CODEWITHSUNDEEP
c#arraysperformanceimage-processingoptimization
Robert Koernke
Robert Koernke

Reputation: 435

How to do this the fastest way? Image arrays

I have a loop that is too slow in C#. I want to know if there is a faster way to process through these arrays. I'm currently working in .NET 2.0. i'm not opposed to upgrading this project. This is part of a theoretical image processing concept involving gray levels.

This function creates an index of those values. hence pgidx.

int[] pgidx = new int[PixCnt];
sw = new Stopwatch();
sw.Start();
for (i = 0; i < PixCnt; i++)
{
    j = 0;
    pgidx[i] = 0;
    while (list_1d[i] != pg[j] && j < g_len) j++;
    if (list_id[i] == pg[j])
        pgidx[i] = j
}
sw.stop();
Debug.WriteLine("PixCnt Loop took" + sw.ElapsedMilliseconds + " ms");

Upvotes: 2

Views: 214

Answers (2)

user1196549
user1196549

Reputation:

If the range of the pixel values in pq allows it (say 16 bpp = 65536 entries), you can create an auxiliary array that maps all possible gray levels to the index value in pg. Filling this array is done with a single pass over pg (after initializing to all zeroes).

Then convert list_1d to pgidx with straight table lookups.

If the table is too big (bigger than the image), then do as @hatchet answered.

Upvotes: 0

hatchet - done with SOverflow
hatchet - done with SOverflow

Reputation: 16257

I think using a dictionary to store what's in the pg array will speed it up. g_len is 4625 elements, so you will likely average around 2312 iterations of the inner while loop. Replacing that with a single hashed look up in a dictionary should be faster. Since the outer loop executes 21 million times, speeding up the body of that loop should reap big rewards. I'm guessing the code below will speed up your time by 100 to 1000 time faster.

var pgDict = new Dictionary<int,int>(g_len);
for (int i = 0; i < g_len; i++) pgDict.Add(pg[i], i);
int[] pgidx = new int[PixCnt];
int value = 0;
for (int i = 0; i < PixCnt; i++) {
    if (pgDict.TryGetValue(list_id[i], out value)) pgidx[i] = value;
 }

Note that setting pgidx[i] to zero when a match isn't found is not necessary, because all elements of the array are already initialized to zero when the array is created.

If there is the possibility for a value in pg to appear more than once, you would want to check first to see if that key has already been added, and skip adding it to the dictionary if it has. That would mimic your current behavior of finding the first match. To do that replace the line where the dictionary is built with this:

for (int i = 0; i < g_len; i++) if (!pgDict.ContainsKey(pg[i])) pgDict.Add(pg[i], i);

Upvotes: 1

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