參數(shù)資料
型號: μSAP70732-B03
廠商: NEC Corp.
英文描述: 8 Bit Single Chip Microcontrollers(8位單片微控制器)
中文描述: 8位單片機微控制器(8位單片微控制器)
文件頁數(shù): 22/166頁
文件大小: 713K
代理商: ΜSAP70732-B03
22
μSAP705100-B03, μSAP70732-B03 USER'S MANUAL
As an example, let’s consider the case in which an image consisting of 640 x 480 pixels is compressed.
To compress this image at a sampling ratio of 4:1:1 (H:V = 2:2), it is divided by 16 pixels both horizontally
and vertically, giving 40 horizontal segments and 30 vertical segments. Six blocks are extracted from each
MCU: four blocks of the Y component, one block of the Cb component, and one block of the Cr component.
Consequently, 7200 blocks (= 40 x 30 x 6) are obtained from the entire image. To these 7200 blocks,
DCT, quantization, and Huffman compression are applied in sequence.
Table 1-2. Sampling Ratio and Block
Sampling ratio
640 x 480 pixels
Number of blocks
per MCU
Total number
of blocks
Horizontal
Vertical
4:1:1
(H:V = 2:2)
40 segments
30 segments
6
7200
4:1:1
(H:V = 4:1)
20 segments
60 segments
6
7200
2:1:1
40 segments
60 segments
4
9600
1:1:1
80 segments
60 segments
3
14400
As is evident from the above table, more blocks are needed at a sampling ratio of 1:1:1 than at 4:1:1.
The greater the number of blocks, the more processing time is required. Moreover, the size of the resulting
JPEG file also increases.
In JPEG compression, processing is performed on a block-by-block basis after sampling. DCT,
quantization, and entropy encoding are performed based on the information to which of Y or Cb/Cr a given
block belongs.
In JPEG expansion, the result is obtained in units of blocks once entropy decoding, reverse quantization,
and reverse DCT have been completed.
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