現在常用衛星影像基本上都是16位影像，如GF，ZY3，Landsat8，WV等，有時我們需要將16位影像降到8位影像，這樣不僅減小了資料量，也便於後期處理。

通常的軟體在處理降位時會存在一些問題，如曝光，出現空值等。因此，我自己根據常用的降位方法，如最簡單的線性拉伸，分段拉伸以及對數變換和指數變換，結合常用影像的特點，使用百分比截斷和指數（冪為0.7）變換將影像從16位降到8位，在抑制高光的同時保證了影像的對比度。

	int imageprocessing::stretch_percent_16to8(const char *inFilename, const char *dstFilename)
	{
		GDALAllRegister();
		//為了支援中文路徑
		CPLSetConfigOption("GDAL_FILENAME_IS_UTF8","NO");
		int src_height = 0; 
		int src_width = 0;
		GDALDataset *poIn = (GDALDataset *)GDALOpen(inFilename,GA_ReadOnly);   //開啟影像
		//獲取影像大小
		src_width = poIn ->GetRasterXSize();
		src_height = poIn ->GetRasterYSize();
		//獲取影像波段數
		int InBands = poIn ->GetRasterCount();
		//獲取影像格式
		GDALDataType eDataType = poIn -> GetRasterBand(1) -> GetRasterDataType();
		//定義儲存影像的空間參考陣列
		double adfInGeoTransform[6] = {0};
		const char *pszWKT = NULL;
		//獲取影像空間參考
		poIn ->GetGeoTransform(adfInGeoTransform);
		pszWKT = poIn ->GetProjectionRef();
		//建立檔案
		GDALDriver *poDriver = (GDALDriver *)GDALGetDriverByName("GTiff");
		GDALDataset *poOutputDS = poDriver -> Create(dstFilename,src_width,src_height,InBands,GDT_Byte,NULL);

		//設定拉伸後圖像的空間參考以及地理座標
		poOutputDS -> SetGeoTransform(adfInGeoTransform);
		poOutputDS -> SetProjection(pszWKT);
		//讀取影像

		cout<<"16位影像降到8位影像處理..."<<endl;
		for(int iBand = 0; iBand < InBands; iBand++ )
		{
			cout<<"正在處理第 "<<iBand + 1<<" 波段"<<endl;
			//讀取影像
			uint16_t *srcData = (uint16_t *)malloc(sizeof(uint16_t) *src_width * src_height *1);
			memset(srcData,0,sizeof(uint16_t ) * 1 *src_width * src_height);
			int src_max = 0, src_min = 65500;
			//讀取多光譜影像到快取
			poIn ->GetRasterBand( iBand + 1) -> RasterIO( GF_Read, 0, 0, src_width, src_height , srcData + 0 * src_width * src_height,src_width,src_height, GDT_UInt16, 0, 0 );
		//}
		//統計最大值
		for (int src_row = 0; src_row < src_height; src_row ++)
		{
			for (int src_col = 0; src_col < src_width; src_col++)
			{
				uint16_t src_temVal = *(srcData + src_row * src_width + src_col);
				if (src_temVal > src_max)
					src_max = src_temVal;
				if(src_temVal < src_min )
					src_min = src_temVal;
			}
		}

		double *numb_pix = (double *)malloc(sizeof(double)*(src_max+1));      //存畫素值直方圖，即每個畫素值的個數
		memset(numb_pix,0,sizeof(double) * (src_max+1));
		//                 -------  統計畫素值直方圖  ------------         //

		for (int src_row = 0; src_row < src_height; src_row ++)
		{
			for (int src_col = 0; src_col < src_width; src_col++)
			{
				uint16_t src_temVal = *(srcData + src_row * src_width + src_col);
				*(numb_pix + src_temVal) += 1;
			}
		}

		double *frequency_val = (double *)malloc(sizeof(double)*(src_max+1));           //畫素值出現的頻率
		memset(frequency_val,0.0,sizeof(double)*(src_max+1));

		for (int val_i = 0; val_i <= src_max; val_i++)
		{
			*(frequency_val + val_i) = *(numb_pix + val_i) / double(src_width * src_height);
		}

		double *accumlt_frequency_val = (double*)malloc(sizeof(double)*(src_max+1));   //畫素出現的累計頻率
		memset(accumlt_frequency_val, 0.0,sizeof(double)*(src_max+1));

		for (int val_i = 0; val_i <= src_max; val_i ++)
		{
			for (int val_j = 0; val_j < val_i; val_j ++ )
			{
				*(accumlt_frequency_val + val_i) += *(frequency_val + val_j);
			}
		}
		//統計畫素兩端截斷值
		int minVal = 0, maxVal = 0;
		for (int val_i = 1; val_i < src_max; val_i++)
		{
			double acc_fre_temVal0 = *(frequency_val + 0);
			double acc_fre_temVal = *(accumlt_frequency_val + val_i);
			if((acc_fre_temVal - acc_fre_temVal0) > 0.0015 )
			{	minVal = val_i;
				break;	}
		}
		for (int val_i = src_max-1; val_i > 0; val_i--)
		{
			double acc_fre_temVal0 = *(accumlt_frequency_val + src_max);
			double acc_fre_temVal = *(accumlt_frequency_val + val_i);
			if(acc_fre_temVal < (acc_fre_temVal0 - 0.00012) )
			{	maxVal = val_i;
				break;	}
		}
		
		for (int src_row = 0; src_row < src_height; src_row ++)
		{
			uint8_t *dstData = (uint8_t*)malloc(sizeof(uint8_t)*src_width);
			memset(dstData, 0, sizeof(uint8_t)*src_width);
			for (int src_col = 0; src_col < src_width; src_col++)
			{
				uint16_t src_temVal = *(srcData + src_row * src_width + src_col);
				double stre_temVal = (src_temVal - minVal) / double(maxVal - minVal) ;
				if(src_temVal < minVal)
				{
					*(dstData + src_col) = (src_temVal) *(20.0/double(minVal)) ;
				}
				else if(src_temVal > maxVal)
				{	stre_temVal = (src_temVal - src_min) / double(src_max - src_min);
					*(dstData + src_col) = 254;	}
				else
					*(dstData + src_col) = pow(stre_temVal,0.7) * 250;

			}
			poOutputDS->GetRasterBand(iBand + 1)->RasterIO(GF_Write, 0,src_row,src_width,1,dstData,src_width,1,GDT_Byte,0,0);
			free(dstData);
		}

		free(numb_pix);
		free(frequency_val);
		free(accumlt_frequency_val);
		free(srcData);
		
		}
		GDALClose(poIn);
		GDALClose(poOutputDS);

		return 0;
	}