GDAL读取影像并插值

影像读取 并缩放

读取大影像某一部分，并缩放到指定大小，我们有时会用如下代码：

#include "gdal.h"
#include "gdal_priv.h"
#include "gdalwarper.h"

void main()
{  
    GDALAllRegister();
    CPLSetConfigOption("GDAL_FILENAME_IS_UTF8","NO");//support Chinese
    
    GDALDataset* readDataSet = (GDALDataset*)GDALOpen(inputFile.c_str(),GA_ReadOnly);
    if (readDataSet == NULL )
    {
        return;
    }
    int width = readDataSet->GetRasterXSize();
    int height = readDataSet->GetRasterYSize();
    int bandCount = readDataSet->GetRasterCount();

    int tilesize = 50;
    unsigned char* data = new unsigned char[tilesize*tilesize*bandCount];
    if (readDataSet->RasterIO(GDALRWFlag::GF_Read,0,0,width,height,data,tilesize,tilesize,GDT_Byte,bandCount,NULL,0,0,0)==CPLErr::CE_Failure)
    {
        delete readDataSet;readDataSet=NULL;  
        delete data; 
        return;  
    }
}

如果这里影像大小，非常大时，几G到几十G，上百G的时候，会可能遇到什么问题？

让我们从RasterIO接口谈起，跟其中代码，得知

1、从影像金字塔取数据。根据缩小到的TileSize，它会从影像金字塔中取相应层级的数据。

2、取到数据后，会进行最近邻插值。

由于最近邻插值效果不佳，会导致得到的数据，都是相应的噪声，麻点现象。

GDAL从2.0版本支持RasterIO接口进行插值算法的配置，仅仅进行简单的配置，就可以解决这个问题。

     GDALRasterIOExtraArg exterArg;
     INIT_RASTERIO_EXTRA_ARG(exterArg);
     exterArg.eResampleAlg = GDALRIOResampleAlg::GRIORA_Bilinear;//配置插值方法
     readDataSet->RasterIO(GDALRWFlag::GF_Read,0,0,width,height,data,tilesize,tilesize,GDT_Byte,bandCount,NULL,0,0,0,&exterArg);

可以选择的插值方法：

    /*! Nearest neighbour */                               GRIORA_NearestNeighbour = 0,
    /*! Bilinear (2x2 kernel) */                           GRIORA_Bilinear = 1,
    /*! Cubic Convolution Approximation (4x4 kernel) */    GRIORA_Cubic = 2,
    /*! Cubic B-Spline Approximation (4x4 kernel) */       GRIORA_CubicSpline = 3,
    /*! Lanczos windowed sinc interpolation (6x6 kernel)*/ GRIORA_Lanczos = 4,
    /*! Average */                                         GRIORA_Average = 5,
    /*! Mode (the value which appears most often) */  　   GRIORA_Mode = 6,
    /*! Gauss blurring */                                  GRIORA_Gauss = 7

影像写出，如下操作：

　　GDALDriver *pDriver;
    pDriver = GetGDALDriverManager()->GetDriverByName("GTiff");
    
    GDALDataset* gdalDataset;
    gdalDataset = pDriver->Create(outputFile.c_str(),tilesize,tilesize,bandCount,GDT_Byte,NULL);
    gdalDataset->RasterIO(GF_Write,0,0,tilesize,tilesize,data,tilesize,tilesize,GDT_Byte,bandCount,NULL,0,0,0);
    GDALClose(gdalDataset);
    gdalDataset = NULL;

参考：https://trac.osgeo.org/gdal/wiki/rfc51_rasterio_resampling_progress