Improve distinction between color and grayscale documents (#36)

app/src/main/java/org/fairscan/app/PostProcessing.kt

@@ -40,21 +40,63 @@ fun enhanceCapturedImage(img: Mat): Mat {
     }
 }
 
-fun isColoredDocument(img: Mat, threshold: Double = 4.0): Boolean {
+fun isColoredDocument(
+    img: Mat,
+    chromaThreshold: Double = 20.0,
+    proportionThreshold: Double = 0.001
+): Boolean {
     val lab = Mat()
     Imgproc.cvtColor(img, lab, Imgproc.COLOR_BGR2Lab)
+
     val channels = ArrayList<Mat>()
     Core.split(lab, channels)
+    val a = channels[1]
+    val b = channels[2]
 
-    val aStd = MatOfDouble()
-    val bStd = MatOfDouble()
-    Core.meanStdDev(channels[1], MatOfDouble(), aStd)
-    Core.meanStdDev(channels[2], MatOfDouble(), bStd)
+    val aFloat = Mat()
+    val bFloat = Mat()
+    a.convertTo(aFloat, CvType.CV_32F)
+    b.convertTo(bFloat, CvType.CV_32F)
 
-    val result = (aStd.toArray()[0] + bStd.toArray()[0]) / 2.0
-    return result > threshold
+    val aShifted = Mat()
+    val bShifted = Mat()
+    Core.subtract(aFloat, Scalar(128.0), aShifted)
+    Core.subtract(bFloat, Scalar(128.0), bShifted)
+
+    val aSq = Mat()
+    val bSq = Mat()
+    Core.multiply(aShifted, aShifted, aSq)
+    Core.multiply(bShifted, bShifted, bSq)
+
+    val sumSq = Mat()
+    Core.add(aSq, bSq, sumSq)
+
+    val chroma = Mat()
+    Core.sqrt(sumSq, chroma)
+
+    val mask = Mat()
+    Imgproc.threshold(chroma, mask, chromaThreshold, 1.0, Imgproc.THRESH_BINARY)
+    val coloredPixels = Core.countNonZero(mask)
+
+    val totalPixels = chroma.rows() * chroma.cols()
+    val proportion = coloredPixels.toDouble() / totalPixels.toDouble()
+
+    lab.release()
+    channels.forEach { it.release() }
+    aFloat.release()
+    bFloat.release()
+    aShifted.release()
+    bShifted.release()
+    aSq.release()
+    bSq.release()
+    sumSq.release()
+    chroma.release()
+    mask.release()
+
+    return proportion > proportionThreshold
 }
 
+
 private fun multiScaleRetinex(img: Mat, kernelSizes: List<Double> = listOf(30.0, 500.0)): Mat {
     // Convert to grayscale (1 channel)
     val gray = Mat()