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Building OpenCV with the WITH_OPENMP flag means that the internal functions will use OpenMP to parallelize some of the algorithms.

In OpenCV, an algorithm can have a sequential (slowest) implementation; a parallel implementation using OpenMP or TBB; and a GPU implementation using OpenCL or CUDA (fastest). You can decide with the WITH_...flags which version to use.

Of course, not every algorithm can be parallelized.

Now, if you want to parallelize your methods with OpenMP, you have to implement it yourself.

A typical image processing function looks like:

for(int y=0;y<image.rows;y++)
    for(int x=0;x<image.cols;x++)
         //do something with pixel (x,y)

Typically it's the outer loop that gets parallelized; so you have to transform the for(y...) loop to a parallel for loop. I never used OpenMP, but it seems it's done with the #pragma omp for directive.

Building OpenCV with the WITH_OPENMP flag means that the internal functions will use OpenMP to parallelize some of the algorithms.

Of course, not every algorithm can be parallelized.

Now, if you want to parallelize your methods with OpenMP, you have to implement it yourself.

A typical image processing function looks like:

for(int y=0;y<image.rows;y++)
    for(int x=0;x<image.cols;x++)
         //do something with pixel (x,y)

directive; something like:

#pragma omp for
for(int y=0;y<image.rows;y++)
    for(int x=0;x<image.cols;x++)
         //do something with pixel (x,y)