Question

How can I calculate the histogram of an image using parallel Computing? Histogram of an image in serial is easy, but I don’t have any idea about calculating it in Parallel.

Any idea, algorithm, source code or a helpful link would be appreciated.

I am using MPI.

Answer 1

Here is the idea:

First you scatter the image rowwise (if you program in C) or columnwise (if you program in Fortran) using MPI_Scatter() or MPI_Scatterv() (with scatterv you can distribute the image to a number of processes that is not an integer divisor of the number of rows/columns).

Then every process computes the histogram of its part of the image. Let the histogram be stored in an integer array local_histogram of 256 elements (I assume the images are grayscale).

At the last step a global reduction with summation is performed with MPI_Reduce():

MPI_Reduce(local_histogram, histogram, 256, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);

Afterwards the full global histogram would be in the histogram array at process with rank 0.