[Qgis-developer] Re: Raster providers

[Radim Blazek]

Or we can just use NaN for float/double and float + NaN for byte/int?

Radim

On Sun, Jan 16, 2011 at 11:29 AM, Radim Blazek <radim.blazek at gmail.com> wrote:
> On Thu, Jan 13, 2011 at 9:04 AM, Marco Hugentobler
> <marco.hugentobler at sourcepole.ch> wrote:
>> With one of my favorite test dataset, I noted a problem with automatic
>> assignment of nodata value and automatic insertion into the transparency list.
>> In that case, the raster is a colortable map (byte) and has no nodata value.
>> So 255 is set to transparent and the map appears completely different than
>> expected...
>
> The problem is that with reprojection we always need null values even
> if not defined by raster. At moment I just assigned the max value for
> each data type which is obviously wrong as you proved.
> The question is how to passed null values from provider to raster
> layer. We could:
>
> 1) scan all values for a not used value, but it can slow down first
> loading and it is not always successful (for short data types all
> values may be occupied)
>
> 2) always use longer data type (int for byte, float for int etc), the
> bigger size probably is not problem because we usually need only a
> block of data for screen (for double it is max around 1600*1200*8 =
> 14MB), but what about Float64 - could/should we use long double, it
> can be 96 or 128 bits and not available on ARM(?) etc...
>
> 3) probably the right solution would be to use separate bit matrix as
> null flags, but that would complicate too much the implementation
>
> Maybe a combination of 1+ 2? 1 for shorter types - byte/int and 2 for
> float/double? How to find quickly a not used value?
>
> I am waiting for your ideas.
> Radim
>