Video Coding Fundamentals

Video Compression

Compression is the act or process of compacting data into a smaller number of bits. Video compression (video coding) is the process of converting digital video into a format suitable for transmission or storage, whilst typically reducing the number of bits. Compression involves a complementary pair of systems, a compressor (encoder) and a decompressor (decoder). Data compression is achieved by removing redundancy, i.e. components that are not necessary for faithful reproduction of the data. Most video coding methods exploit both temporal and spatial redundancy to achieve compression.

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Pie de foto: : Encoder/Decoder Pair

Prediction Model

The goal of the prediction model is to reduce redundancy by forming a prediction of the data and subtracting this prediction from the current data. The prediction may be formed from previously coded frames (a temporal prediction) or from previously coded image samples in the same frame (a spatial prediction).

Temporal Prediction: prediction from the previous video frame: The simplest method of temporal prediction is to use the previous frame as the predictor for the current frame Motion estimation and compensation Block based sub pixel based​​​​​​​         

Block-Based Motion Estimation

The method of motion compensation is to compensate for movement of rectangular sections or ‘blocks’ of the current frame. Procedure for motion estimation: It is carried out for all MxN blocks Search an area in the reference frame, past or future frame, to find a similar MxN-sample region. The chosen candidate region becomes the predictor for the current MxN block (a motion compensated prediction) and is subtracted from the current block to form a residual MxN block (motion compensation). The residual block is encoded and transmitted and the offset between the current block and the position of the candidate region (motion vector) is also transmitted.    

Types of M

Prediction Model

Spatial model: intra prediction The prediction for the current block of image samples is created from previously-coded samples in the same frame Assuming that the blocks of image samples are coded in raster-scan order, which is not always the case, the upper/left shaded blocks are available for intra prediction. Once the prediction has been generated, it is subtracted from the current block to form a residual in a similar way to inter prediction.

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Pie de foto: : Intra Prediction