Question

I am running an image analysis code on an array storing information about the image. Unfortunately the code is very heavy and takes an average of 25s to run through a single frame. The main problem I see is the array addressing. Which is the fastest to run through a 2d array and are there at all any differences in

horizontal then vertical

for (int y = 0; y < array.Length; ++y)
    for (int x = 0; x < array[].Length; ++x)
        //Code using array[y][x]

and vertical then horrizontal?

for (int x = 0; x < array[].Length; ++x)
    for (int y = 0; y < array.Length; ++y)
        //Code using array[y][x]

Furthermore, I tried to avoid direct addressing and use pointers instead.

for (int y = 0; y < array.Length; ++y)
    int* ptrArray = (int*)array[0];
    for (int x = 0; x < array[].Length; ++x, ++ptrArray)
        //Code using ptrArray for array[y][x]

or

for (int x = 0; x < array[].Length; ++x)
    int* ptrArray = (int*)array[0];
    for (int y = 0; y < array.Length; ++y, ptrArray += array[].Length)
        //Code using ptrArray for array[y][x]

Any help is greatly appreciated.
Max

Answer 1

One option is to use reverse looping (start your for() loop from array.Length down to 0)

That’ll speed things up abit.

for example,

for (int x = array[].Length-1; x >= 0; --x)
    int* ptrArray = (int*)array[0];
    for (int y = array.Length-1; y >= 0 ; --y, ptrArray += array[].Length)
        //Code using ptrArray for array[y][x]