文档介绍：教你辨别房屋出租骗子方法
Compression
Store more images
Transmit image to PC in less time
JPEG (Joint Photographic Experts Grouing example
Pixel frequencies on left
Pixel value –1 occurs 15 times
Pixel value 14 occurs 1 time
Build Huffman tree from bottom up
Create one leaf node for each pixel value and assign frequency as node’s value
Create an internal node by joining any two nodes whose sum is a minimal value
This sum is internal nodes value
Repeat until complete binary tree
Traverse tree from root to leaf to obtain binary code for leaf’s pixel value
Append 0 for left traversal, 1 for right traversal
Huffman encoding is reversible
No code is a prefix of another code
144
5
3
2
1
0
-2
-1
-10
-5
-3
-4
-8
-9
6
14
1
1
2
1
1
2
1
2
2
4
3
5
4
6
5
9
5
10
5
11
5
14
6
17
8
18
15
29
35
64
1
Pixel frequencies
Huffman tree
Huffman codes
12
Archive step
Record starting address and image size
Can use linked list
One possible way to archive images
If max number of images archived is N:
Set aside memory for N addresses and N image-size variables
Keep a counter for location of next available address
Initialize addresses and image-size variables to 0
Set global memory address to N x 4
Assuming addresses, image-size variables occupy N x 4 bytes
First image archived starting at address N x 4
Global memory address updated to N x 4 + (compressed image size)
Memory requirement based on N, image size, and average compression ratio
13
Uploading to PC
When connected to PC and upload command received
Read images from memory
Transmit serially using UART
While transmitting
Reset pointers, image-size variables and global memory pointer accordingly
14
Requirements Specification
System’s requirements – what system should do
Nonfunctional requirements
Constraints on design metrics (., “should use watt or less”)
Functional requirements
System’s behavior (., “output X should be input Y times 2”)
Initial specification may be very genera