaves
    Jan 21 at 14:37
  • @Llaves you were totally right! :)
    – root676
    Jan 21 at 14:38
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    Unoptimised working code is better than no working code. ;) I don't think miracles are possible and you either need to read often or cache parts.
    – Al rl
    Mar 20 at 22:49

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