Optical Flow Donovan Parks What is optical flow
- Slides: 14
Optical Flow Donovan Parks
What is optical flow? • a method for estimating the motion of objects within an image sequence • answers the question “how are my pixels (objects) moving? ”
Where is optical flow used? • widely used in computer vision: • • motion detection camera jitter correction video compression video segmentation • basis on many special effects used in the movie industry
Why is calculating optical flow difficult? The Aperture Problem • often insufficient information in a local area to fully determine motion
Apparent vs. real motion
How is optical flow calculated?
Motion constraint equation • Ix, Iy, and It are just intensity derivatives • easily calculated for images • H&S suggest: • vx, vy are the pixel velocities we want to find Problem One equation with two unknowns!
What constraint do Horn and Schunck suggest? Global smoothness constraint on velocity field ¡ Why? ¡ l l Neighbouring pixels from a rigid object must have similar velocities Therefore, the velocity field in the image should vary smoothly in most places
Horn-Schunck cont… • λ controls the influence of the smoothness constraint • Iterative solution exists to find the minimum • all the ugly details are in the paper!
Example: Optical Flow in Practice
Example: Sparse Optical Flow Example by David M. Stavens (http: //ai. stanford. edu/~dstavens/)
Discussion: A word of caution ¡ Brightness constancy assumption requires: l l ¡ constant illumination conditions objects to be at least locally rigid Higher level processing often needed to “weed out” usable information from an optical flow algorithm
Conclusion ¡ ¡ Major contribution of H&S: first to propose formal method for calculating optical flow Critical point in calculating optical flow is finding another “good” equation to go along with the motion constraint equation
Questions? Thank you!
