DPCM编码进行图像压缩

一、实验要求

本实验的目标是验证DPCM编码的编码效率。首先读取一个256级的灰度图像，采用自己设定的预测方法计算预测误差（本次实验报告使用左向预测），并对预测误差进行8比特均匀量化（基本要求）。还可对预测误差进行1比特、2比特和4比特的量化设计（提高要求）。

在DPCM编码器实现的过程中可同时输出预测误差图像和重建图像。将预测误差图像写入文件并将该文件输入Huffman编码器，得到输出码流、给出概率分布图并计算压缩比。将原始图像文件输入Huffman编码器，得到输出码流、给出概率分布图并计算压缩比。最后比较两种系统（1.DPCM+熵编码和2.仅进行熵编码）之间的编码效率（压缩比和图像质量）。压缩质量以PSNR进行计算。

二、实验原理

三、关键代码

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;
        }
    }
}
           

计算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量化原始图片、输出出来的重建图片以及预测差值图片

调用了Huffman编码进行压缩的图像文件：

原图大小为96kb，运用了Huffman编码后压缩后图像大小为72.2KB，再将转化过的图像文件用Huffman码编码，压缩后文件大小如下：

四、完整代码

main函数

#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<<"打开yuv文件失败"<<endl;
    }
    FILE* yuvsave = fopen(yuv2addr,"w");
    if(yuvsave == NULL){
        cout<<"创建yuv空白文件失败"<<endl;
    }
    FILE* yuvsave2 = fopen(yuverraddr,"w");
    if(yuvsave2 == NULL){
        cout<<"创建yuv空白文件失败"<<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;
}