Activity and Motion Detection in Videos

8x8 Blocks

Block size relative to image size

• Block 24x28
• 1728 blocks per frame
• Image Size:
• 36x48 blocks

Motion Measure Computation

• We use spatial-temporal blocks to represent videos
• Each block consists of NBLOCK x NBLOCK pixels from 3 consecutive frames
• Those pixel values are reduced to K principal components using PCA (Kahrunen-Loeve trans.)
• In our applications, NBLOCK=4, K=10
• Thus, we project 48 gray level values to a texture vector with 10 PCA components

3D Block Projection with PCA (Kahrunen-Loeve trans.)

• 48-component block vector (4*4*3)

• -0.5221 -0.0624 -0.1734 -0.2221 -0.2621 -0.4739 -0.4201 -0.4224 -0.0734 -0.1386

• 10 principal components

• t+1 t t-1

• 4*4*3 spatial-temporal block
• Location I=24, J=28,
• time t-1, t, t+1

• Motion Measure Computation

Texture of spatiotemporal blocks works better than color pixel values

• More robust
• Faster

• We illustrate this with texture trajectories.

• 499

• 624

• 863

• 1477

Trajectory of block (24,8) (Campus 1 video)

• Space of
• spatiotemporal
• block vectors

• Moving blocks corresponds
• to regions of high local variance,
• i.e., higher spread

• Campus 1 video
• block I=24, J=28

• Standardized PCA components of RGB pixel values at pixel location (185,217) that is inside of block (24,28).

• Comparison to the trajectory of a pixel inside block (24,8)