[gdal-dev] Writing data to "CreateCopy" files

[Cole, Derek]

I think I may have tried to go down this road before, but perhaps someone can elighten me.

I am using CreateCopy on this file so that I get a copy of the file on disk with all the same metadata and projection info, etc. 

Is it possible for me to spawn CreateCopy in another thread which would presumably write the headers and such immediately. Then I could use my host thread or whatever to start writing my own data into the newly created file? It seems like it shows up on disk as the right size immediately when CreateCopy is called, so I assume all the delay is CreateCopy moving the raster data over (which I dont care about at this point, because I am overwriting it).

Am I maybe just better off creating a new file from the get-go with the required size parameters? If I was going to do that what is the best route to go to ensure that I copy all of the important metadata and projection, coordinates, etc?

Thanks

Derek
________________________________________
From: fwarmerdam at gmail.com [fwarmerdam at gmail.com] on behalf of Frank Warmerdam [warmerdam at pobox.com]
Sent: Friday, July 15, 2011 10:22 PM
To: Cole, Derek
Cc: gdal-dev at lists.osgeo.org
Subject: Re: [gdal-dev] Writing data to "CreateCopy" files

On Fri, Jul 15, 2011 at 3:57 PM, Cole, Derek <dcole at integrity-apps.com> wrote:
> I have a couple of questions regarding this.
>
> Is the dataset that is returned from CreateCopy able to be used immediately?
> I would like to use it to get the first band from it, and use RasterIO on
> that band.

Derek,

Yes, you should be able to read the resulting file
with the returned GDALDataset object immediately.
Whether you can write to the resulting file will depend
on the format.  Some formats are readonly (with
CreateCopy being the only way to accomplishing
writing).


> Secondly - the data I am trying to write with RasterIO is unsigned char, but
> when I was reading the data, I had to read it with a GDT_Float32, which was
> allocated like so:
>
>     float * floatData = (float *)
> CPLMalloc(sizeof(float)*bufSizeX*bufSizeY);
>
> and that seemed to be working well. Will my copy have to be written via a
> GDT_Float32 data buffer like the one I read it into initially, or can I just
> change the RasterIO parameter on the write to be GDT_Byte?


You can read into a float buffer and write back out using
an unsigned char buffer as long as you use the appropriate
data type (GDT_Byte or GDT_Float32) in the RasterIO call.
RasterIO wlil take care of on the fly conversion of the data.

Be aware that writing a float buffer to a Byte file will not
cause the file on disk to be upgraded to store floating
point values.  Instead RasterIO will convert the floating
point values to unsigned char.  When down converting
types out of range values are clamped to the max and min
of the target type and otherwise a floor() convertion is
applied when going to integer types.

Best regards,
--
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