Why Onion Skinning Is Every Animator’s Secret Weapon

Animators are time travellers: they need to visualize past, present, and future frames to create convincing animations.

Modern tools help quickly visualize the flow of a character’s motion to identify what needs editing before the entire sequence plays out―onion skinning is one of them.

Read on to learn how to see multiple points in time simultaneously to make better animations, faster.

What is Onion Skinning?

Onion skinning is a tool used in animation to see multiple frames of an action at once.

The term "onion skinning" originates from the metaphor of peeling back layers of an onion: by displaying frames before and after the current one in a translucent manner with a faint overlay, onion skinning shows the progression from one frame to the next―the past, present, and potential future frames merge together on screen.

Why Onion Skinning

Onion skinning used to be done manually by layering translucent sheets of paper (called "onionskin"), with each sheet containing a different frame.

This method provided animators with a tactile, intuitive understanding of motion, but it was time-consuming and displayed a limited capacity for revisions: changes often required redrawing entire sequences!

Modern digital onion skinning revolutionizes the process.

First, there is the obvious productivity benefit: by allowing animators to see multiple frames at once, onion skinning provides a visual reference that enables smoother and more consistent motion throughout the animation sequence. It saves time and effort that would otherwise be spent constantly toggling back and forth between frames. You can make corrections more easily, and you also get advanced user interface features like adjustable transparency and color-coded frames.

Onion skinning also enables a whole new level of precision in an animator's work: you can more accurately determine the position and timing of elements and ensure a smoother progression of movement. It's especially important in complex scenes with fast motion.

Since animators can instantly preview how their edits affect the overall sequence, they can identify and correct mistakes early on, minimizing the need for extensive revisions later and thus reducing costs.

Onion Skinning In Blender

Onion skinning comes with potential challenges: animators can encounter software limitations or face visual clutter due to multiple overlays when using this technique, and animators need to master onion skinning features to overcome these issues.

In Blender, onion skinning provides visual cues or "ghosts" of frames in an animation sequence, both before and after the current frame.

You can toggle the main onion skinning visibility through the Viewport Overlays. For a more customized experience, particularly when using Grease Pencil, onion skinning can also be activated on a per-layer basis from the layer list.

You can, of course, customize the onion skinning depending on your workflow:

• The "Mode" selection determines how the frames to be ghosted are picked. By choosing "Keyframes," Blender will show keyframes in the range specified by your "Before" and "After" settings. If you select "Frames," it will show the frames based on the same range settings. The "Selected" mode will display only those keyframes that you've manually selected in the Dope Sheet, offering precise control over which frames are ghosted.
• Opacity lets you control how prominently the ghost frames appear. This can be crucial for maintaining clarity and helping you focus on either the primary animation or its surrounding context. The "Filter by Type" option refines what kinds of frames are included in the onion skinning display, allowing for more specific frames to be visualized.
• To help distinguish between past and future frames, Blender provides the option to color-code them: previous frames are shown in one color (often red) and future frames in another (often green or blue). This color distinction makes it easier to differentiate between frames quickly and helps in planning the movement of objects and characters.

• For display tweaks, the "Fade" setting gradually decreases the opacity of ghost frames the further they are from the current frame to focus your attention appropriately.
• The "Show Start Frame" feature is particularly beneficial for loop animations: it allows the animator to visualize the first keyframe or frame as a ghost when working on the last frame of an animation, essentially enabling a seamless loop cycle.

Use Cases

Onion skinning plays a key role in integrating several animation principles during production. Depending on the task at hand, you'll probably need to tweak the onion skinning settings to focus on relevant frames:

• Timing - Onion skinning allows animators to visualize the spacing of frames, which is key to work on the timing of the animation more effectively. By observing the sequence of frames laid on top of each other, animators can discern if the motion is too fast or too slow and make precise edits.
• Anticipation - Anticipation is about preparing the audience for an action. Using onion skinning, animators can see preceding and following frames to ensure that anticipation actions (like a character crouching before jumping) are effectively depicted.
• Follow-through and overlapping actions - Follow-through are secondary actions that continue after the primary action has been completed. Onion skinning allows animators to keep track of these secondary motions by observing how they evolve frame by frame. Filtering frames helps animators fine-tune overlapping actions like hair, clothing, or appendages that move at a slightly delayed timing relative to the main action.
• Slow in/out - Slow in and slow out relate to the easing of animations where actions start slowly, pick up speed, and then slow down again toward the end. Through onion skinning, animators make sure more frames are used at the beginning and end of an action for smooth deceleration or acceleration. It's a visual representation of how densely frames are packed together.
• Pose-to-pose - Pose-to-pose animation involves drawing key poses and then filling in the in-between actions. With onion skinning, animators can efficiently create these in-betweens manually or automatically through software by observing how their key poses transition into one another.

Last but not least, onion skinning can be used to create an illusion of motion blur. By examining the ghosted images, animators strategically animate intermediate frames that simulate streaks or blurred edges to create an illusion of speed:

Conclusion

Onion skinning remains an essential technique that bridges both the rich history of traditional animation and the innovation of modern digital methods. It provides animators with the ability to view multiple frames simultaneously to create smooth movements and transitions.

While it might seem like just another tool, its impact is far-reaching across various workflows. Make sure to leverage it! For example, you could create custom keyboard shortcuts in your DCC tool to quickly navigate between frames or toggle settings while using onion skinning.