Feature Documentation:

Onion Skinning:

Onion Object

• Set Onion As Selected: If the selected object is a mesh that either has its own keyframes or has a skeleton with keyframes, that object will be set as the object that the onion skins are on, or Onion Object
• Update: Updates the skins of the onion object. This will account for all aspects of the object, including keyframe interpolation, shape keys, etc.
• Clear Selected: Clears the current onion object and all data associated with it.

Onion Skinning Modes

• Per-Frame: Creates an onion skin for every single frame of the animation.
• Per-Frame Stepped: Creates an onion skin for every nth frame of the animation using the step value.
• Direct Keys: Creates an onion skin only on frames that have keyframed values on them.

Onion Skinning Adjustments

• Amount: The amount of onion skins visible on either side of any frame. This is based on the frame the onion skin is on, and not the order of the onion skin relative to others.
• Past/Future Color: The color of that set of onion skins.
• Past/Future Start Opacity: The starting opacity of that set of onion skins relative to the scene's current frame. The nearest visible onion skin of the set will have this opacity.
• Past/Future End Opacity: The ending opacity of that set of onion skins relative to the scene's current frame. The furthest visible onion skin of the set will have this opacity.
• Past/Future Enabled: Determines whether or not that set of onion skins is visible or not.
• Use X-Ray: Disables depth on onion skins, making them visible from anywhere and through anything.
• Flat Colors: makes all onion skins solidly colored.
• Draw/Stop Drawing: Toggles the visibility of all onion skins