Detecting white margin in HF Fax mode and correcting image shift when saving

app/src/main/java/xdsopl/robot36/BaseMode.java

@@ -1,4 +1,8 @@
 package xdsopl.robot36;
 
 public abstract class BaseMode implements Mode {
+    @Override
+    public int getEstimatedHorizontalShift() {
+        return 0;
+    }
 }

app/src/main/java/xdsopl/robot36/HFFax.java

@@ -1,19 +1,21 @@
 package xdsopl.robot36;
 
+import android.graphics.Color;
+
 public class HFFax extends BaseMode {
     private final ExponentialMovingAverage lowPassFilter;
-    private final int smallPictureMaxSamples;
-    private final int mediumPictureMaxSamples;
     private final String name;
 
     private final int sr;
 
+    private final float[] cumulated;
+    private int horizontalShift = 0;
+
     HFFax(String name, int sampleRate) {
         this.name = name;
-        smallPictureMaxSamples = (int) Math.round(0.125 * sampleRate);
-        mediumPictureMaxSamples = (int) Math.round(0.175 * sampleRate);
         lowPassFilter = new ExponentialMovingAverage();
         this.sr = sampleRate;
+        cumulated = new float[getWidth()];
     }
 
     private float freqToLevel(float frequency, float offset) {
@@ -55,6 +57,11 @@ public class HFFax extends BaseMode {
         return sr / 2;
     }
 
+    @Override
+    public int getEstimatedHorizontalShift() {
+        return horizontalShift;
+    }
+
     @Override
     public void reset() {
     }
@@ -63,22 +70,38 @@ public class HFFax extends BaseMode {
     public boolean decodeScanLine(PixelBuffer pixelBuffer, float[] scratchBuffer, float[] scanLineBuffer, int scopeBufferWidth, int syncPulseIndex, int scanLineSamples, float frequencyOffset) {
         if (syncPulseIndex < 0 || syncPulseIndex + scanLineSamples > scanLineBuffer.length)
             return false;
-        int horizontalPixels = scopeBufferWidth;
-        if (scanLineSamples < smallPictureMaxSamples)
-            horizontalPixels /= 2;
-        if (scanLineSamples < mediumPictureMaxSamples)
-            horizontalPixels /= 2;
+        int horizontalPixels = getWidth();
         lowPassFilter.cutoff(horizontalPixels, 2 * scanLineSamples, 2);
         lowPassFilter.reset();
         for (int i = 0; i < scanLineSamples; ++i)
-            scratchBuffer[i] = lowPassFilter.avg(scanLineBuffer[syncPulseIndex + i]);
+            scratchBuffer[i] = lowPassFilter.avg(scanLineBuffer[i]);
         lowPassFilter.reset();
         for (int i = scanLineSamples - 1; i >= 0; --i)
             scratchBuffer[i] = freqToLevel(lowPassFilter.avg(scratchBuffer[i]), frequencyOffset);
         for (int i = 0; i < horizontalPixels; ++i) {
             int position = (i * scanLineSamples) / horizontalPixels;
-            pixelBuffer.pixels[i] = ColorConverter.GRAY(scratchBuffer[position]);
+            int color = ColorConverter.GRAY(scratchBuffer[position]);
+            pixelBuffer.pixels[i] = color;
+
+            cumulated[i] *= 0.99f; //decay old data
+            cumulated[i] += Color.luminance(color);
         }
+
+        //try to detect "sync": thick white margin
+        int bestIndex = 0;
+        float bestValue = 0;
+        for (int x = 0; x < getWidth(); ++x)
+        {
+            float val = cumulated[x];
+            if (val > bestValue)
+            {
+                bestIndex = x;
+                bestValue = val;
+            }
+        }
+
+        horizontalShift = bestIndex;
+
         pixelBuffer.width = horizontalPixels;
         pixelBuffer.height = 1;
         return true;

app/src/main/java/xdsopl/robot36/MainActivity.java

@@ -15,6 +15,7 @@ import android.content.SharedPreferences;
 import android.content.pm.PackageManager;
 import android.content.res.Configuration;
 import android.graphics.Bitmap;
+import android.graphics.Canvas;
 import android.graphics.Color;
 import android.media.AudioFormat;
 import android.media.AudioRecord;
@@ -846,6 +847,19 @@ public class MainActivity extends AppCompatActivity {
 		Bitmap bmp = Bitmap.createBitmap(scopeBuffer.pixels, offset, stride, width, height, Bitmap.Config.ARGB_8888);
 		if (currentMode == null || !currentMode.equals("HF Fax")) {
 			bmp = Bitmap.createScaledBitmap(bmp, width / 3, height / 3, true);
+		} else {
+			Mode mode = decoder.currentMode;
+			int shift = mode.getEstimatedHorizontalShift();
+			if (shift > 0) {
+				Bitmap part1 = Bitmap.createBitmap(bmp, 0, 0, shift, bmp.getHeight());
+				Bitmap part2 = Bitmap.createBitmap(bmp, shift, 0, mode.getWidth() - shift, bmp.getHeight());
+
+				Bitmap bmpMutable = Bitmap.createBitmap(mode.getWidth(), bmp.getHeight(), Bitmap.Config.ARGB_8888);
+				Canvas canvas = new android.graphics.Canvas(bmpMutable);
+				canvas.drawBitmap(part2, 0, 1, null);
+				canvas.drawBitmap(part1, mode.getWidth() - shift, 0, null);
+				bmp = bmpMutable;
+			}
 		}
 		storeBitmap(bmp);
 	}

app/src/main/java/xdsopl/robot36/Mode.java

@@ -42,6 +42,11 @@ public interface Mode {
 	 */
 	int getScanLineSamples();
 
+	/**
+	 * @return number of pixels of horizontal shift based on recent data, nonzero for HF Fax
+	 */
+	int getEstimatedHorizontalShift();
+
 	/**
 	 * Reset internal state.
 	 */