Motion estimation
Aug 1, 2007 12:00 PM, BY JOE ZALLER
Reducing artifacts delivers a cleaner picture and simplifies compression and decompression.
Motion compensation is used extensively in video processing, where it constitutes an essential enabling technology for meeting quality expectations in the multiformat digital delivery world. But not every technology in the motion-compensated category delivers equal quality, a fact of increasing significance as more processes requiring motion compensation are concatenated in the video distribution chain.
It is common for a signal to pass through three or more compression stages and two or more conversion stages before it reaches the viewer. Each process is motion-compensated, none are lossless, and all introduce their own set of artifacts. The higher the quality of the motion estimators in the concatenated signal chain, the better the overall picture quality delivered to the viewer.
Broadly speaking, there are three main types of motion estimation: block matching, gradient and phase correlation.
Block matching
Figure 1. An example of block matching
Click image to enlarge.
In block matching, an image on the screen is divided into a grid of blocks. (See Figure 1.) One block of the image is compared a pixel at a time with a block in the same place in the next image. If there is no motion, there is a high correlation between the two blocks.
If something has shifted, the same place position in the next field will not provide good correlation, and it will be necessary to search for the best correlation in the next image. The position that gives the best correlation is assumed to be the new location of a moving object.
Block matchers work at the pixel level, so they are poor at tracking larger objects and high-motion speeds. To overcome this, hierarchical techniques are used, where block matching is initially carried out with large blocks. Then the process is repeated at subdivided blocks all the way down to pixel resolution.
Such hierarchical techniques try to balance the ability to track large objects versus small objects. In practice, however, they fall short on accuracy when tracking small, fast-moving objects, such credit roll text. They also tend to introduce spurious motion vectors on cuts, fades, sources with noise and other irregular material.
Gradient
The gradient method is based on analyzing the relationship between the spatial and temporal luminance gradients, where the spatial luminance gradient is a property of the current image, and the temporal luminance gradient is the gradient between successive images.
This technique showed great promise when first adopted on professional equipment in the early 1990s, and it performs well for processing relatively clean, continuous video feeds. It runs into problems, however, in the real world of irregular moving pictures, where the technique may mistake a different object in the next frame for a spatial gradient.
Phase correlation
Phase correlation is the most powerful and accurate motion estimation technique for video processing. It is also the most technologically sophisticated, and an understanding of how it works requires some familiarity with frequency domain analysis and the use of Fourier transforms.
Figure 2. An example of the shift theory
Click image to enlarge.
The main principle behind phase correlation is the shift theory. (See Figure 2.) In the shift theory, a simple single sine wave is sampled at different times, t = 0 and t = +1. In the interval between these two points in time, the sine wave has moved, but there is more than one way to represent this movement. It can be understood as a displacement of 20 pixels or as a phase difference of 45°. Shift theory shows that the displacement in the time domain equates to the phase shift in the frequency domain.
While this is easy to grasp when dealing with a single sine wave, TV signals are obviously more complex. For this reason, a Fourier transform needs to break down the waveform of the video signal into a series of sine waves. (See Figure 3.) In this instance, a square wave pattern is broken down into sine waves. For each sine wave, the phase is provided. If the phase information is available from successive images, the motion can be measured.
Figure 3. Fourier transform of a square wave pattern
Click image to enlarge.
This principle is the basis for the complete phase correlation system shown in Figure 4. Spectral analysis is performed on two successive fields, and then all of the individual phase components are subtracted. The phase differences are then subjected to a reverse transform. The outputs of the reverse transform provide a correlation surface that contains peaks, the position of which corresponds to the motion between successive images.
The phase correlation system is highly accurate. One aspect of this accuracy is its sensitivity to the effects of variations in noise and lighting, which ensures high-quality performance on fades, objects moving in and out of the shade, and flashes of light.
Motion estimation applications
Motion estimation technology is used for several video processing tasks, including deinterlacing, SD-to-HD upconversion and HD-to-HD conversion. All of these require temporal interpolation to enable full-resolution, artifact-free outputs. The main benefit of reducing artifacts is, of course, a cleaner and more pleasing picture for the viewer. However, artifact elimination also simplifies subsequent compression and decompression processes, as less processing power is wasted on noise.
Within the compression systems themselves, motion estimation technology plays a fundamental role in exploiting temporal redundancy. For the same level of output quality, non-motion-compensation methods are limited to a compression ratio of 5:1, whereas motion compensation allows compression ratios of 100:1.
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