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

alex alexhighviz at hotmail.com
Tue Sep 13 05:40:56 PDT 2016

```>Ari Wrote:
>At FOSS4G my conclusion was that my first approach was not good for many reasons, most >importantly because it did not scale to several bands in one operation. So I've trying with the >following ideas/decisions:
>
>The problem is to compute y=f(x1, x2, ...), where y is a new dataset with one band or an existing >dataset, into which a new band is added.
>x1, x2, ... are existing bands. f is an expression. The goal is to be able to create an expression >object, with which one can write

Hi Ari,

Perhaps it is worthwhile looking back at the email I wrote earlier in the process:

At the time, I suggested using expression objects and to allow functions with more than 2 arguments and gave some suggestions on the implementation.

In my experience the key first step is to develop pixel iterators for raster bands. Once you have that, raster bands can be wrapped as regular ranges and you can make use of 'regular' tools and methods to develop your expression objects, e.g. as in the Range v3 library by Eric Niebler.

Hence, my repeated suggestion for GDAL: create iterators for raster bands :)

As you may recall, the code below works just fine (but requires C++11).

With kind regards, Alex

int my_function(int w, int x, int y, int z)
{
return w * x + y * z;
}

int main()
{

auto w = ma::create_from_model<double>("output_1.tif", a);
auto x = ma::create_from_model<double>("output_2.tif", a);
auto y = ma::create_from_model<double>("output_3.tif", a);
auto z = ma::create_from_model<double>("output_4.tif", a);

// Example 1: Using operators
w = a + 3 * b;

// Example 2: Using assigning operators
x = 1;
x *= a;
x += 3 * b;

// Example 3: Map algebra using cell-by-cell functions
y = ma::apply(my_function, 1, a, 3, b);

// Example 4: Combination
z = b + 3 * ma::apply(my_function, 1, a, 2, b);

return 0;
}

```