DPCM基本原理

DPCM即查分預測編碼調制，將一個像素與上一個像素的差量化，得到預測誤差。
本實驗中采用右側減左側得到差值來預測，量化器采用8比特均勻量化。

編碼代碼

/* y_buffer:存儲y分量的指針 x_dim:圖像水平個數 y_dim:圖像垂直個數 y_new_out[k + 1]:上圖的yk' differ[k+1]:上圖的^ek */ for (int j = 0; j < y_dim; j++) {y = y_buffer + j * x_dim;y_new_out[j * x_dim] = *y;differ[j * x_dim] = 0;for (int i = 0; i < x_dim-1; i++){differ[j * x_dim + i + 1] = ((*(y + 1) - y_new_out[j * x_dim + i]) - (*(y + 1) - y_new_out[j * x_dim + i]) % 2) / 2;y_new_out[j * x_dim + i + 1] = y_new_out[j * x_dim + i] + differ[j * x_dim + i + 1];y++;}}for (int i = 0; i<size; i++)differ[i] = differ[i] + 128;

圖像測試

原圖：
復原圖

差值圖：

原圖：

復原圖：

差值圖：

采用huffman編碼器進行編碼：

原圖原圖壓縮原圖壓縮比差值圖差值圖壓縮差值圖壓縮比

Birds	1,153KB	1,099KB	1.049	576KB	191KB	3.016
Lena	193KB	184KB	1.049	96KB	48KB	2.000

計算psnr判斷壓縮質量：
PSNR：峰值信噪比
MSE：均方誤差
PSNR越大，圖像失真越小，即壓縮質量越好

計算psnr的代碼：

void CountPSNR(unsigned char* oldBuf, unsigned char* newBuf, int n) {double mse;double psnr;double sum = 0;for (int i = 0; i < n; i++){sum = sum + (oldBuf[i] - newBuf[i]) * (oldBuf[i] - newBuf[i]);}mse = sum / n;psnr = 10 * log10(255 * 255 / mse);printf("psnr=%lf", psnr); }

經過計算，Birds的psnr=33.500681
Lena的psnr=27.042175

由數據可知，圖像在經過DPCM前使用huffman編碼壓縮效果很差幾乎無法壓縮，經過DPCM編碼后數據更容易被壓縮，壓縮效率大幅提高；經過DPCM后恢復出的圖片壓縮質量較好。

for (int i = 0; i < x_dim-1; i++){differ[j * x_dim + i + 1] = ((*(y + 1) - y_new_out[j * x_dim + i]) - (*(y + 1) - y_new_out[j * x_dim + i]) % 4) / 4;//修改此處“% 2) / 2”中的兩個2y_new_out[j * x_dim + i + 1] = y_new_out[j * x_dim + i] + differ[j * x_dim + i + 1];y++;}

通過修改上述代碼編碼代碼中的2為32、128、256可將8比特量化改為4、2、1比特量化。（將2改為（2^n）即為9-n比特量化）

完整代碼

rgb2yuv.h：

int RGB2YUV(int x_dim, int y_dim, void* bmp, void* y_out, void* u_out, void* v_out, int flip,unsigned char*y_new_out,unsigned char*differ_out);void InitLookupTable();#pragma once

rgb2yuv.cpp:

