BITMAP图片压缩算法三则--bilinear、nearest、cubic

原文：http://blog.chinaunix.net/uid-253932-id-3037805.html

工作需要，要弄截图且缩小。截图倒是好说，WIN API可以搞定，但是缩小且尽量不失真，这个对我来说难度太大了吧。这里主要说说缩小的算法。我从网上找到两个算法分别是bilinear和nearest。但是效果看上去太差了。我先贴上处理后的三种算法效果图，然后再贴算法。

bilinear

nearest

cubic

当然大家可以看的出来，效果最好的自然是Cubic，提供一些该算法的资料，不过我自己是没看懂http://en.wikipedia.org/wiki/Cubic_function。cubic是我们公司多媒体方面的牛人帮我写的。

以下是代码：

#include "stdafx.h"
#include <stdio.h>
#include <string>
#include <windows.h>
#include <math.h>
using namespace std;
enum StretchMode
{
    nearest, //最临近插值算法
    bilinear, //双线性内插值算法
    cubic
};
const int CUBIC_SIZE = 3;
int getSincVec(float *pfVec, float fPos, float fScale)
{
    int nPos = (int)fPos;
    fPos -= (float)nPos;
    memset(pfVec, 0, sizeof(float) * CUBIC_SIZE);
    float fSum = 0.f;
    for (int i=0; i<CUBIC_SIZE; i++)
    {
        float fTmp = (i - 1.f + fPos) * 3.14159265358979323846f;
        if (fTmp > 0.00001 || fTmp < -0.00001)
        {
            pfVec[i] = sinf(fTmp * fScale) / fTmp;
        }
        else
        {
            pfVec[i] = 1.f;
        }
        fSum += pfVec[i];
    }
    fSum = 1.f / fSum;
    for (int i=0; i<CUBIC_SIZE; i++)
    {
        pfVec[i] *= fSum;
    }
    return nPos;
}
void calcSincMat(float *pfMat, float *pfVecY, float *pfVecX)
{
    for (int i=0; i<CUBIC_SIZE; i++)
    {
        for (int j=0; j<CUBIC_SIZE; j++)
        {
            pfMat[i * CUBIC_SIZE + j] = pfVecY[i] * pfVecX[j];
        }
    }
}
void Stretch(const string& srcFile,const string& desFile,int desW,int desH,StretchMode mode)
{
    BITMAPFILEHEADER bmfHeader;
    BITMAPINFOHEADER bmiHeader;
    FILE *pFile;
    if ((pFile = fopen(srcFile.c_str(),"rb")) == NULL)
    {
        printf("open bmp file error.");
        exit(-1);
    }
    //读取文件和Bitmap头信息
    fseek(pFile,0,SEEK_SET);
    fread(&bmfHeader,sizeof(BITMAPFILEHEADER),1,pFile);
    fread(&bmiHeader,sizeof(BITMAPINFOHEADER),1,pFile);
    //先不支持小于16位的位图
    int bitCount = bmiHeader.biBitCount;
    if (bitCount < 16)
    {
        exit(-1);
    }
    int srcW = bmiHeader.biWidth;
    int srcH = bmiHeader.biHeight;
    int lineSize = bitCount * srcW / 8;
    //偏移量，windows系统要求每个扫描行按四字节对齐
    int alignBytes = ((bmiHeader.biWidth * bitCount + 31) & ~31) / 8L
        - bmiHeader.biWidth * bitCount / 8L;
    //原图像缓存
    int srcBufSize = lineSize * srcH;
    BYTE* srcBuf = new BYTE[srcBufSize];
    int i,j;
    //读取文件中数据
    for (i = 0; i < srcH; i++)
    {
        fread(&srcBuf[lineSize * i],lineSize,1,pFile);
        fseek(pFile,alignBytes,SEEK_CUR);
    }
    //目标图像缓存
    int desBufSize = ((desW * bitCount + 31) / 32) * 4 * desH;
    int desLineSize = ((desW * bitCount + 31) / 32) * 4;
    BYTE *desBuf = new BYTE[desBufSize];
    double rateH = (double)srcH / desH;
    double rateW = (double)srcW / desW;
    //最临近插值算法
    if (mode == nearest)
    {
        for (i = 0; i < desH; i++)
        {
            //选取最邻近的点
            int tSrcH = (int)(rateH * i + 0.5);
            for (j = 0; j < desW; j++)
            {
                int tSrcW = (int)(rateW * j + 0.5);
                memcpy(&desBuf[i * desLineSize] + j * bmiHeader.biBitCount / 8,&srcBuf[tSrcH * lineSize] + tSrcW * bmiHeader.biBitCount / 8,bmiHeader.biBitCount / 8);
            }
        }
    }
    //双线型内插值算法
    else if (mode == bilinear)
    {
        for (i = 0; i < desH; i++)
        {
            int tH = (int)(rateH * i);
            int tH1 = min(tH + 1,srcH - 1);
            float u = (float)(rateH * i - tH);
            for (j = 0; j < desW; j++)
            {
                int tW = (int)(rateW * j);
                int tW1 = min(tW + 1,srcW - 1);
