如何使用GDAL重采样图像 .

  在编写重采样图像时，可以使用GDAL来读写图像，然后自己编写重采样算法（最邻近像元法，双线性内插法，三次立方卷积法等）【关于这采样算法有时间我会单独写一篇文章来说明原理的】将计算的结果写入图像中来实现。

    在GDAL的算法中，已经提供了五种重采样算法，其定义如下（位置gdalwarper.h 的46行）：

/*! Warp Resampling Algorithm */
typedef enum {
  /*! Nearest neighbour (select on one input pixel) */ GRA_NearestNeighbour=0,
  /*! Bilinear (2x2 kernel) */                         GRA_Bilinear=1,
  /*! Cubic Convolution Approximation (4x4 kernel) */  GRA_Cubic=2,
  /*! Cubic B-Spline Approximation (4x4 kernel) */     GRA_CubicSpline=3,
  /*! Lanczos windowed sinc interpolation (6x6 kernel) */ GRA_Lanczos=4
} GDALResampleAlg;

在查看Gdalwarp的源代码发现，warp的功能非常强大，可以用来做投影转换，重投影，投影定义，重采样，镶嵌，几何精校正和影像配准等。一句话，很好很强大。下面就看看其中的一点点皮毛，使用warp来编写一个重采样的接口，代码如下：

/**
* 重采样函数(GDAL)
* @param pszSrcFile        输入文件的路径
* @param pszOutFile        写入的结果图像的路径
* @param fResX             X转换采样比，默认大小为1.0，大于1图像变大，小于1表示图像缩小
* @param fResY             Y转换采样比，默认大小为1.0
* @param nResampleMode     采样模式，有五种，具体参见GDALResampleAlg定义，默认为双线性内插
* @param pExtent           采样范围，为NULL表示计算全图
* @param pBandIndex        指定的采样波段序号，为NULL表示采样全部波段
* @param pBandCount        采样的波段个数，同pBandIndex一同使用，表示采样波段的个数
* @param pszFormat         写入的结果图像的格式
* @param pProgress         进度条指针
* @return 成功返回0，否则为其他值
*/
int ResampleGDAL(const char* pszSrcFile, const char* pszOutFile, float fResX , float fResY, LT_ResampleMode nResampleMode,
    LT_Envelope* pExtent, int* pBandIndex, int *pBandCount, const char* pszFormat,  LT_Progress *pProgress)
{
    if(pProgress != NULL)
    {
        pProgress->SetProgressCaption("重采样");
        pProgress->SetProgressTip("正在执行重采样...");
    }

    GDALAllRegister();
    GDALDataset *pDSrc = (GDALDataset *)GDALOpen(pszSrcFile, GA_ReadOnly);
    if (pDSrc == NULL)
    {
        if(pProgress != NULL)
            pProgress->SetProgressTip("指定的文件不存在，或者该格式不被支持！");

        return RE_NOFILE;
    }

    GDALDriver *pDriver = GetGDALDriverManager()->GetDriverByName(pszFormat);
    if (pDriver == NULL)
    {
        if(pProgress != NULL)
            pProgress->SetProgressTip("不能创建该格式的文件！");

        GDALClose((GDALDatasetH) pDSrc);
        return RE_CREATEFILE;
    }

    int iBandCount = pDSrc->GetRasterCount();
    string strWkt = pDSrc->GetProjectionRef();//返回坐标系统
    GDALDataType dataType = pDSrc->GetRasterBand(1)->GetRasterDataType();

    double dGeoTrans[6] = {0};
    pDSrc->GetGeoTransform(dGeoTrans);   //六个参数其实是图像行列号坐标和地理坐标转换系数

    int iNewBandCount = iBandCount;
    if (pBandIndex != NULL && pBandCount != NULL)
    {
        int iMaxBandIndex = pBandIndex[0];    //找出最大的波段索引序号
        for (int i=1; i<*pBandCount; i++)
        {
            if (iMaxBandIndex < pBandIndex[i])
                iMaxBandIndex = pBandIndex[i];
        }

        if(iMaxBandIndex > iBandCount)
        {
            if(pProgress != NULL)
                pProgress->SetProgressTip("指定的波段序号超过图像的波段数，请检查输入参数！");

            GDALClose((GDALDatasetH) pDSrc);
            return RE_PARAMERROR;
        }
        
        iNewBandCount = *pBandCount;
    }

    LT_Envelope enExtent;
    enExtent.setToNull();

    if (pExtent == NULL)    //全图计算

    {
        double dPrj[4] = {0};    //x1,x2,y1,y2
        ImageRowCol2Projection(dGeoTrans, 0, 0, dPrj[0], dPrj[2]); //将图像左上角行列号（0,0）坐标转为地理坐标
        ImageRowCol2Projection(dGeoTrans, pDSrc->GetRasterXSize(), pDSrc->GetRasterYSize(), dPrj[1], dPrj[3]);//将图像右下角的坐标转为地理坐标
        enExtent.init(dPrj[0], dPrj[1], dPrj[2], dPrj[3]);

