Motion Estimation

Question 1

What is displaced frame difference

Answer 1

The DFD measures the reconstruction error

Question 2

What are the two basic assumptions of block matching

Answer 2

1. Constant translational motion over small blocks.
2. Maximum pre-determined range for the horizontal and vertical components of the motion vector at each pixel site.

Question 3

What is motion compensation?

Answer 3

Motion compensation is when the frame is reconstructed from the previous frame matching blocks as found by motion estimation.

Question 4

How can the aperture problem be mitigated?

Answer 4

1. Increasing the block size.
2. Greater chance that the block contains unique features.
3. More likely that the vector minimises the DFD and is the correct motion vector.

HOWEVER,

Increasing the block size lowers the resolution of the motion field.

Question 5

What is gradient-based motion estimation. Give the advantage and drawback of this approach.

Answer 5

1. Gradient-based motion estimation allows to efficiently find the optimal displacement, provided that the displacement is small.
2. Sub-pixelic accuracy can be obtained with a few iterations = no need to make full (2w+1)^2 of full search BM.
3. The taylor series expansion is only valid over very small distances.

Question 6

Based on the drawback of the gradient-based motion estimation, what is a potential solution?

Answer 6

1. A multi-resolution approach is usually adopted, starting at coarser resolution of the images, where large displacements correspond to fewer pixels.
2. Problem is that it discards pixels