                float v = (float)(rateW * j - tW);
                //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)
                for (int k = 0; k < 3; k++)
                {
                    desBuf[i * desLineSize + j * bitCount / 8 + k] =
                        (1 - u)*(1 - v) * srcBuf[tH * lineSize + tW * bitCount / 8 + k] +
                        (1 - u)*v*srcBuf[tH1 * lineSize + tW * bitCount / 8+ k] +
                        u * (1 - v) * srcBuf[tH * lineSize + tW1 * bitCount / 8 + k] +
                        u * v * srcBuf[tH1 * lineSize + tW1 * bitCount / 8 + k];
                }
            }
        }
    }
    else
    {
        float pfVecX[CUBIC_SIZE], pfVecY[CUBIC_SIZE], pfMat[CUBIC_SIZE][CUBIC_SIZE];
        int nPixelBytes = bitCount / 8;
        for (i=1; i<desH-1; i++)
        {
            int nSrcY = getSincVec(pfVecY, rateH * i, 1.f / rateH);
            for (j = 1; j < desW-1; j++)
            {
                int nSrcX = getSincVec(pfVecX, rateW * j, 1.f / rateW);
                calcSincMat(pfMat[0], pfVecY, pfVecX);
                BYTE *pbyTmpDes = desBuf + i * desLineSize + j * nPixelBytes;
                BYTE *pbyTmpSrc = srcBuf + nSrcY * lineSize + nSrcX * nPixelBytes;
                for (int k = 0; k < 3; k++)
                {
                    float fTmp = pbyTmpSrc[0] * pfMat[1][1];
                    fTmp += pbyTmpSrc[-lineSize] * pfMat[0][1];
                    fTmp += pbyTmpSrc[lineSize] * pfMat[2][1];
                    fTmp += pbyTmpSrc[-nPixelBytes] * pfMat[1][0];
                    fTmp += pbyTmpSrc[-nPixelBytes-lineSize] * pfMat[0][0];
                    fTmp += pbyTmpSrc[-nPixelBytes+lineSize] * pfMat[2][0];
                    fTmp += pbyTmpSrc[nPixelBytes] * pfMat[1][2];
                    fTmp += pbyTmpSrc[nPixelBytes-lineSize] * pfMat[0][2];
                    fTmp += pbyTmpSrc[nPixelBytes+lineSize] * pfMat[2][2];
                    if (fTmp < 0.f) fTmp = 0.f;
                    if (fTmp > 255.f) fTmp = 255.f;
                    *pbyTmpDes++ = (BYTE)fTmp;
                    pbyTmpSrc++;
                }
            }
        }
    }
    //创建目标文件
    HFILE hfile = _lcreat(desFile.c_str(),0);
    //文件头信息
    BITMAPFILEHEADER nbmfHeader;
    nbmfHeader.bfType = 0x4D42;
    nbmfHeader.bfSize = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER)
        + desW * desH * bitCount / 8;
    nbmfHeader.bfReserved1 = 0;
    nbmfHeader.bfReserved2 = 0;
    nbmfHeader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
    //Bitmap头信息
    BITMAPINFOHEADER bmi;
    bmi.biSize=sizeof(BITMAPINFOHEADER);
    bmi.biWidth=desW;
    bmi.biHeight=desH;
    bmi.biPlanes=1;
    bmi.biBitCount=bitCount;
    bmi.biCompression=BI_RGB;
    bmi.biSizeImage=0;
    bmi.biXPelsPerMeter=0;
    bmi.biYPelsPerMeter=0;
    bmi.biClrUsed=0;
    bmi.biClrImportant=0;
    //写入文件头信息
    _lwrite(hfile,(LPCSTR)&nbmfHeader,sizeof(BITMAPFILEHEADER));
    //写入Bitmap头信息
    _lwrite(hfile,(LPCSTR)&bmi,sizeof(BITMAPINFOHEADER));
    //写入图像数据
    _lwrite(hfile,(LPCSTR)desBuf,desBufSize);
    _lclose(hfile);
}
int main(int argc, char* argv[])
{
    FILE *pFile;
    if ((pFile = fopen("e://t.bmp","rb")) == NULL)
    {
        printf("open bmp file error.");
        return -1;
    }
    string srcFile("e://t.bmp");
    string desFileN("e://nearest.bmp");
    string desFileB("e://bilinear.bmp");;
    string desFileC("e://cubic.bmp");
    //Stretch(srcFile,desFileN,235,136,nearest);
    //Stretch(srcFile,desFileB,235,136,bilinear);
    Stretch(srcFile,desFileC,235,136,cubic);
    //int alignBytes = ~31;
    //printf("alignbytes : %d",alignBytes);
    return 0;
}