[Qgis-developer] Re: Raster providers

[Marco Hugentobler]

Hi Radim

> Why do you consider it problematic, because it occupies too much space?
> Regarding precision, int to double is OK, I believe, but long to
> double is not because double has only 52bits to store mantissa. GDAL
> however does not define long,
> the longest integer is GDT_Int32

My concern was because of precision. But maybe its safe. Anyway, your 
suggestion below seems good to me.

>So I suggest:
> 
> source_type  -> qgis_type
> byte -> byte (if null value is defined) / short (if null value is not
> defined) short -> short  (if null value is defined) / int (if null value
> is not defined) int -> int, if null value is not defined use max value for
> null
> float -> float, use NaN for null values
> double -> double, use NaN for null values

+1

Marco

Am Dienstag, 18. Januar 2011, um 10.29:00 schrieb Radim Blazek:
> On Tue, Jan 18, 2011 at 9:19 AM, Marco Hugentobler
> 
> <marco.hugentobler at sourcepole.ch> wrote:
> > oops, that should be 'Agreed, int to double conversion could be
> > problematic'
> 
> Why do you consider it problematic, because it occupies too much space?
> Regarding precision, int to double is OK, I believe, but long to
> double is not because double has only 52bits to store mantissa. GDAL
> however does not define long,
> the longest integer is GDT_Int32
> 
> > What about an approach where the out-of-bound values are assigned the
> > last value only if reprojection is used (and if the layer does not have
> > a null value)?
> 
> Still the problem remains for projected data.
> 
> I think that usually with common data and wider data type like int,
> float and double the max value is never used except for some special
> values like null. The problem occurs
> with shorter types like byte and short. So I suggest:
> 
> source_type  -> qgis_type
> byte -> byte (if null value is defined) / short (if null value is not
> defined) short -> short  (if null value is defined) / int (if null value
> is not defined) int -> int, if null value is not defined use max value for
> null
> float -> float, use NaN for null values
> double -> double, use NaN for null values
> 
> This is still not 100% solution, but it should cover most common cases.
> The only code complication is, that we need a new srcDataType()
> provider method,
> to return the true data source type.
> 
> Radim
> 
> > This would be quite similar to your current approach, except that people
> > will have the appropriate raster appearance if they don't use reproj.
> > And if they do, they have an undesired border, but still the right
> > colors in the raster area.
> > 
> > Regards,
> > Marco
> > 
> > Am Montag, 17. Januar 2011, um 17.34:22 schrieb Radim Blazek:
> >> On Mon, Jan 17, 2011 at 10:06 AM, Marco Hugentobler
> >> <marco.hugentobler at sourcepole.ch> > Using NaN sounds like a good idea
> >> and Qt has platform independent support for
> >> 
> >> > it (qIsNan & co.).
> >> > All other solutions I can think of seem to be more complicated (e.g.
> >> > force a transparency value only if raster is reprojected).
> >> > 
> >> >>float + NaN for byte/int?
> >> >>
> >> > This is not clear to me. Could you explain your approach for byte/int?
> >> 
> >> I thought that if data type of the source is integer the provider
> >> could represent them as floating point. Byte can be represented as
> >> integer. Bad solution however.
> >> 
> >> Radim
> > 
> > --
> > Dr. Marco Hugentobler
> > Sourcepole -  Linux & Open Source Solutions
> > Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
> > marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
> > Technical Advisor QGIS Project Steering Committee


-- 
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee