w8 gemini Flashcards
List the constraints typically applied to solving the video correspondence problem.
Spatial coherence (neighboring points have similar optical flow) and Small motion (optical flow vectors have small magnitude).
Note circumstances in which the spatial coherence constraint fails.
It fails at discontinuities between surfaces at different depths, or surfaces with different motion.
Note circumstances in which the small motion constraint fails.
It fails if relative motion is fast or frame rate is slow.
Define what is meant by the “aperture problem”.
The aperture problem refers to the fact that the direction of motion of a small image patch can be ambiguous.
Suggest how the aperture problem can be overcome.
- Integrating information from many local motion detectors / image patches, or 2. by giving preference to image locations where image structure provides unambiguous information about optic flow (e.g. corners).
Consider a traditional barber’s pole. When the pole rotates on its axis, in which direction is the motion field?
Horizontal.
Consider a traditional barber’s pole. When the pole rotates on its axis, in which direction is the optic flow?
Vertical.
Explain why the optic flow of a rotating barber’s pole appears vertical, despite the actual motion being horizontal, in the context of the aperture problem.
The stripes of the pole provide ambiguous information. Only where the stripes meet the top and bottom and the sides of the pole are corners present, and hence, there is (seemingly) unambiguous information.
What is video in the context of computer vision?
A series of N images, or frames, acquired at discrete time instants tk=to+kΔt, where Δt is a fixed time interval and k=0,1,…,N-1.
How is video similar to stereo vision?
Both deal with more than one image.
What are some advantages of using video compared to static images or stereo vision?
Inference of 3D structure, segmentation of objects from background without recovery of depth, and inference of self and object motion.
What are the two main types of motion that cause changes in the projection of a scene point in a video?
Object motion and camera motion (or ‘ego motion’).
What is optic flow?
The apparent motion of objects, surfaces, and edges in a visual scene caused by the relative motion between an observer (an eye or a camera) and the scene.
Distinguish between an optic flow vector and an optic flow field.
An optic flow vector represents the image motion of a single scene point. An optic flow field is the collection of all optic flow vectors in an image.
What are the two main categories of optic flow fields?
Sparse (vectors defined only for specified features) and dense (vectors defined everywhere).
What is the analogy between optic flow vectors and disparity vectors?
Optic flow vectors are analogous to disparity vectors in stereo vision.
What is required to measure optic flow?
Finding correspondences between images.
What is the motion field?
The true image motion of a scene point, which is the actual projection of the relative motion between the camera and the 3D scene.
Explain why optic flow is often an approximation of the motion field.
Optic flow is what we can measure in the image, but it might not perfectly represent the actual 3D motion due to factors like illumination changes or non-Lambertian surfaces.
Give an example where the motion field is non-zero, but the optic flow is zero.
A smooth, Lambertian, uniform sphere rotating around a diameter.
Give an example where the motion field is zero, but the optic flow is non-zero.
A stationary, specular sphere and a moving light source.
Give an example where the motion field and optic flow differ.
A barber’s pole: motion field is horizontal, optic flow is vertical.
Why must we estimate the motion field by observing the optic flow?
Because the motion field cannot be directly observed.
What is the video correspondence problem?
The problem of finding corresponding points in different frames of a video to measure optic flow.