[gdal-dev] C# bindings for (fast) reproject, resample, and send to DB

[mladen-g at distributel.net]

  Hello,

Can you be more specific about what steps you are suggesting I take,  
and how that would improve performance?
Are you saying that gdalwarp-ing to gpkg is more performant than  
writing to VRT?

I looked at the geopackage docs and it seems that all results are  
encoded as ARGB, so in the case where I do not have Float32 rasters, I  
think I would have some inefficiency.  Also I guess I would have to  
specify PNG as the output, since I would have information loss with  
JPG?  In general I don't understand it enough to try, it would help if  
you could expand a bit.

What I have tried so far:

1)  Retrieve the dataset using "Gdal.wrapper_GDALWarpDestName(...)"  
(which works correctly):

Dataset ds = Gdal.wrapper_GDALWarpDestName(memFilename, 1, dss, new  
GDALWarpAppOptions(options), null, null);

2)  Retrieve the projection, GeoTransform, and GCPs (GCPs were null)

string projection = ds.GetProjection();
...
double[] geoTransform = new double[6];
ds.GetGeoTransform(resultGeoTransform);
...
GCP[] resGcps = ds.GetGCPs();

3)  Retrieve the width and height of band 1 of "ds"

Band band = ds.GetRasterBand(1);
int xSize = band.XSize;
int ySize = band.YSize;

4)  Create a new dataset with 1 band of the same width & height as  
retrieved in #3 (x size and y size were both 256, I just entered the  
literals here).

Dataset newData = Gdal.GetDriverByName("VRT").Create(memFilename, 256,  
256, 1, DataType.GDT_Float32, null);

5)  Set the projection and Geotransform as retrieved in #2, which seem  
to be the only two settable attributes (except GCPs which were null in  
the original)

newData.SetProjection(projection);
                    
var newGeoTransform = new double[6];
newGeoTransform[0] =geoTransform[0];
newGeoTransform[1] = geoTransform[1];
newGeoTransform[3] = geoTransform[3];
newGeoTransform[5] = geoTransform[5];
newData.SetGeoTransform(newGeoTransform);

6)  Use this dataset as the input to Gdal.wrapper_GDALWarpDestDS(...):

int status = Gdal.wrapper_GDALWarpDestDS(newData, 1, dss, new  
GDALWarpAppOptions(options), null, null);

This gave me a result of all null values :/

Not sure where I am going wrong?

Quoting Geospatial Information Technology Solutions  
<geospatialsolutions at gmail.com>:

> Why not build an OGC gpkg GEOPACKAGE it supports 4326 tiles        
> you can extract the tile_data blob if you need to
>         
>
>      On Thu, Oct 10, 2019, 5:51 PM <mladen-g at distributel.net> wrote:
>
>> Hello,
>>
>> I've been using GDAL for a while, but I am still a newbie to the API.   
>> I see there are some experts on this group, so apologies if I'm asking 
>> something simple (my searches did not reveal the answers I was looking 
>> for).
>>
>> Here is my problem:
>>
>> I need to take an input raster, split into areas of various sizes, 
>> resample each area into a 256x256 tile, reproject to EPSG:4326 and 
>> send the resulting pixels to a DB.
>>
>> GDAL is perfect for this, but I am using the C# bindings and I'm not 
>> sure what is the most performant way to do it.
>>
>> I used the following two references to create something that works, 
>> but I am wondering if it can be improved:
>> https://gis.stackexchange.com/questions/297831/using-gdalwarp-with-c-bindings
>> https://lists.osgeo.org/pipermail/gdal-dev/2017-February/046046.html
>>
>> How I do it right now:
>>
>> 1)  Copy the input grid to RAM disk for fast access
>> 2)  Invoke Gdal.wrapper_GDALWarpDestName() for reprojection, 
>> resampling, and output to a /vsimem/ memory-mapped file
>> 3)  Read the memory-mapped file into a Dataset (returned by 
>> wrapper_GDALWarpDestName())
>> 4)  Write the Dataset values to DB
>>
>> What I would like to know:
>>
>> 1)  To skip the /vsimem file creation, how can I use 
>> Gdal.wrapper_GDALWarpDestDs() instead of -DestName?  It requires a 
>> dataset as a parameter, but I am not sure how to initialize it 
>> correctly.  I've tried Gdal.GetDriverByName("VRT").Create(...) and 
>> .GetDriverByName("MEM").Create(...), but all the values end up being 0 
>> when I pass the dataset to wrapper_GDALWarpDestDs()?
>>
>> 2)  Is there a more direct, efficient way to do what I'm doing?  Here 
>> is the relevant code
>>
>> <snip>
>> using (Dataset rasterInput = Gdal.Open(inputRasterPath, Access.GA_ReadOnly))
>> {
>>    IntPtr[] ptr = {Dataset.getCPtr(inputRasterPath).Handle};
>>    GCHandle gcHandle = GCHandle.Alloc(ptr, GCHandleType.Pinned);
>> ...
>>     var dss = new 
>> SWIGTYPE_p_p_GDALDatasetShadow(gcHandle.AddrOfPinnedObject(), false, 
>> null);
>> ..
>>     foreach (var tileExtent in overlappingTileExtents) // tileExtent 
>> just contains minX/minY/maxX/maxY
>>     {
>>     ...
>>
>>        string vsiMemFilePath = 
>> $"/vsimem/{destinationTableName}#{extentName}#tile{indexNumber}.vrt";
>>        string[] options = {
>>                                  "-of", "VRT",
>>                                  "-srcnodata", "99999.9",
>>                                  "-dstnodata", "99999.9",
>>                                  "-ot", "Float32",
>>                                  "-t_srs", "EPSG:4326",
>>                                  "-r", "average"),
>>                                  "-te", tileExtent.xmin.ToString(), 
>> tileExtent.ymin.ToString(), tileExtent.xmax.ToString(), 
>> tileExtent.ymax.ToString(),
>>                                  "-ts", "256", "256"
>>                              };
>>
>>         using (Dataset ds = 
>> Gdal.wrapper_GDALWarpDestName(vsiMemFilePath, 1, dss, new 
>> GDALWarpAppOptions(options), null, null))
>>         {
>> ...
>>           //read the geotransform + first band and write each pixel 
>> lat/long + value to the DB
>> ...
>>         }
>>
>>      Gdal.Unlink(vsiMemFilePath)
>>
>> ....
>>
>> Regards,
>> Mladen
>>
>> _______________________________________________
>> gdal-dev mailing list
>> gdal-dev at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/gdal-dev
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