Allow arbitrary rotations by being able to specify an anchor point [qtblend]

src/modules/qt/filter_qtblend.cpp

@@ -166,12 +166,23 @@ static int filter_get_image(mlt_frame frame,
     if (mlt_properties_get(properties, "rotation")) {
         double angle = mlt_properties_anim_get_double(properties, "rotation", position, length);
         if (angle != 0.0) {
-            if (mlt_properties_get_int(properties, "rotate_center")) {
+            if (mlt_properties_get(properties, "rotate_anchor")) {
+                mlt_rect anchor
+                    = mlt_properties_anim_get_rect(properties, "rotate_anchor", position, length);
+                // Use custom anchor point (x,y are normalized 0-1 coordinates) where 0, 0 is top left and 1, 1 is bottom right
+                // negative values are allowed so its possible to rotate around a point outside the rectangle
+                double anchor_x = anchor.x * rect.w;
+                double anchor_y = anchor.y * rect.h;
+                transform.translate(anchor_x, anchor_y);
+                transform.rotate(angle);
+                transform.translate(-anchor_x, -anchor_y);
+            } else if (mlt_properties_get_int(properties, "rotate_center")) {
+                // old style rotation (from center) to keep compatibility, equivalent to rotate_anchor = 0.5, 0.5
                 transform.translate(rect.w / 2.0, rect.h / 2.0);
                 transform.rotate(angle);
                 transform.translate(-rect.w / 2.0, -rect.h / 2.0);
             } else {
-                // old style rotation (from top left corner) to keep compatibility
+                // old style rotation (from top left corner) to keep compatibility, equivalent to rotate_anchor = 0, 0
                 transform.rotate(angle);
             }
             hasAlpha = true;

src/modules/qt/filter_qtblend.yml

@@ -2,7 +2,7 @@ schema_version: 7.0
 type: filter
 identifier: qtblend
 title: Composite and transform
-version: 2
+version: 3
 copyright: Meltytech, LLC
 creator: Jean-Baptiste Mardelle
 license: LGPLv2.1
@@ -56,6 +56,16 @@ parameters:
     mutable: yes
     widget: checkbox
 
+  - identifier: rotate_anchor
+    title: Rotation anchor point
+    type: rect
+    description: >
+      Keyframable anchor point for rotation. X and Y coordinates define the rotation center.
+      Values are normalized (0-1 for inside rectangle, negative or >1 for outside).
+      Default is center (0.5, 0.5). Takes precedence over rotate_center when set.
+    mutable: yes
+    animation: yes
+
   - identifier: background_color
     title: Background color
     type: integer