GDAL讀取影像並插值

影像讀取 並縮放

讀取大影像某一部分，並縮放到指定大小，我們有時會用如下代碼：

#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;  
    }
}

如果這里影像大小，非常大時，幾G到幾十G，上百G的時候，會可能遇到什么問題？

讓我們從RasterIO接口談起，跟其中代碼，得知

1、從影像金字塔取數據。根據縮小到的TileSize，它會從影像金字塔中取相應層級的數據。

2、取到數據后，會進行最近鄰插值。

由於最近鄰插值效果不佳，會導致得到的數據，都是相應的噪聲，麻點現象。

GDAL從2.0版本支持RasterIO接口進行插值算法的配置，僅僅進行簡單的配置，就可以解決這個問題。

 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);

可以選擇的插值方法：

    /*! 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

影像寫出，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;

 

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