#include "stdlib.h" #include "rgb2yuv.h" #include <math.h> #include <stdio.h> static float RGBYUV02990[256], RGBYUV05870[256], RGBYUV01140[256]; static float RGBYUV01684[256], RGBYUV03316[256]; static float RGBYUV04187[256], RGBYUV00813[256];int RGB2YUV(int x_dim, int y_dim, void* bmp, void* y_out, void* u_out, void* v_out, int flip,unsigned char* y_new_out,unsigned char* differ) {static int init_done = 0;long i, j, size;unsigned char* r, * g, * b;unsigned char* y, * u, * v;unsigned char* pu1, * pu2, * pv1, * pv2, * psu, * psv;unsigned char* y_buffer, * u_buffer, * v_buffer;unsigned char* sub_u_buf, * sub_v_buf;if (init_done == 0){InitLookupTable();init_done = 1;}// check to see if x_dim and y_dim are divisible by 2if ((x_dim % 2) || (y_dim % 2)) return 1;size = x_dim * y_dim;// allocate memoryy_buffer = (unsigned char*)y_out;sub_u_buf = (unsigned char*)u_out;sub_v_buf = (unsigned char*)v_out;u_buffer = (unsigned char*)malloc(size * sizeof(unsigned char));v_buffer = (unsigned char*)malloc(size * sizeof(unsigned char));if (!(u_buffer && v_buffer)){if (u_buffer) free(u_buffer);if (v_buffer) free(v_buffer);return 2;}b = (unsigned char*)bmp;y = y_buffer;u = u_buffer;v = v_buffer;// convert RGB to YUVif (!flip) {for (j = 0; j < y_dim; j++){y = y_buffer + (y_dim - j - 1) * x_dim;u = u_buffer + (y_dim - j - 1) * x_dim;v = v_buffer + (y_dim - j - 1) * x_dim;for (i = 0; i < x_dim; i++) {g = b + 1;r = b + 2;*y = (unsigned char)(RGBYUV02990[*r] + RGBYUV05870[*g] + RGBYUV01140[*b]);*u = (unsigned char)128;*v = (unsigned char)128;b += 3;y++;u++;v++;}}}else {for (j = 0; j < y_dim; j++){for (i = 0; i < x_dim; i++){g = b + 1;r = b + 2;*y = (unsigned char)(RGBYUV02990[*r] + RGBYUV05870[*g] + RGBYUV01140[*b]);*u = (unsigned char)128;*v = (unsigned char)128; b += 3;y++;u++;v++;}}}for (int j = 0; j < y_dim; j++) {y = y_buffer + j * x_dim;y_new_out[j * x_dim] = *y;differ[j * x_dim] = 0;for (int i = 0; i < x_dim-1; i++){differ[j * x_dim + i + 1] = ((*(y + 1) - y_new_out[j * x_dim + i]) - (*(y + 1) - y_new_out[j * x_dim + i]) % 2) / 2;y_new_out[j * x_dim + i + 1] = y_new_out[j * x_dim + i] + differ[j * x_dim + i + 1];y++;}}for (int i = 0; i<size; i++)differ[i] = differ[i] + 128;// subsample UVfor (j = 0; j < y_dim / 2; j++){psu = sub_u_buf + j * x_dim / 2;psv = sub_v_buf + j * x_dim / 2;pu1 = u_buffer + 2 * j * x_dim;pu2 = u_buffer + (2 * j + 1) * x_dim;pv1 = v_buffer + 2 * j * x_dim;pv2 = v_buffer + (2 * j + 1) * x_dim;for (i = 0; i < x_dim / 2; i++){*psu = (*pu1 + *(pu1 + 1) + *pu2 + *(pu2 + 1)) / 4;*psv = (*pv1 + *(pv1 + 1) + *pv2 + *(pv2 + 1)) / 4;psu++;psv++;pu1 += 2;pu2 += 2;pv1 += 2;pv2 += 2;}}free(u_buffer);free(v_buffer);return 0; }void InitLookupTable() {int i;for (i = 0; i < 256; i++) RGBYUV02990[i] = (float)0.2990 * i;for (i = 0; i < 256; i++) RGBYUV05870[i] = (float)0.5870 * i;for (i = 0; i < 256; i++) RGBYUV01140[i] = (float)0.1140 * i;for (i = 0; i < 256; i++) RGBYUV01684[i] = (float)0.1684 * i;for (i = 0; i < 256; i++) RGBYUV03316[i] = (float)0.3316 * i;for (i = 0; i < 256; i++) RGBYUV04187[i] = (float)0.4187 * i;for (i = 0; i < 256; i++) RGBYUV00813[i] = (float)0.0813 * i; }

main.cpp:

