两种图像缩放算法的对比与实现
f(i+u,j+v) = (1-u)(1-v)f(i,j) + (1-u)vf(i,j+1) + u(1-v)f(i+1,j) + uvf(i+1,j+1)
其中U和V是浮点坐标的小数部分，显然离目标点距离越近的点的权重越大，这也正符合目标点的值与离他最近的点最接近这一事实。关于这两种算法的详细说明，网上很多，这里就不多说了。下面是实现程序，这里除了两个算法之外，还涉及到位图的读写操作。具体看注释吧，需要注意的是位图中有涉及到调色板，这里为了简便，原图像最好是不带有调色板的24位位图。
1 #include "stdafx.h"
2 #include <stdio.h>
3 #include <string>
4 #include <windows.h>
5 using namespace std;
6
7 enum StretchMode
8 {
9 nearest, //最临近插值算法
10 bilinear //双线性内插值算法
11 };
12
13 void Stretch(const string& srcFile,const string& desFile,int desW,int desH,StretchMode mode)
14 {
15 BITMAPFILEHEADER bmfHeader;
16 BITMAPINFOHEADER bmiHeader;
17
18 FILE *pFile;
19 if ((pFile = fopen(srcFile.c_str(),"rb")) == NULL)
20 {
21 printf("open bmp file error.");
22 exit(-1);
23 }
24 //读取文件和Bitmap头信息
25 fseek(pFile,0,SEEK_SET);
26 fread(&bmfHeader,sizeof(BITMAPFILEHEADER),1,pFile);
27 fread(&bmiHeader,sizeof(BITMAPINFOHEADER),1,pFile);
28 //先不支持小于16位的位图
29 int bitCount = bmiHeader.biBitCount;
30 if (bitCount < 16)
31 {
32 exit(-1);
33 }
34 int srcW = bmiHeader.biWidth;
35 int srcH = bmiHeader.biHeight;
36
37 int lineSize = bitCount * srcW / 8;
38 //偏移量，windows系统要求每个扫描行按四字节对齐
39 int alignBytes = ((bmiHeader.biWidth * bitCount + 31) & ~31) / 8L
40 - bmiHeader.biWidth * bitCount / 8L;
41 //原图像缓存
42 int srcBufSize = lineSize * srcH;
43 BYTE* srcBuf = new BYTE[srcBufSize];
44 int i,j;
45 //读取文件中数据
46 for (i = 0; i < srcH; i++)
47 {
48 fread(&srcBuf[lineSize * i],lineSize,1,pFile);
49 fseek(pFile,alignBytes,SEEK_CUR);
50 }
51
52 //目标图像缓存
53 int desBufSize = ((desW * bitCount + 31) / 32) * 4 * desH;
54 int desLineSize = ((desW * bitCount + 31) / 32) * 4;
55 BYTE *desBuf = new BYTE[desBufSize];
56 double rateH = (double)srcH / desH;
57 double rateW = (double)srcW / desW;
58 //最临近插值算法
59 if (mode == nearest)
60 {
61 for (i = 0; i < desH; i++)
62 {
63 //选取最邻近的点
64 int tSrcH = (int)(rateH * i + 0.5);
65 for (j = 0; j < desW; j++)
66 {
67 int tSrcW = (int)(rateW * j + 0.5);
68 memcpy(&desBuf[i * desLineSize] + j * bmiHeader.biBitCount / 8,&srcBuf[tSrcH * lineSize] + tSrcW * bmiHeader.biBitCount / 8,bmiHeader.biBitCount / 8);
69 }
70 }
71 }
72 //双线型内插值算法
73 else
74 {
75 for (i = 0; i < desH; i++)
76 {
77 int tH = (int)(rateH * i);
78 int tH1 = min(tH + 1,srcH - 1);
79 float u = (float)(rateH * i - tH);
80 for (j = 0; j < desW; j++)
81 {
82 int tW = (int)(rateW * j);
83 int tW1 = min(tW + 1,srcW - 1);
84 float v = (float)(rateW * j - tW);
85
86 //f(i+u,j+v) = (1-u)(1-v)f(i,j) + (1-u)vf(i,j+1) + u(1-v)f(i+1,j) + uvf(i+1,j+1)
87 for (int k = 0; k < 3; k++)
88 {
89 desBuf[i * desLineSize + j * bitCount / 8 + k] =
90 (1 - u)*(1 - v) * srcBuf[tH * lineSize + tW * bitCount / 8 + k] +
91 (1 - u)*v*srcBuf[tH1 * lineSize + tW * bitCount / 8+ k] +
92 u * (1 - v) * srcBuf[tH * lineSize + tW1 * bitCount / 8 + k] +
93 u * v * srcBuf[tH1 * lineSize + tW1 * bitCount / 8 + k];
94 }
95 }
96 }
97 }
98
99 //创建目标文件
100 HFILE hfile = _lcreat(desFile.c_str(),0);
101 //文件头信息
102 BITMAPFILEHEADER nbmfHeader;
103 nbmfHeader.bfType = 0x4D42;
104 nbmfHeader.bfSize = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER)
105 + desW * desH * bitCount / 8;
106 nbmfHeader.bfReserved1 = 0;
107 nbmfHeader.bfReserved2 = 0;
108 nbmfHeader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
109 //Bitmap头信息
110 BITMAPINFOHEADER bmi;
111 bmi.biSize=sizeof(BITMAPINFOHEADER);
112 bmi.biWidth=desW;
113 bmi.biHeight=desH;
114 bmi.biPlanes=1;
115 bmi.biBitCount=bitCount;
116 bmi.biCompression=BI_RGB;
117 bmi.biSizeImage=0;
118 bmi.biXPelsPerMeter=0;
119 bmi.biYPelsPerMeter=0;
120 bmi.biClrUsed=0;
121 bmi.biClrImportant=0;
122
123 //写入文件头信息
124 _lwrite(hfile,(LPCSTR)&nbmfHeader,sizeof(BITMAPFILEHEADER));
125 //写入Bitmap头信息
126 _lwrite(hfile,(LPCSTR)&bmi,sizeof(BITMAPINFOHEADER));
127 //写入图像数据
128 _lwrite(hfile,(LPCSTR)desBuf,desBufSize);
129 _lclose(hfile);
130 }
131
132 int main(int argc, char* argv[])
133 {
134 FILE *pFile;
135 if ((pFile = fopen("e://t.bmp","rb")) == NULL)
136 {
137 printf("open bmp file error.");
138 return -1;
139 }
140 string srcFile("e://t.bmp");
141 string desFileN("e://nearest.bmp");
142 string desFileB("e://bilinear.bmp");
143 Stretch(srcFile,desFileN,800,600,nearest);
144 Stretch(srcFile,desFileB,800,600,bilinear);
145 //int alignBytes = ~31;
146 //printf("alignbytes : %d",alignBytes);
147
148 system("pause");
149 return 0;
150 }
结论：通过生成的两幅图像对比，双线性内插值算法缩放效果比最邻近插值算法好很多。