Onion skinning is a technique used in animation that lets artists see multiple frames at once, making it easier to create smooth, fluid motion. The term comes from the translucent paper animators originally drew on, which resembled the thin layers of an onion. Outside of animation, “onion skinning” also appears in medicine, where doctors use it to describe layered patterns they see in bone, blood vessels, and organ tissue under a microscope or on imaging.
Onion Skinning in Animation
Animators at Walt Disney Studios in the 1920s drew individual frames on semi-transparent sheets called cels. By stacking these sheets on a light source, they could see the previous and next drawings beneath the one they were currently working on. This let them position characters precisely from frame to frame, ensuring that movement looked natural rather than jerky. The animators nicknamed the transparent sheets “onion skins” because of how they layered together, and the workflow became known as onion skinning.
The technique was especially important for “inbetweeners,” the artists responsible for drawing the transitional frames between key poses. If a character’s arm is raised in frame one and lowered in frame five, the inbetweener needs to draw frames two through four so the motion flows. Seeing the surrounding frames through the translucent paper made this work far more accurate than guessing.
How It Works in Digital Tools
Modern animation software replicates this layering digitally. When you turn on onion skinning in programs like Adobe Animate, Procreate, or Clip Studio Paint, the software overlays faint versions of neighboring frames on your current canvas. Previous frames typically appear in one color (often red or blue), and upcoming frames appear in another, so you can tell at a glance which direction the motion is heading.
Most tools let you adjust how many frames are visible at once, how opaque they appear, and how far ahead or behind you want to see. Lowering the opacity makes the ghost frames subtler so they don’t distract from your current drawing. Increasing the frame range gives you a wider view of the motion arc, which is helpful for complex movements like a character spinning or jumping. These controls make digital onion skinning more flexible than the original paper method, where you were limited by how many physical sheets you could stack before the image became too faint to read.
Onion Skinning in Bone Imaging
In radiology, an “onion skin” pattern describes a specific type of bone reaction visible on X-rays. When something irritates the outer layer of bone (the periosteum), the body responds by laying down new bone in concentric layers, one on top of another. Each new layer that forms away from the original bone surface triggers the growth of yet another layer, producing a ringed appearance that resembles the cross-section of an onion.
This layered reaction happens when bone is being deposited quickly, usually in response to a fast-growing tumor or a severe infection. On an X-ray, it shows up as alternating bands of dense bone and darker spaces. The darker gaps may or may not contain tumor or inflammatory tissue.
Ewing sarcoma, a bone cancer that most often affects children and young adults, classically produces this onion skin appearance on plain radiographs. The tumor creates a “moth-eaten” look in the bone itself while stimulating the periosteum to build those characteristic concentric layers around it. While the pattern is strongly associated with Ewing sarcoma, it is not exclusive to cancer. Bone infections (osteomyelitis) and other aggressive processes can produce a similar layered reaction, which is why doctors use additional imaging and biopsies to reach a final diagnosis.
Onion Skin Patterns in Other Organs
Pathologists use the same “onion skin” label when they see concentric rings of scar tissue (fibrosis) forming around structures in other parts of the body. Two of the most recognized examples involve the bile ducts and small blood vessels.
Bile Duct Fibrosis
In primary sclerosing cholangitis (PSC), a chronic liver disease, scar tissue wraps around the bile ducts in tight, concentric rings. Under a microscope, these rings look like the layers of an onion surrounding the duct opening. As the fibrosis progresses, the rings can completely close off the duct. This “onion skin” fibrosis is considered the hallmark finding of PSC, though it only shows up on biopsy in about 30 to 40% of patients with the condition.
Small Blood Vessel Changes
In cases of severe, uncontrolled high blood pressure, the walls of small arteries can thicken in a layered pattern as cells in the vessel lining multiply and fibrous tissue builds up. This narrows the inside of the vessel, restricting blood flow. When seen in kidney biopsies, these onion skin lesions are associated with dangerously high blood pressure readings, sometimes as extreme as 210/120 mmHg. The layered thickening can eventually cause the tiny blood vessels downstream to collapse, leading to kidney damage.
Why One Term Covers So Many Things
The common thread is the visual pattern: concentric, translucent, or semi-transparent layers stacked on top of each other. Whether it is an animator’s drawing paper, a bone reaction on an X-ray, scar tissue around a bile duct, or thickened artery walls under a microscope, the layered structure calls to mind the same everyday object. The term is purely descriptive, borrowed from what the layers look like rather than from any shared biological or technical mechanism. Context tells you which meaning applies. In a conversation about motion graphics, it is the animation technique. In a radiology report or pathology slide, it is describing that distinctive ringed pattern in tissue or bone.