How Do I Lock an Image Plane in Blender?

To "lock" the transformation of an object, such as an image plane, to a specific plane in Blender during a transform operation (like moving or scaling), you use the Plane Locking feature with specific hotkeys.

Understanding Plane Locking

Plane Locking is a powerful transformation constraint in Blender. It works by locking the transformation to two axes simultaneously, effectively creating a plane on which the object can move or scale freely. Conversely, this means the transformation is forbidden along the one axis perpendicular to the chosen plane.

For instance, locking to the XY plane means movement is restricted to the horizontal plane (along the X and Y axes), while movement along the Z (up/down) axis is prevented.

How to Use Plane Locking Hotkeys

The provided reference details the hotkeys for Plane Locking: Shift-X, Shift-Y, Shift-Z, or Shift-MMB. These hotkeys are used after initiating a transformation (like Move, Scale, or Rotate) but before confirming it.

Here's how to apply Plane Locking to your image plane object:

1. Select the image plane object you want to transform.

2. Initiate a transform operation:

   • Press G for Move (Grab).
   • Press S for Scale.
   • Press R for Rotate.

3. While the transformation is active, press one of the Plane Locking hotkeys:

   • Press Shift-X to lock transformation to the YZ Plane (forbids movement/scaling along the X-axis).
   • Press Shift-Y to lock transformation to the XZ Plane (forbids movement/scaling along the Y-axis).
   • Press Shift-Z to lock transformation to the XY Plane (forbids movement/scaling along the Z-axis).
   • Alternatively, press Shift-MMB (Middle Mouse Button) after moving the mouse slightly in the desired general direction. Blender will attempt to guess the plane based on your initial movement direction.

4. Move or scale the object within the restricted plane.

5. Confirm the transformation with LMB (Left Mouse Button) or press Enter.

6. Cancel the transformation with RMB (Right Mouse Button) or press Esc.

Plane Locking Hotkeys Summary

Hotkey	Plane Locked To	Axis Forbidden	Common Use Case
Shift-X	YZ Plane	X-axis	Moving/Scaling freely in the side view.
Shift-Y	XZ Plane	Y-axis	Moving/Scaling freely in the front/back view.
Shift-Z	XY Plane	Z-axis	Moving/Scaling freely on a horizontal plane.
Shift-MMB	Auto-detected	Auto-detected	Quick locking based on mouse gesture direction.

Using these hotkeys allows you to precisely control the movement or scaling of your image plane object, keeping it constrained to a specific 2D plane in your 3D scene during the transformation process.