Forensic Applications of Computer Vision

Peter KovesiDepartment of Computer Science

& Software EngineeringThe University of Western Australia

Application areas

• Image enhancement

• Image metrology

• Biometric identification

Image Enhancement

Video surveillance images are typically of poor quality

camera video recorder

monitor

The quality of the display on the monitor can be misleading.The video recorder runs 24 hours/day. The tape gets worn and the recording heads get dirty. The images that are recorded can be very poor.

Image Enhancement

Common problems in images :

• Poor resolution, especially in video images.

• Poor contrast due to under or over-exposure.

• Corruption with noise.

• Motion blur or poor focus.

• Misalignment of rows from line jitter and interlacing of motion in video images.

Poor resolution:Zooming in does not help!

Create a `higher resolution’ image by mathematically interpolating pixel values

Super-resolution from multiple low resolution images

Images must be aligned accurately.Object should be planar. Irani and Peleg 1991

Capel and Zisserman 1998

Results from synthetic data can be impressive

Super-resolution from multiplelow resolution images:

Real data: Car number plate - low resolution + blur and noise.

Super-resolution from multiplelow resolution images:

Number plate reconstruction

Denoising

original image

Current method of choice is wavelet shrinkage denoising.

Donoho and Johnson 1995

Denoising

Grey scale enhanced image

original deinterlaced

wavelet denoised and contrast enhanced

original deinterlaced

wavelet denoised and contrast enhanced

Image Metrology

Calibration targets allow views of flat surfaces to be rectified.

Rectified views allow measurements to be taken.

Image Metrology

Rectified views of the fence and ground.

Criminisi, Reid and Zisserman 1999

Image Metrology If a 3D calibration frame is placed in a scene and photographed from two or more directions 3D measurements can be made.

However, care is needed to get accurate results.

Image Metrology

Stereo reconstruction

3D Measurements can be taken from the photographs long after the scene has been destroyed.

Image Metrology

Some measurements can be made in an uncalibrated image if you can determine vanishing points and the horizon.

Criminisi, Reid and Zisserman 1999

An unknown height can be determined if you know the horizon, the vertical vanishing point and a reference height.

Conclusion

Advances that have been made in Computer Vision, Photogrammetry and Projective Geometry over the last 10 years provide new opportunities for forensic science.