Question

in C++, what is the indexing value for a W * H * D sized 3D array?

for a particular i, j, k is this the correct indexing:

i*W*H+j*W+k

Answer 1

What you have written is equivalent to the pointer arithmetic that this would do:

T x[D][H][W];

x[i][j][k];  // Pointer arithmetic done here

Obviously, depending on how you order D, H and W (or i, j, k), the calculation will differ.