一、實驗要求
本實驗的目标是驗證DPCM編碼的編碼效率。首先讀取一個256級的灰階圖像，采用自己設定的預測方法計算預測誤差（本次實驗報告使用左向預測），并對預測誤差進行8比特均勻量化（基本要求）。還可對預測誤差進行1比特、2比特和4比特的量化設計（提高要求）。
在DPCM編碼器實作的過程中可同時輸出預測誤差圖像和重建圖像。将預測誤差圖像寫入檔案并将該檔案輸入Huffman編碼器，得到輸出碼流、給出機率分布圖并計算壓縮比。将原始圖像檔案輸入Huffman編碼器，得到輸出碼流、給出機率分布圖并計算壓縮比。最後比較兩種系統（1.DPCM+熵編碼和2.僅進行熵編碼）之間的編碼效率（壓縮比和圖像品質）。壓縮品質以PSNR進行計算。
二、實驗原理
三、關鍵代碼
void DpcmEn(unsigned char* yBuff, unsigned char* preerr, unsigned char* level, int h, int w, int q){
int prediction;
int err;
int i;
int j;
int a;
int b;
for (i = 0; i < h; i++){
prediction = 128;
err = yBuff[i*w] - prediction;
a = (err + 128)/pow(2, 8 - q);
if(a > pow(2, q) - 1){
a = pow(2, q) - 1;
}
if(a < 0){
a = 0;
}
preerr[i*w] = a;
b = preerr[i*w]*pow(2, 8 - q) - 128 + prediction;
if(b > 255){
b = 255;
}
if(b < 0){
b = 0;
}
level[i*w] = b;
for (j = 1; j < w; j++){
prediction = level[i*w + j - 1];
err = yBuff[i*w + j] - prediction;
a = (err + 255)/pow(2, 9 - q);
if(a > pow(2, q) - 1){
a = pow(2, q) - 1;
}
if(a < 0){
a = 0;
}
preerr[i*w + j] = a;
b = preerr[i*w + j]*pow(2, 9 - q) - 255 + prediction;
if(b > 255){
b = 255;
}
if(b < 0){
b = 0;
}
level[i*w + j] = b;
}
}
}
計算PSNR
void PrintPSNR(unsigned char* ybuffer, unsigned char* levelbuffer, int w, int h) {
double mse;
double psnr;
double sum = 0;
double temp;
int i;
for (i = 0; i < w*h; i++) {
temp = pow((ybuffer[i] - levelbuffer[i]), 2);
sum += temp;
}
mse = sum/(w * h);
psnr = 10*log10((pow(2,8)-1)*(pow(2,8)-1)/mse);
cout<<"the psnr is: "<<psnr<<endl;
}
8bit量化原始圖檔、輸出出來的重建圖檔以及預測內插補點圖檔
調用了Huffman編碼進行壓縮的圖像檔案：
原圖大小為96kb，運用了Huffman編碼後壓縮後圖像大小為72.2KB，再将轉化過的圖像檔案用Huffman碼編碼，壓縮後檔案大小如下：
四、完整代碼
main函數
#include"iostream"
#include"math.h"
#include"stdio.h"
#include"malloc.h"
#include"DCPM.h"
using namespace std;
int main(int argc, char* argv[]){
char* yuvaddr = argv[1];
char* yuv2addr = argv[2];
int W = atoi(argv[3]);
int H = atoi(argv[4]);
char* yuverraddr = argv[5];
int imgsize = W*H*3/2;
int q = atoi(argv[6]);
unsigned char* yuvbuffer = new unsigned char[imgsize];
unsigned char* ybuffer = new unsigned char[imgsize*2/3];
unsigned char* ubuffer = new unsigned char[imgsize/6];
unsigned char* vbuffer = new unsigned char[imgsize/6];
unsigned char* preerrbuffer = new unsigned char[imgsize*2/3];
unsigned char* levelbuffer = new unsigned char[imgsize*2/3];
FILE* imgopen = fopen(yuvaddr,"rb");
if(imgopen == NULL){
cout<<"打開yuv檔案失敗"<<endl;
}
FILE* yuvsave = fopen(yuv2addr,"w");
if(yuvsave == NULL){
cout<<"建立yuv空白檔案失敗"<<endl;
}
FILE* yuvsave2 = fopen(yuverraddr,"w");
if(yuvsave2 == NULL){
cout<<"建立yuv空白檔案失敗"<<endl;
}
fread(yuvbuffer, sizeof(unsigned char), imgsize, imgopen);
int i;
for(i = 0; i < imgsize*2/3; i++){
ybuffer[i] = yuvbuffer[i];
}
for(i = 0; i < imgsize/6; i++){
ubuffer[i] = yuvbuffer[i+imgsize*2/3];
}
for(i = 0; i < imgsize/6; i++){
vbuffer[i] = yuvbuffer[i+imgsize*2/3+imgsize/6];
}
DpcmEn(ybuffer, preerrbuffer, levelbuffer, H, W, q);
fwrite(levelbuffer, sizeof(unsigned char), W*H, yuvsave);
fwrite(ubuffer, sizeof(unsigned char), W*H/4, yuvsave);
fwrite(vbuffer, sizeof(unsigned char), W*H/4, yuvsave);
for(i = 0; i < imgsize/6; i++){
ubuffer[i] = 128;
}
for(i = 0; i < imgsize/6; i++){
ubuffer[i] = 128;
}
fwrite(preerrbuffer, sizeof(unsigned char), W*H, yuvsave2);
fwrite(ubuffer, sizeof(unsigned char), W*H/4, yuvsave2);
fwrite(vbuffer, sizeof(unsigned char), W*H/4, yuvsave2);
PrintPSNR(ybuffer, levelbuffer, W, H);
}
DPCM.h
#ifndef DCPM_H_INCLUDED
#define DCPM_H_INCLUDED
#include"iostream"
#include"math.h"
#include"stdio.h"
#include"malloc.h"
using namespace std;
void DpcmEn(unsigned char* yBuff, unsigned char* preerr, unsigned char* level, int h, int w, int q){
int prediction;
int err;
int i;
int j;
int a;
int b;
for (i = 0; i < h; i++){
prediction = 128;
err = yBuff[i*w] - prediction;
a = (err + 128)/pow(2, 8 - q);
if(a > pow(2, q) - 1){
a = pow(2, q) - 1;
}
if(a < 0){
a = 0;
}
preerr[i*w] = a;
b = preerr[i*w]*pow(2, 8 - q) - 128 + prediction;
if(b > 255){
b = 255;
}
if(b < 0){
b = 0;
}
level[i*w] = b;
for (j = 1; j < w; j++){
prediction = level[i*w + j - 1];
err = yBuff[i*w + j] - prediction;
a = (err + 255)/pow(2, 9 - q);
if(a > pow(2, q) - 1){
a = pow(2, q) - 1;
}
if(a < 0){
a = 0;
}
preerr[i*w + j] = a;
b = preerr[i*w + j]*pow(2, 9 - q) - 255 + prediction;
if(b > 255){
b = 255;
}
if(b < 0){
b = 0;
}
level[i*w + j] = b;
}
}
}
void PrintPSNR(unsigned char* ybuffer, unsigned char* levelbuffer, int w, int h) {
double mse;
double psnr;
double sum = 0;
double temp;
int i;
for (i = 0; i < w*h; i++) {
temp = pow((ybuffer[i] - levelbuffer[i]), 2);
sum += temp;
}
mse = sum/(w * h);
psnr = 10*log10((pow(2,8)-1)*(pow(2,8)-1)/mse);
cout<<"the psnr is: "<<psnr<<endl;
}