[gdal-dev] GDAL ReadRaster thread-safe?

[Eric Domazlicky]

This looks like a great way to go but can anyone explain exactly what the
bandmap and bandspace parameters are? I know BandMap is an array but what is
it supposed to represent? I want the image data to be read and put into my
destination buffer in ARGB format, the source data is in RGBA format. I
tried making an array with elements in order: 4,1,2,3 but the destination
data is transparent so I'm thinking the alpha value isn't getting read
right.

On Wed, Jan 7, 2009 at 4:12 PM, Tamas Szekeres <szekerest at gmail.com> wrote:

> Eric,
>
> The most performant option I'm aware of is reading the dataset directly
> into a bitmap buffer something like:
>
> try
> {
>         int stride = bitmapData.Stride;
>         IntPtr buf = bitmapData.Scan0;
>
>         ds.ReadRaster(xOff, yOff, width, height, buf, imageWidth,
> imageHeight, dataType,
>         channelCount, bandMap, pixelSpace, stride, 1);
> }
> finally
> {
>         bitmap.UnlockBits(bitmapData);
> }
>
> In this case there's no internal buffers in the marshalling code used by
> the C# wrappers. The only challenge here is to ensure that the bitmap buffer
> should have the same structure as the gdal dataset represents. You could
> refer to the complete example in the C# wrapper for the details:
>
> http://trac.osgeo.org/gdal/browser/trunk/gdal/swig/csharp/apps/GDALDatasetRasterIO.cs
>
> http://trac.osgeo.org/gdal/browser/trunk/gdal/swig/csharp/apps/GDALReadDirect.cs
>
> I don't think if triggering multiple reads for different sections in
> multiple threads is currently supported by gdal, it might also be driver
> dependent how the reads are actually implemented by the drivers, however you
> might anyway try defining separate Dataset objects for the different when
> performing the read operations, but there might be locks in different places
> causing the operations to be serialized in effect.
>
> Best regards,
>
> Tamas
>
>
>
> 2009/1/8 Eric Domazlicky <edomazlicky at gmail.com>
>
>>  I am developing a tiling application and I was hoping to up the speed a
>> bit by doing some multithreading. What I want to do is start each of 3
>> ReadRaster calls in seperate threads (using BeginInvoke delegates), then
>> wait for them to complete and build my RGB image (using EndInvoke). I am
>> using the C# wrapper to the GDAL library. This multithreaded approach is
>> showing clear signs of race conditions, with some of the channels not
>> appearing, random crashes etc. If I ensure each thread completes before I go
>> on to read the next Raster Band it works fine.
>>
>> Is there anyway to make ReadRaster thread-safe? Maybe by disabling caching
>> which I suspect is the problem.
>>
>>
>>
>>
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>>
>
>
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