[gdal-dev] Update on RFC 62: Raster algebra

[Richard Barnes]

In building tile/block managers, I too have found it difficult to use 
iterators or design algorithms without specifically considering both 
tiles/blocks and cells. Without doing so, it is very easy to write code 
which is (extremely) cache inefficient.

I'm not sure if flow algebras have arisen in the discussion yet, but 
they come to mind when I think of raster algebras. They permit 
operations in which the values of "downstream" cells are functions of 
upstream cells. In such a case, efficient calculations are then driven 
both by blocking and by the data itself. In recent work, I've found that 
a number of flow algebra functions can be written by considering only 
one block at a time (link 
<http://www.sciencedirect.com/science/article/pii/S0098300416301704>, 
link <https://arxiv.org/abs/1608.04431>). I'm working on generalizing 
the concept now and can imagine it forming an easy way to quickly add 
general terrain analysis functionality.

-Richard
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