Basic postprocessing with OpenCV

app/src/main/java/org/mydomain/myscan/DocumentDetection.kt

@@ -91,10 +91,17 @@ fun extractDocument(originalBitmap: Bitmap, quad: Quad): Bitmap {
     val inputMat = Mat()
     Utils.bitmapToMat(originalBitmap, inputMat)
     val outputMat = Mat()
-    Imgproc.warpPerspective(inputMat, outputMat, transform, Size(maxWidth.toDouble(), maxHeight.toDouble()))
+    val outputSize = Size(maxWidth.toDouble(), maxHeight.toDouble())
+    Imgproc.warpPerspective(inputMat, outputMat, transform, outputSize)
 
-    val outputBitmap = createBitmap(maxWidth, maxHeight)
-    Utils.matToBitmap(outputMat, outputBitmap)
+    val enhanced = enhanceCapturedImage(outputMat)
+
+    return toBitmap(enhanced, maxWidth, maxHeight)
+}
+
+private fun toBitmap(mat: Mat, width: Int, height: Int): Bitmap {
+    val outputBitmap = createBitmap(width, height)
+    Utils.matToBitmap(mat, outputBitmap)
     return outputBitmap
 }
 

app/src/main/java/org/mydomain/myscan/MainViewModel.kt

@@ -18,7 +18,6 @@ import android.content.Context
 import android.graphics.Bitmap
 import android.graphics.BitmapFactory
 import android.graphics.Matrix
-import android.graphics.pdf.PdfDocument
 import androidx.camera.core.ImageProxy
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.ViewModelProvider

app/src/main/java/org/mydomain/myscan/PostProcessing.kt

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2025 Pierre-Yves Nicolas
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option)
+ * any later version.
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.mydomain.myscan
+
+import android.util.Log
+import org.opencv.core.Core
+import org.opencv.core.CvType
+import org.opencv.core.Mat
+import org.opencv.core.MatOfDouble
+import org.opencv.core.Scalar
+import org.opencv.core.Size
+import org.opencv.imgproc.Imgproc
+
+fun enhanceCapturedImage(img: Mat): Mat {
+    return if (isColoredDocument(img)) {
+        Log.i("PostProcessing", "color document")
+        val result = Mat()
+        Core.convertScaleAbs(img, result, 1.2, 10.0)
+        result
+    } else {
+        Log.i("PostProcessing", "grayscale document")
+        val gray = correctLighting(img)
+        val contrastedGray = enhanceContrastGray(gray)
+        val result = Mat()
+        Imgproc.cvtColor(contrastedGray, result, Imgproc.COLOR_GRAY2BGR)
+        result
+    }
+}
+
+fun isColoredDocument(img: Mat, threshold: Double = 4.0): Boolean {
+    val lab = Mat()
+    Imgproc.cvtColor(img, lab, Imgproc.COLOR_BGR2Lab)
+    val channels = ArrayList<Mat>()
+    Core.split(lab, channels)
+
+    val aStd = MatOfDouble()
+    val bStd = MatOfDouble()
+    Core.meanStdDev(channels[1], MatOfDouble(), aStd)
+    Core.meanStdDev(channels[2], MatOfDouble(), bStd)
+
+    val result = (aStd.toArray()[0] + bStd.toArray()[0]) / 2.0
+    return result > threshold
+}
+
+// TODO the radius should depend on the image size
+fun correctLighting(img: Mat, radius: Int = 100): Mat {
+    val gray = Mat()
+    Imgproc.cvtColor(img, gray, Imgproc.COLOR_BGR2GRAY)
+
+    val blur = Mat()
+    val kernelSize = 2 * radius + 1
+    Imgproc.GaussianBlur(gray, blur, Size(kernelSize.toDouble(), kernelSize.toDouble()), 0.0)
+
+    val normalized = Mat()
+    Core.divide(gray, blur, normalized, 255.0)
+    return normalized
+}
+
+fun enhanceContrastGray(img: Mat): Mat {
+    val flat = img.reshape(0, 1)
+    val sorted = Mat()
+    Core.sort(flat, sorted, Core.SORT_ASCENDING)
+
+    val totalPixels = sorted.cols()
+    val pLow = sorted[0, (totalPixels * 0.01).toInt()][0]
+    val pHigh = sorted[0, (totalPixels * 0.95).toInt()][0]
+
+    val result = Mat(img.size(), img.type())
+    img.convertTo(result, CvType.CV_32F)
+    Core.subtract(result, Scalar(pLow), result)
+    Core.multiply(result, Scalar(255.0 * 1.03 / (pHigh - pLow)), result)
+    Core.min(result, Scalar(255.0), result)
+    Core.max(result, Scalar(0.0), result)
+    result.convertTo(result, CvType.CV_8U)
+    return result
+}