image_processing/binary_thresholding.cpp
/*******************************************************
* Copyright (c) 2015, ArrayFire
* All rights reserved.
*
* This file is distributed under 3-clause BSD license.
* The complete license agreement can be obtained at:
* https://arrayfire.com/licenses/BSD-3-Clause
********************************************************/
#include <arrayfire.h>
#include <cmath>
#include <cstdio>
#include <cstdlib>
using namespace af;
array threshold(const array& in, float thresholdValue) {
int channels = in.dims(2);
array ret_val = in.copy();
if (channels > 1) ret_val = colorSpace(in, AF_GRAY, AF_RGB);
ret_val =
(ret_val < thresholdValue) * 0.0f + 255.0f * (ret_val > thresholdValue);
return ret_val;
}
array otsu(const array& in) {
array gray;
int channels = in.dims(2);
if (channels > 1)
gray = colorSpace(in, AF_GRAY, AF_RGB);
else
gray = in;
unsigned total = gray.elements();
array hist = histogram(gray, 256, 0.0f, 255.0f);
array wts = range(256);
array wtB = accum(hist);
array wtF = total - wtB;
array sumB = accum(wts * hist);
array meanB = sumB / wtB;
float lastElemInSumB;
sumB(seq(255, 255, 1)).host((void*)&lastElemInSumB);
array meanF = (lastElemInSumB - sumB) / wtF;
array mDiff = meanB - meanF;
array interClsVar = wtB * wtF * mDiff * mDiff;
float max = af::max<float>(interClsVar);
float threshold2 = where(interClsVar == max).scalar<unsigned>();
array threshIdx = where(interClsVar >= max);
float threshold1 =
threshIdx.elements() > 0 ? threshIdx.scalar<unsigned>() : 0.0f;
return threshold(gray, (threshold1 + threshold2) / 2.0f);
}
int main(int argc, char** argv) {
try {
int device = argc > 1 ? atoi(argv[1]) : 0;
af::setDevice(device);
array bimodal =
loadImage(ASSETS_DIR "/examples/images/noisy_square.png", false);
bimodal = resize(0.75f, bimodal);
array bt = threshold(bimodal, 180.0f);
array ot = otsu(bimodal);
array bimodHist = histogram(bimodal, 256, 0, 255);
array smooth = convolve(bimodal, gaussianKernel(5, 5));
array smoothHist = histogram(smooth, 256, 0, 255);
af::Window wnd(1536, 1024, "Binary Thresholding Algorithms");
printf("Press ESC while the window is in focus to proceed to exit\n");
wnd.grid(3, 3);
wnd(0, 1).setAxesTitles("Bins", "Frequency");
wnd(1, 1).setAxesTitles("Bins", "Frequency");
wnd(2, 1).setAxesTitles("Bins", "Frequency");
while (!wnd.close()) {
wnd(0, 0).image(bimodal / 255, "Input Image");
wnd(1, 0).image(bimodal / 255, "Input Image");
wnd(2, 0).image(smooth / 255, "Input Smoothed by Gaussian Filter");
wnd(0, 1).hist(bimodHist, 0, 255, "Input Histogram");
wnd(1, 1).hist(bimodHist, 0, 255, "Input Histogram");
wnd(2, 1).hist(smoothHist, 0, 255, "Smoothed Input Histogram");
wnd(0, 2).image(bt, "Simple Binary threshold");
wnd(1, 2).image(ot, "Otsu's Threshold");
wnd(2, 2).image(otsu(smooth), "Otsu's Threshold on Smoothed Image");
wnd.show();
}
} catch (af::exception& e) {
fprintf(stderr, "%s\n", e.what());
throw;
}
return 0;
}