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Frame Prediction
There are two important kinds of
frames in Video Coding: I (intra) frames and P (predicted) frames.
P-frames contributes significantly towards the high compression
ratios. Depends upon the type of prediction, one more frame predication method is also
widely used, named B (bidirectional) frames.
Intra frames do not refer to
other frames, making them suitable as key frames. They are,
essentially, self-contained compressed images. Consider the
following as a film strip.
Intra Frame (I-Frame)
This First frame will be coded independently.
Predicted Frame (P-Frame)
Second frame will be code only with
differences from the first frame
Predicted Frame (P-Frame)
Third frame will be code only with
differences from the second frame
Predicted Frame (P-Frame)
Fourth frame will be code only with
differences from the third frame
In this film strip, the first
frame is an I-frame. The following frames are predicted from the
first-frame and hence these frames are predicted frames and is
known as P-frames.
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I frame
Intra frame is essentially the
first frame to encode but with less amount of compression. This
frame is also known as ‘key frame’ because the preceding frames are
encoded using the information available from this frame. Intra-prediction utilizes
spatial correlation in each frame to reduce the amount of
transmission data necessary to represent the picture.
Intra-frame is more or less
similar to image compression like JPEG or GIF. They is coded without
any dependencies to other frames. This type of frame is in which a complete image
is stored in the data stream. Thus an Intra-frame can be decoded
of its own without referring other frames. This kind of frames
are formed using Intra-Predication methods, which are discussed
below.
H.264 view:-
H.264 performs
intra-prediction on two different sized blocks: 16x16 (the
entire macroblock) and 4x4. 16x16 prediction is generally chosen
for areas of the picture that are smooth. 4x4 prediction, on the
other hand, is useful for predicting more detailed sections of
the frame. In the following picture some of the locations are
pointed out, where we can use 16x16 and 4x4.
The general idea is to predict a block, whether it be
a 4x4 or 16x16 block, based on surrounding pixels using a mode
that results in a prediction that most closely resembles the
actual pixels in that block.
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P-frame
Inter
prediction
P-frames are
predicted by using the previous P or I-frame. The frames 2 to 4 are
Predicted frames.
The Inter frames are encoded
from the second frame onwards from the incoming frames. This
type of frames is responsible for the most reduction of the
video stream. This is possible by extracting only the motion
information from frames.
Motion
estimation
The motion estimation
algorithms are used in the encoding of Inter frames.H.264
encoding supports sub-pixel resolution formotion vectors,
meaning that the reference block is actually calculated by
interpolating inside a block of real pixels. The motion vectors
for luma blocks are expressed at quarter-pixel resolution, and
for chroma blocks the accuracy can be eighth-pixel accuracy.
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B-frame
B-frames are bidirectional predicted frames. As
the name suggests, B-frames rely on the frames preceding and
following them. B-frames contain only the data that have changed
from the preceding frame or are different from the data in the
very next frame. The following figure shows
frame number 2 and 3 are B-frames.
B frames are interesting for
two facts. First they have a slightly better prediction. And
second and more important, they do not impact the quality of
following frames, so they can be coded with lower quality
without degrading the whole sequence.
Since B-frames depend on both
past and future picture, the decoder have to be fed with future
I-P frames before being able to decode them.
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