Video
compression technologies are about reducing and removing redundant video data so
that a digital video file can be effectively sent over a network. With
efficient compression techniques, a significant reduction in file size can be
achieved with little or no adverse effect on the visual quality. The video
quality can be affected if the file size is further lowered by raising the
compression level for a given compression technique.
Run
length coding is the video compression method sometimes called as Run Length
used in video processing. It is also used in many image formats. The role of RLE is to reduce data
redundancy. It is based on the repetition of consecutive elements. The basic
principle is to replace the sequence of the same symbol by a number of its
occurrences. Thus, according to this principle, “HHHHHYYYYYYYYYYYYYY” when
compressed yields “5H14Y”. The compression gain is thus (19-5)/19, that is,
approximately 73.7 %. Run length coding is lossless compression, because all the information
is retained after decoding.
Generally
RLE is used after quantization. RLE step re-arranges the quantized data in a
format that reduces data redundancy in bit stream. For run length encoding, the
8x8 block should be scanned into 64-entry array in a zigzag fashion, as shown
in figure1.
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Figure1: Zigzag scanning
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The
block, which has now become a 64 entry, should next be manipulated using a
modified run-length encoding algorithm transforming it into a series of integer
pairs consisting of a run component and a signed level component. The run
integer represents the number of zero entries preceding a level value in the
array. The final run of zero entries in the array ends at the 64th
block entry and not with a signed level- this reality is encoded as End of
Block. The syntax as shown in Figure2.
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Figure 2: Sample block Run length coding.
Author - Hemika Yadav
Intern Design Engineer at Silicon Mentor)
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