In this case the encoder would make an exception and send a raw encoding for that specific block.
#IMATE FRAME PLUS#
Thus the overall size of motion vector plus prediction error will be greater than the raw encoding. If the block matching algorithm fails to find a suitable match the prediction error will be considerable.If everything goes fine, the algorithm will be able to find a matching block with little prediction error so that, once transformed, the overall size of motion vector plus prediction error is lower than the size of a raw encoding.This kind of prediction has some pros and cons:
In this case, there has been an illumination change between the block at the reference frame and the block which is being encoded: this difference will be the prediction error to this block. The following image shows the whole process graphically: Using both elements, the decoder will be able to recover the raw pixels of the block. To sum up, if the encoder succeeds in finding a matching block on a reference frame, it will obtain a motion vector pointing to the matched block and a prediction error. Those residual values are known as the prediction error and need to be transformed and sent to the decoder. This is why the encoder will compute the differences between them. In most cases the encoder will succeed, but the block found is likely not an exact match to the block it is encoding. The process of motion vector determination is called motion estimation. If the encoder succeeds on its search, the block could be encoded by a vector, known as motion vector, which points to the position of the matching block at the reference frame. This process is done by a block matching algorithm. After that, instead of directly encoding the raw pixel values for each block, the encoder will try to find a block similar to the one it is encoding on a previously encoded frame, referred to as a reference frame. 4.2 Resolution of up to ¼ pixel motion compensationĪn inter coded frame is divided into blocks known as macroblocks.
4 H.264 Inter frame prediction improvements.3 Typical Group Of Pictures (GOP) structure.
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