From 26d6f24e97583e033c79d8df84b199748fcaa436 Mon Sep 17 00:00:00 2001
From: Pacien TRAN-GIRARD
Date: Tue, 8 Dec 2015 11:32:48 +0100
Subject: Fix and simplify filter convolution

---
 src/main/java/Filter.java | 61 ++++++++++++++++++++++-------------------------
 1 file changed, 29 insertions(+), 32 deletions(-)

diff --git a/src/main/java/Filter.java b/src/main/java/Filter.java
index 7e55f04..d88ea07 100644
--- a/src/main/java/Filter.java
+++ b/src/main/java/Filter.java
@@ -41,6 +41,32 @@ public final class Filter {
         return gray[row][col];
     }
 
+    /**
+     * Combine neighbour pixels using the given kernel.
+     *
+     * @param gray   a HxW float array
+     * @param kernel a MxN float array, with M and N odd
+     * @param row    Y coordinate
+     * @param col    X coordinate
+     * @return the pixel value
+     */
+    private static float combineNeighbours(float[][] gray, float[][] kernel, int row, int col) {
+        int kernelWidth = kernel[0].length;
+        int kernelHeight = kernel.length;
+
+        int rowOffset = row - kernelWidth / 2; // int division
+        int colOffset = col - kernelHeight / 2; // int division
+
+        float pixelValue = 0;
+
+        for (int kernelRow = 0; kernelRow < kernelHeight; ++kernelRow)
+            for (int kernelCol = 0; kernelCol < kernelWidth; ++kernelCol)
+                pixelValue += kernel[kernelRow][kernelCol]
+                        * Filter.at(gray, rowOffset + kernelRow, colOffset + kernelCol);
+
+        return pixelValue;
+    }
+
     /**
      * Convolve a single-channel image with specified kernel.
      *
@@ -52,40 +78,11 @@ public final class Filter {
         int width = gray[0].length;
         int height = gray.length;
 
-        int kernelWidth = kernel[0].length;
-        int kernelHeight = kernel.length;
-
-        float pixelNeighbors[][] = new float[kernelHeight][kernelWidth];
         float[][] filteredImage = new float[height][width];
 
-        for (int row = 0; row < height; ++row) {
-            for (int col = 0; col < width; ++col) {
-
-                int pixelRow = -kernelHeight / 2;
-                int pixelCol = -kernelWidth / 2;
-
-                for (int kernelRow = 0; kernelRow < kernelWidth; kernelRow++) {
-                    for (int kernelCol = 0; kernelCol < kernelHeight; kernelCol++) {
-                        pixelNeighbors[kernelRow][kernelCol] = Filter.at(gray, row + pixelRow, col + pixelCol);
-
-                        pixelCol += 1;
-                        if (pixelCol > kernelWidth / 2) {
-                            pixelCol = -kernelWidth / 2;
-                        }
-                    }
-                    pixelRow += 1;
-                    if (pixelRow > kernelHeight / 2) {
-                        pixelRow = -kernelHeight / 2;
-                    }
-                }
-
-                for (int i = 0; i < kernelWidth; i++) {
-                    for (int j = 0; j < kernelHeight; j++) {
-                        filteredImage[row][col] += kernel[i][j] * pixelNeighbors[i][j];
-                    }
-                }
-            }
-        }
+        for (int row = 0; row < height; ++row)
+            for (int col = 0; col < width; ++col)
+                filteredImage[row][col] = Filter.combineNeighbours(gray, kernel, row, col);
 
         return filteredImage;
     }
-- 
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