support rotation snapping

lib/image-editor/src/main/java/org/signal/imageeditor/core/ElementScaleEditSession.java

@@ -1,3 +1,8 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
 package org.signal.imageeditor.core;
 
 import android.graphics.Matrix;
@@ -9,13 +14,16 @@
 
 final class ElementScaleEditSession extends ElementEditSession {
 
-  private ElementScaleEditSession(@NonNull EditorElement selected, @NonNull Matrix inverseMatrix) {
+  private final float initialRotationRadians;
+
+  private ElementScaleEditSession(@NonNull EditorElement selected, @NonNull Matrix inverseMatrix, float initialRotationRadians) {
     super(selected, inverseMatrix);
+    this.initialRotationRadians = initialRotationRadians;
   }
 
   static ElementScaleEditSession startScale(@NonNull ElementDragEditSession session, @NonNull Matrix inverseMatrix, @NonNull PointF point, int p) {
     session.commit();
-    ElementScaleEditSession newSession = new ElementScaleEditSession(session.selected, inverseMatrix);
+    ElementScaleEditSession newSession = new ElementScaleEditSession(session.selected, inverseMatrix, session.selected.getLocalRotationAngle());
     newSession.setScreenStartPoint(1 - p, session.endPointScreen[0]);
     newSession.setScreenEndPoint(1 - p, session.endPointScreen[0]);
     newSession.setScreenStartPoint(p, point);
@@ -38,6 +46,7 @@ public void movePoint(int p, @NonNull PointF point) {
       editorMatrix.postScale(scale, scale);
 
       double angle = angle(endPointElement[0], endPointElement[1]) - angle(startPointElement[0], startPointElement[1]);
+      angle = RotationSnap.snapToAngle(initialRotationRadians, angle);
 
       if (!selected.getFlags().isRotateLocked()) {
         editorMatrix.postRotate((float) Math.toDegrees(angle));

lib/image-editor/src/main/java/org/signal/imageeditor/core/RotationSnap.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.imageeditor.core;
+
+final class RotationSnap {
+
+  private static final double SNAP_ANGLE_RADIANS     = Math.PI / 2d; // 90 degrees in radians
+  private static final double SNAP_THRESHOLD_RADIANS = Math.toRadians(5d);
+
+  private RotationSnap() {}
+
+  private static double snapToAngle(double angleRadians) {
+    double snappedAngle = Math.rint(angleRadians / SNAP_ANGLE_RADIANS) * SNAP_ANGLE_RADIANS;
+
+    if (Math.abs(angleRadians - snappedAngle) <= SNAP_THRESHOLD_RADIANS) {
+      return snappedAngle;
+    }
+
+    return angleRadians;
+  }
+
+  static double snapToAngle(double baseAngleRadians, double relativeAngleRadians) {
+    double absoluteAngle        = baseAngleRadians + relativeAngleRadians;
+    double snappedAbsoluteAngle = snapToAngle(absoluteAngle);
+    return snappedAbsoluteAngle - baseAngleRadians;
+  }
+}

lib/image-editor/src/main/java/org/signal/imageeditor/core/ThumbDragEditSession.java

@@ -1,3 +1,8 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
 package org.signal.imageeditor.core;
 
 import android.graphics.Matrix;
@@ -13,6 +18,7 @@ class ThumbDragEditSession extends ElementEditSession {
   private final PointF  oppositeControlPoint                = new PointF();
   private final float[] oppositeControlPointOnControlParent = new float[2];
   private final float[] oppositeControlPointOnElement       = new float[2];
+  private final float   initialRotationRadians;
 
   @NonNull
   private final          ThumbRenderer.ControlPoint controlPoint;
@@ -21,11 +27,13 @@ class ThumbDragEditSession extends ElementEditSession {
   private ThumbDragEditSession(@NonNull EditorElement selected,
                                @NonNull ThumbRenderer.ControlPoint controlPoint,
                                @NonNull Matrix inverseMatrix,
-                               @NonNull Matrix thumbContainerRelativeMatrix)
+                               @NonNull Matrix thumbContainerRelativeMatrix,
+                               float initialRotationRadians)
   {
     super(selected, inverseMatrix);
     this.controlPoint                 = controlPoint;
     this.thumbContainerRelativeMatrix = thumbContainerRelativeMatrix;
+    this.initialRotationRadians       = initialRotationRadians;
   }
 
   static EditSession startDrag(@NonNull EditorElement selected,
@@ -36,7 +44,7 @@ static EditSession startDrag(@NonNull EditorElement selected,
   {
     if (!selected.getFlags().isEditable()) return null;
 
-    ElementEditSession elementDragEditSession = new ThumbDragEditSession(selected, controlPoint, inverseViewModelMatrix, thumbContainerRelativeMatrix);
+    ElementEditSession elementDragEditSession = new ThumbDragEditSession(selected, controlPoint, inverseViewModelMatrix, thumbContainerRelativeMatrix, selected.getLocalRotationAngle());
     elementDragEditSession.setScreenStartPoint(0, point);
     elementDragEditSession.setScreenEndPoint(0, point);
     return elementDragEditSession;
@@ -72,6 +80,7 @@ public void movePoint(int p, @NonNull PointF point) {
       editorMatrix.postTranslate(-oppositeControlPoint.x, -oppositeControlPoint.y);
       editorMatrix.postScale(scale, scale);
       double angle = angle(endPointElement[0], oppositeControlPoint) - angle(startPointElement[0], oppositeControlPoint);
+      angle = RotationSnap.snapToAngle(initialRotationRadians, angle);
       rotate(editorMatrix, angle);
       editorMatrix.postTranslate(oppositeControlPoint.x, oppositeControlPoint.y);
     } else {
@@ -142,4 +151,4 @@ private static float getDistanceSquared(@NonNull PointF a, @NonNull PointF b) {
     float dy = a.y - b.y;
     return dx * dx + dy * dy;
   }
-}
+}