        pExtent = &enExtent;
    }
    
    dGeoTrans[0] = pExtent->getMinX();
    dGeoTrans[3] = pExtent->getMaxY();   //dGeoTrans[0],dGeoTrans[3]表示的是左上角的地理坐标
    dGeoTrans[1] = dGeoTrans[1] / fResX; 
    dGeoTrans[5] = dGeoTrans[5] / fResY;  //dGeoTrans[1],dGeoTrans[5]表示的是图像横向和纵向的分辨率

    int iNewWidth  = static_cast<int>( (pExtent->getMaxX() - pExtent->getMinX() / ABS(dGeoTrans[1]) + 0.5) );
    int iNewHeight = static_cast<int>( (pExtent->getMaxX() - pExtent->getMinX() / ABS(dGeoTrans[5]) + 0.5) );

    GDALDataset *pDDst = pDriver->Create(pszOutFile, iNewWidth, iNewHeight, iNewBandCount, dataType, NULL);
    if (pDDst == NULL)
    {
        if(pProgress != NULL)
            pProgress->SetProgressTip("创建输出文件失败！");

        GDALClose((GDALDatasetH) pDSrc);
        return RE_CREATEFILE;
    }

    pDDst->SetProjection(strWkt.c_str());
    pDDst->SetGeoTransform(dGeoTrans);

    GDALResampleAlg eResample = (GDALResampleAlg) nResampleMode;

    if(pProgress != NULL)
    {
        pProgress->SetProgressTip("正在执行重采样...");
        pProgress->SetProgressTotalStep(iNewBandCount*iNewHeight);
    }

    int *pSrcBand = NULL;
    int *pDstBand = NULL;
    int iBandSize = 0;
    if (pBandIndex != NULL && pBandCount != NULL)
    {
        iBandSize = *pBandCount;
        pSrcBand = new int[iBandSize];
        pDstBand = new int[iBandSize];

        for (int i=0; i<iBandSize; i++)
        {
            pSrcBand[i] = pBandIndex[i];
            pDstBand[i] = i+1;
        }
    }
    else
    {
        iBandSize = iBandCount;
        pSrcBand = new int[iBandSize];
        pDstBand = new int[iBandSize];

        for (int i=0; i<iBandSize; i++)
        {
            pSrcBand[i] = i+1;
            pDstBand[i] = i+1;
        }
    }
    
    void *hTransformArg = NULL, *hGenImgPrjArg = NULL;
    hTransformArg = hGenImgPrjArg = GDALCreateGenImgProjTransformer2((GDALDatasetH) pDSrc, (GDALDatasetH) pDDst, NULL);
    if (hTransformArg == NULL)
    {
        if(pProgress != NULL)
            pProgress->SetProgressTip("转换参数错误！");

        GDALClose((GDALDatasetH) pDSrc);
        GDALClose((GDALDatasetH) pDDst);
        return RE_PARAMERROR;
    }
    
    GDALTransformerFunc pFnTransformer = GDALGenImgProjTransform;
    GDALWarpOptions *psWo = GDALCreateWarpOptions();

    psWo->papszWarpOptions = CSLDuplicate(NULL);
    psWo->eWorkingDataType = dataType;
    psWo->eResampleAlg = eResample;

    psWo->hSrcDS = (GDALDatasetH) pDSrc;
    psWo->hDstDS = (GDALDatasetH) pDDst;

    psWo->pfnTransformer = pFnTransformer;
    psWo->pTransformerArg = hTransformArg;

    psWo->pfnProgress = GDALProgress;
    psWo->pProgressArg = pProgress;

    psWo->nBandCount = iNewBandCount;
    psWo->panSrcBands = (int *) CPLMalloc(iNewBandCount*sizeof(int));
    psWo->panDstBands = (int *) CPLMalloc(iNewBandCount*sizeof(int));
    for (int i=0; i<iNewBandCount; i++)
    {
        psWo->panSrcBands[i] = pSrcBand[i];
        psWo->panDstBands[i] = pDstBand[i];
    }

    RELEASE(pSrcBand);
    RELEASE(pDstBand);

    GDALWarpOperation oWo;
    if (oWo.Initialize(psWo) != CE_None)
    {
        if(pProgress != NULL)
            pProgress->SetProgressTip("转换参数错误！");

        GDALClose((GDALDatasetH) pDSrc);
        GDALClose((GDALDatasetH) pDDst);

        return RE_PARAMERROR;
    }

    oWo.ChunkAndWarpImage(0, 0, iNewWidth, iNewHeight);
    
    GDALDestroyGenImgProjTransformer(psWo->pTransformerArg);
    GDALDestroyWarpOptions( psWo );
    GDALClose((GDALDatasetH) pDSrc);
    GDALClose((GDALDatasetH) pDDst);

    if(pProgress != NULL)
        pProgress->SetProgressTip("重采样完成！");

    return RE_SUCCESS;
}