[Gdal-dev] Nodata bitmasks / null processing

[Simon Perkins]

Frank Warmerdam wrote:
> Simon Perkins wrote:
>> So, would the default implementation return a universal RasterBand 
>> derived class that implements IReadBlock() and IWriteBlock() in terms 
>> of the underlying format RasterIO() method in conjunction with NODATA 
>> values?
>
> Simon,
>
> I think this is quite plausible, with the default implementation of
> GetBitmask() returning NULL if there is no nodata value set.

That's reasonable. It could also just return a bitmask containing all 
ones so that code doesn't have to handle this special case. Then we 
could have a separate method "HasBitmask()" (or the BitmaskType() method 
mentioned below) so that code that wants to optimize this case out can 
do so. Don't have a strong feeling.


>> One thing though - if the GetBitmask() method is a RasterBand method, 
>> then presumably invoking that method on a RasterBand returned by 
>> GetBitmask() wouldn't make sense. Either this could just raise some 
>> kind of error. Or we could make GetBitmask() a method of the Dataset 
>> instead.
>
> It is up to the drivers creating custom classes for their GetBitmask()
> result to ensure those don't in turn return bitmasks, and don't have
> a nodata value set on them.
>
> I think GetBitmask() being a method on GDALRasterBand, instead of
> the dataset is important as it clearly establishes the association.

That's reasonable.


> I am wondering, a bit how we would represent the case where there is
> one nodata bitmask for a whole dataset.  That it, one mask applying
> to all bands.  Perhaps all bands just return the same object?

Since users might be deleting the Bitmask RasterBand when they're done 
with it, it probably shouldn't be _exactly_ the same object, but it 
could be a copy of a stub that shares a lot of the underlying state / 
cache, etc. Might be worth adding a method  "BitmaskType()" method 
returning NONE, ONE_PER_DATASET, or ONE_PER_BAND , or similar. This 
would also subsume the "HasBitmask()" method.

Cheers,

Simon