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## 运动补偿

(2007-07-26 16:03:28)

## 全局运动补偿

• 该模型仅仅使用少数的参数对全局的运行进行描述，参数所占用的码率基本上可以忽略不计。
• 该方法不对帧进行分区编码，这避免了分区造成的块效应。
• 在时间方向的一条直线的点如果在空间方向具有相等的间隔，就对应了在实际空间中连续移动的点。其它的运动估计算法通常会在时间方向引入非连续性。

## 重叠分块运动补偿

Overlapped block motion compensation (OBMC) is a good solution to these problems because it not only increases prediction accuracy but also avoids blocking artifacts. 重叠分块运动补偿(OBMC for Overlapped block motion compensation)是一种更好的解决方案，它不但能增加预测精度，而且能够避免块失真。

When using OBMC,blocks are typically twice as big in each dimension and overlap quadrant-wise with all 8 neighbouring blocks.

Thus, each pixel belongs to 4 blocks. In such a scheme, there are 4 predictions for each pixel which are summed up to a weighted mean. 因此，每个像素第属于4个分块。基于此方案，每个像素的4个预测值求和后得到一个加权平均数。

For this purpose, blocks are associated with a window function that has the property that the sum of 4 overlapped windows is equal to 1 everywhere. 为此目的，分块被关联到一个窗口函数，该窗口函数具有任何地方的4个重叠窗口的总和为1的属性。

Studies of methods for reducing the complexity of OBMC have shown that the contribution to the window function is smallest for the diagonally-adjacent block. Reducing the weight for this contribution to zero and increasing the other weights by an equal amount leads to a substantial reduction in complexity without a large penalty in quality. In such a scheme, each pixel then belongs to 3 blocks rather than 4, and rather than using 8 neighboring blocks, only 4 are used for each block to be compensated. Such a scheme is found in the H.263 Annex F Advanced Prediction mode.

## 运动估计

Motion estimation (BME, OBME) is the process of finding optimal or near-optimal motion vectors. The amount of prediction error for a block is often measured using the mean squared error (MSE) or sum-of-absolute-differences (SAD) between the predicted and actual pixel values over all pixels of the motion-compensated region.

To find optimal motion vectors, one basically has to calculate the block prediction error for each motion vector within a certain search range and pick the one that has the best compromise between the amount of error and the number of bits needed for motion vector data. The motion estimation technique of simply exhaustively testing all possible motion representations to perform such an optimization is called full search. 发现最优向量，一个最基本的方法是不得不为在固定探测范围内，给每一个运动向量，计算块的预测误差計算鄰近禎之中找尋前後frame之中相似的Macro Block，兩者之間的差異值。以及估算表示此Motion Vector所需的位元數目，和在错误数和比特数之间挑选一个最折中作为运动向量值。运动估计技术尽量简单的测试在执行前一個簡單的探測測試技術為：估計所有可能的运动表现，比如這樣的最优化被称做全探测。

A faster and sub-optimal method is to use a coarse search grid for a first approximation and to refine the grid in the surrounding of this approximation in further steps. 一个稍快但不是最优的方法是用第一个近似值作为一个粗略探测栅格，然后在接下来的步骤里在近似值的周围精确栅格。

One common method is the 3-step search, which uses search grids of 3×3 motion vectors and 3 refinement steps to get an overall search range of 15×15 pixel. 一个通用办法是3步探测，用3次探测栅格；3个运动向量和3个精确步骤来得到15次15个像素范围内的全面探测。

For OBME, the pixel-wise prediction errors of a block and its overlapping neighbouring blocks have to be weighted and summed according to the window function before being squared. As in the process of successively finding/refining motion vectors some neighbouring MVs are not known yet, the corresponding prediction errors can be ignored (not added) as a sub-optimal solution. 对于分块运动估计，一个块的像素预测误差和它的附近搭接块，根据此前自乘的窗函数，都被测重和求和。

The major disadvantages of OBMC are increased computational complexity of OBME, and the fact that prediction errors and, thus, also the optimal motion vectors depend on neighbouring blocks/motion vectors. 分块运动估计最主要的缺点是增加计算的复杂性，和实际的预测误差，因而最友向量依靠于临近运动块向量。

Therefore, there is no algorithm with polynomial computational complexity that guarantees optimal motion vectors. 因此，没有一个多项式（计算的复杂性）算法可以保证最优运动向量。

However, there are near-optimal iterative and non-iterative methods with acceptable computational complexity. 然而，在可接受的计算的复杂性上，存在最接近最理想迭代和非迭代方法。

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