运动估计( Motion Estimation)
运动估计的应用有很多，最初的应用的领域是视频的编码。
运动估计算法一般分为: 像素递归法pel-recursive algorithm (PRA)和块匹配法 block-matching algorithm (BMA)。块匹配法是把图像分成若干矩形块，事先假定块做平移运动, 按照不同的准则函数对块进行匹配；像素递归法则是对每一个像素的位移进行估计。相对像素递归法，块匹配法的精确度比较较低, 但它的位移跟踪能力强, 在实际中很容易实现, 得到了广泛地应用。像素递归法中因为每个像素都参与，所以精度很高，但由于像素间距较小，因而实际使用中位移跟踪能力略逊一筹。
块匹配算法的流程
图一 块匹配算法的流程
在一个块匹配算法中，每一帧都将分为许多宏块，每个宏块由亮度块（luminance block）和色度块（chrominance block）组成。通常为了编码效率，只是在亮度块上进行运动估计。当前帧的宏块将与参考帧中指定范围内的候选宏块一一匹配，这些候选块可以看成是原始宏块对应不同位移量之后的宏块，最佳的匹配快将查找出来并记录。
由于位移是在三维空间的，而图像只是三维空间的实物在二维平面的投影，所以运动估计是一个病态问题（an ill-posed problem）。
运动补偿(Motion compensation)
用当前帧图像的运动矢量来合成下一帧图像的变换就称为运动补偿（Motion compensation）。
运动补偿的例子：
The following is a simplistic illustrated explanation of how motion compensation works. Two successive frames were captured from the movie
Elephants Dream
. As can be seen from the images, the bottom (motion compensated) difference between two frames contains significantly less detail than the prior images, and thus compresses much better than the rest.
| Type | Example Frame | Description |
| Original |
|
Full original frame, as shown on screen. |
| Difference |
|
Differences between the original frame and the next frame. |
| Motion compensated difference |
|
Differences between the original frame and the next frame, shifted right by 2 pixels. Shifting the frame compensates for the panning of the camera, thus there is greater overlap between the two frames. |
