Changed Undo arrow color based on the selected background

app/src/main/java/com/simplemobiletools/draw/Utils.java

@@ -1,10 +1,28 @@
 package com.simplemobiletools.draw;
 
 import android.content.Context;
+import android.graphics.Color;
 import android.widget.Toast;
 
 public class Utils {
+    private final static double BRIGHTNESS_CUTOFF = 130.0;
+
     public static void showToast(Context cxt, int msgId) {
         Toast.makeText(cxt, cxt.getResources().getString(msgId), Toast.LENGTH_SHORT).show();
     }
+
+    // Used to determine the best foreground color (black or white) given a background color
+    public static boolean shouldUseWhite(int color) {
+        float r, g, b;
+        r = Color.red(color);
+        g = Color.green(color);
+        b = Color.blue(color);
+
+        double brightness = Math.sqrt(
+                r * r * .299 +
+                g * g * .587 +
+                b * b * .114);
+
+        return brightness < BRIGHTNESS_CUTOFF;
+    }
 }

app/src/main/java/com/simplemobiletools/draw/activities/MainActivity.java

@@ -19,6 +19,7 @@ import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.widget.EditText;
+import android.widget.ImageView;
 import android.widget.Toast;
 
 import com.simplemobiletools.draw.Config;
@@ -98,6 +99,11 @@ public class MainActivity extends SimpleActivity implements MyCanvas.PathsChange
 
                             @Override
                             public void onOk(AmbilWarnaDialog dialog, int pickedColor) {
+                                if (Utils.shouldUseWhite(pickedColor)) {
+                                    ((ImageView)mUndoBtn).setImageResource(R.mipmap.undo_white);
+                                } else {
+                                    ((ImageView)mUndoBtn).setImageResource(R.mipmap.undo_black);
+                                }
                                 mMyCanvas.setBackgroundColor(pickedColor);
                             }
                         });

app/src/main/res/layout/activity_main.xml

@@ -23,7 +23,7 @@
         android:layout_alignParentRight="true"
         android:layout_below="@id/color_picker"
         android:padding="@dimen/icon_padding"
-        android:src="@mipmap/undo"
+        android:src="@mipmap/undo_black"
         android:visibility="gone"/>
 
 </RelativeLayout>