Structure similarity example

- Added two images to check structure similarity
- Added example code to calculate structure similarity

Author: emelnychenko

data/ssim-1.png

@@ -0,0 +1,54 @@
+import {CV_32F, imread, Size} from '../';
+
+// Ported from https://docs.opencv.org/2.4/doc/tutorials/gpu/gpu-basics-similarity/gpu-basics-similarity.html
+function getStructureSimilarity(i1, i2) {
+    const C1 = 6.5025, C2 = 58.5225;
+    /***************************** INITS **********************************/
+    const d = CV_32F;
+
+    const I1 = i1.convertTo(d);    // cannot calculate on one byte large values
+    const I2 = i2.convertTo(d);
+
+    const I2_2  = I2.hMul(I2);     // I2^2
+    const I1_2  = I1.hMul(I1);     // I1^2
+    const I1_I2 = I1.hMul(I2);     // I1 * I2
+
+    /*************************** END INITS **********************************/
+
+    const mu1 = I1.gaussianBlur(new Size(11, 11), 1.5);
+    const mu2 = I2.gaussianBlur(new Size(11, 11), 1.5);
+
+    const mu1_2   = mu1.hMul(mu1);
+    const mu2_2   = mu2.hMul(mu2);
+    const mu1_mu2 = mu1.hMul(mu2);
+
+    let sigma1_2 = I1_2.gaussianBlur(new Size(11, 11), 1.5);
+    sigma1_2 = sigma1_2.sub(mu1_2);
+
+    let sigma2_2 = I2_2.gaussianBlur(new Size(11, 11), 1.5);
+    sigma2_2 = sigma2_2.sub(mu2_2);
+
+    let sigma12 = I1_I2.gaussianBlur(new Size(11, 11), 1.5);
+    sigma12 = sigma12.sub(mu1_mu2);
+
+    ///////////////////////////////// FORMULA ////////////////////////////////
+
+    let t1 = mu1_mu2.convertTo(-1, 2, C1);
+    let t2 = sigma12.convertTo(-1, 2, C2);
+    let t3 = t1.hMul(t2);
+
+    t1 = mu1_2.addWeighted(1.0, mu2_2, 1.0, C1);
+    t2 = sigma1_2.addWeighted(1.0, sigma2_2, 1.0, C2);
+    t1 = t1.hMul(t2);
+
+    const ssim_map = t3.hDiv(t1);
+    const { y, x, w } = ssim_map.mean();
+    return [y, x, w].reduce((a, b) => a + b) / 3;
+}
+
+const i1 = imread('../data/ssim-1.png');
+const i2 = imread('../data/ssim-2.png');
+
+const structureSimilarity = getStructureSimilarity(i1, i2);
+
+console.log('SSIM: '+structureSimilarity); // Output: SSIM: 0.717