#include <stdio.h> #include <stdlib.h> #include <malloc.h> #include <windows.h> #include "rgb2yuv.h" #include"math.h" BITMAPFILEHEADER File_header; BITMAPINFOHEADER Info_header; #pragma warning(disable:4996) #define u_int8_t unsigned __int8 #define u_int unsigned __int32 #define u_int32_t unsigned __int32 #define FALSE false #define TRUE true/** rgb2yuv* required arg1 should be the input RAW RGB24 file* required arg2 should be the output RAW YUV12 file*/ int main() {/* variables controlable from command line */u_int frameWidth; /* --width=<uint> */u_int frameHeight; /* --height=<uint> */bool flip = FALSE; /* --flip */unsigned int i;void Write(const char* bmpflie, const char* yuvfile, u_int8_t * rgbBuf, u_int8_t * yBuf, u_int8_t * uBuf, u_int8_t * vBuf, u_int frameWidth, u_int frameHeight);void CountPSNR(unsigned char* oldBuf, unsigned char* newBuf, int n);/* internal variables */const char* bmpFileName = NULL;const char* yuvFileName1 = NULL;const char* yuvFileName2 = NULL;FILE* bmpFile = NULL;FILE* yuvFile1 = NULL;FILE* yuvFile2 = NULL;u_int8_t* rgbBuf = NULL;u_int8_t* yBuf = NULL;u_int8_t* yNewBuf = NULL;u_int8_t* differBuf = NULL;u_int8_t* uBuf = NULL;u_int8_t* vBuf = NULL;u_int32_t videoFramesWritten = 0;/* begin process command line *//* point to the specified file names */bmpFileName = "Birds.bmp";yuvFileName1 = "Birds_out.yuv";yuvFileName2 = "Birds_out_re.yuv";//yuvFileName = "Birds_outwhole.yuv";/* open the RGB file */bmpFile = fopen(bmpFileName, "rb");if (bmpFile == NULL){printf("cannot find bmp file\n");exit(1);}if (fread(&File_header, sizeof(BITMAPFILEHEADER), 1, bmpFile) != 1){printf("read file header error!");exit(0);}if (File_header.bfType != 0x4D42){printf("Not bmp file!");exit(0);}if (fread(&Info_header, sizeof(BITMAPINFOHEADER), 1, bmpFile) != 1){printf("read info header error!");exit(0);}printf("The input bmp file is %s\n", bmpFileName);frameWidth = Info_header.biWidth;frameHeight = Info_header.biHeight;printf("width: %d , height: %d\n ", frameWidth, frameHeight);/* open the RAW file */yuvFile1 = fopen(yuvFileName1, "wb");yuvFile2 = fopen(yuvFileName2, "wb");if (yuvFile1 == NULL|| yuvFile2 == NULL){printf("cannot find yuv file\n");exit(1);}else{printf("The output yuv file is %s and %s\n", yuvFileName1, yuvFileName2);}/* get an input buffer for a frame */rgbBuf = (u_int8_t*)malloc(frameWidth * frameHeight * 3);/* get the output buffers for a frame */yBuf = (u_int8_t*)malloc(frameWidth * frameHeight);yNewBuf = (u_int8_t*)malloc(frameWidth * frameHeight);differBuf = (u_int8_t*)malloc(frameWidth * frameHeight);uBuf = (u_int8_t*)malloc((frameWidth * frameHeight) / 4);vBuf = (u_int8_t*)malloc((frameWidth * frameHeight) / 4);if (rgbBuf == NULL || yBuf == NULL || uBuf == NULL || vBuf == NULL){printf("no enought memory\n");exit(1);}while (fread(rgbBuf, 1, frameWidth * frameHeight * 3, bmpFile)){if (RGB2YUV(frameWidth, frameHeight, rgbBuf, yBuf, uBuf, vBuf, flip,yNewBuf,differBuf)){printf("error");return 0;}fwrite(differBuf, 1, frameWidth * frameHeight, yuvFile1);fwrite(uBuf, 1, (frameWidth * frameHeight) / 4, yuvFile1);fwrite(vBuf, 1, (frameWidth * frameHeight) / 4, yuvFile1);fwrite(yNewBuf, 1, frameWidth * frameHeight, yuvFile2);fwrite(uBuf, 1, (frameWidth * frameHeight) / 4, yuvFile2);fwrite(vBuf, 1, (frameWidth * frameHeight) / 4, yuvFile2);}CountPSNR(yBuf, yNewBuf, frameWidth * frameHeight);/* cleanup */free(rgbBuf);free(yBuf);free(uBuf);free(vBuf);fclose(bmpFile);fclose(yuvFile1);fclose(yuvFile2);return(0); } void CountPSNR(unsigned char* oldBuf, unsigned char* newBuf, int n) {double mse;double psnr;double sum = 0;for (int i = 0; i < n; i++){sum = sum + (oldBuf[i] - newBuf[i]) * (oldBuf[i] - newBuf[i]);}mse = sum / n;psnr = 10 * log10(255 * 255 / mse);printf("psnr=%lf", psnr); }

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