Changed: Use black or white cursor color based on terminal background instead of always white if colors.properties didn't have cursor color set

Credit for algorithm link belong to @Jamie-Landeg-Jones

Closes #2653

terminal-emulator/src/main/java/com/termux/terminal/TerminalColorScheme.java

@@ -71,6 +71,7 @@ public final class TerminalColorScheme {
 
     public void updateWith(Properties props) {
         reset();
+        boolean cursorPropExists = false;
         for (Map.Entry<Object, Object> entries : props.entrySet()) {
             String key = (String) entries.getKey();
             String value = (String) entries.getValue();
@@ -82,6 +83,7 @@ public final class TerminalColorScheme {
                 colorIndex = TextStyle.COLOR_INDEX_BACKGROUND;
             } else if (key.equals("cursor")) {
                 colorIndex = TextStyle.COLOR_INDEX_CURSOR;
+                cursorPropExists = true;
             } else if (key.startsWith("color")) {
                 try {
                     colorIndex = Integer.parseInt(key.substring(5));
@@ -98,6 +100,27 @@ public final class TerminalColorScheme {
 
             mDefaultColors[colorIndex] = colorValue;
         }
+
+        if (!cursorPropExists)
+            setCursorColorForBackground();
+    }
+
+    /**
+     * If the "cursor" color is not set by user, we need to decide on the appropriate color that will
+     * be visible on the current terminal background. White will not be visible on light backgrounds
+     * and black won't be visible on dark backgrounds. So we find the perceived brightness of the
+     * background color and if its below the threshold (too dark), we use white cursor and if its
+     * above (too bright), we use black cursor.
+     */
+    public void setCursorColorForBackground() {
+        int backgroundColor = mDefaultColors[TextStyle.COLOR_INDEX_BACKGROUND];
+        int brightness = TerminalColors.getPerceivedBrightnessOfColor(backgroundColor);
+        if (brightness > 0) {
+            if (brightness < 130)
+                mDefaultColors[TextStyle.COLOR_INDEX_CURSOR] = 0xffffffff;
+            else
+                mDefaultColors[TextStyle.COLOR_INDEX_CURSOR] = 0xff000000;
+        }
     }
 
 }

terminal-emulator/src/main/java/com/termux/terminal/TerminalColors.java

@@ -1,5 +1,7 @@
 package com.termux.terminal;
 
+import android.graphics.Color;
+
 /** Current terminal colors (if different from default). */
 public final class TerminalColors {
 
@@ -73,4 +75,22 @@ public final class TerminalColors {
         if (c != 0) mCurrentColors[intoIndex] = c;
     }
 
+    /**
+     * Get the perceived brightness of the color based on its RGB components.
+     *
+     * https://www.nbdtech.com/Blog/archive/2008/04/27/Calculating-the-Perceived-Brightness-of-a-Color.aspx
+     * http://alienryderflex.com/hsp.html
+     *
+     * @param color The color code int.
+     * @return Returns value between 0-255.
+     */
+    public static int getPerceivedBrightnessOfColor(int color) {
+        return (int)
+            Math.floor(Math.sqrt(
+                Math.pow(Color.red(color), 2) * 0.241 +
+                    Math.pow(Color.green(color), 2) * 0.691 +
+                    Math.pow(Color.blue(color), 2) * 0.068
+            ));
+    }
+
 }