[gdal-dev] Implementing driver to support-of output format

[Even Rouault]

On dimanche 5 mars 2017 22:47:54 CET Mateusz Loskot wrote:
> Hi,
> 
> I have a question about "-of output format" handling in a raster
> driver implementation:
> 
> 1. src_byte.tif is single band of Byte data type.
> 
> 2. gdal_translate -of GTiff -ot UInt16 src_byte.tif dst_uint16.tif
> 
> 3. In an implementation of `CreateCopy`
> 
> CreateCopy(char const* pszFilename, GDALDataset* poSrcDS, int bStrict, ...)
> {
>     ...
>     eDT = poSrcDS->GetRasterBand(1)->GetRasterDataType();
>     ...
>     poSrcDS->RasterIO(GF_Read, ..., eDT, ...);
>     ...
> }
> 
> a) The source dataset band type is UInt16, while band in the source
> src_byte.tif file is Byte.
> b) RasterIO requests Uint16 from the src_byte.tif with Byte band.
> 
> Which, if any, part is responsible for the magic that changes the
> pixel data form Byte to Uint16 during the RasterIO?

Hi Mateusz,

if you look at apps/gdal_translate_lib.cpp, you'll see that, except cases without any 
arguments like gdal_translate in out -of output_format where the 'in' dataset is passed as the 
source dataset of CretaeCopy(), as soon as you add some transformation like changing data 
type, subsetting, etc, a in-memory VRT will be created to do that transformation. And it is 
that in-memory VRT that is passed as the source dataset to the CreateCopy() implementation 
of the output driver. That should answer all your following questions.

Even

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