Add better description of CFloat64, Float64, etc.

[yjacolin]

See this PR to see the "problem" https://github.com/qgis/QGIS-Documentation/pull/2963

Proposition:

• create an appendice with a full description
• add a reference to it wherever needed

[yjacolin]

@rouault do you have some input to fill in this table? Complex types documentation is difficult to find on Internet :)

Type	What does it mean?	Python2/3	Number domain
Byte	8-bit int
UInt16	Unsigned 16-bit int	Int	0 to 65,535
Int16	signed 16-bit int	Int	–32,768 to 32,767
UInt32	unsigned 32-bit int	Int	0 to 4,294,967,295
Int32	signed 32-bit int	Int	–2,147,483,648 to 2,147,483,647
Float32	32-bit IEEE float	float	-3.4E38 to 3.4E38
Float64	64-bit IEEE float	float	-1.7E308 to 1.7E308
CInt16	complex 16-bit int	Int	–32,768 to 32,767
CInt32	complex 32-bit int	Int	–2,147,483,648 to 2,147,483,647
CFloat32	complex 32-bit IEEE float	float	-3.4E38 to 3.4E38
CFloat64	complex 64-bit IEEE float	float	-1.7E308 to 1.7E308

[rouault]

I'm not sure how QGIS maps those complex data types on the Python side. With GDAL numpy support, CInt16, CInt32 and CFloat32 are mapped to numpy.complex64 and CFloat64 to numpy.complex64 As far as the number domain, this is the same as the corresponding real type, for the real and imaginary parts, so for CInt16 this is [-32768,32767]x[-32768,32767], etc.

[yjacolin]

@rouault thanks, another question: do you know any web page or so that explain the difference between complex and real type? Which usage?

I want to document this as algorithms in QGIS Processing are not using Python and seems to use C Types (processing feature is now in C/C++).

[rouault]

do you know any web page or so that explain the difference between complex and real type? Which usage?

Documentation on this in GDAL is more than sparse. Basically, complex is complex numbers in the mathematical meaning of the term, ie "x + i * y" where x and y are real number and i the imaginary unit whose square value is -1. Complex data types in raster are mostly used for SAR (Synthetic-aperture radar) images.

[havatv]

This is not specific to QGIS. Those that are interested in this can look it up elsewhere. I suggest that we close this issue.