opencv讀寫圖像問題！

一、OpenCV中支持讀取的圖像格式：

1.  Windows bitmaps     \*.bmp, \*.dib

2.  JPEG files         \*.jpeg, \*.jpg, \*.jpe

3.  JPEG 2000 files      \*.jp2 

4.   Portable Network Graphics       \*.png 

5.   WebP       \*.webp 

6.   Portable image format      \*.pbm, \*.pgm, \*.ppm \*.pxm, \*.pnm 

7.   Sun rasters       \*.sr, \*.ras 

8.   TIFF files        \*.tiff, \*.tif 

9.   OpenEXR Image files        \*.exr

10.   Radiance HDR        \*.hdr, \*.pic 

11.   Raster and Vector geospatial data supported by GDAL 

二、讀取方式

cv2.imread(filename, flags=None)

其中：
flags = -1, 讀取原始數據
flags = 0,  讀取圖像並將其轉換爲單通道灰度圖，並且結果可能與使用cvtColor()函數的結果不同
flags = 1,  讀取圖像並將其轉換爲3通道圖像，通道順序爲B、G、R
flags = 2,  讀取並返回16位或32位圖像（前提是圖像必須有對應的深度，否則將轉化爲int8位圖像返回

三、保存圖像

cv2.imwrite(filename, img, params=None)

Imwrite flags
enum ImwriteFlags {
       IMWRITE_JPEG_QUALITY        = 1,  //!< For JPEG, it can be a quality from 0 to 100 (the higher is the better). Default value is 95.
       IMWRITE_JPEG_PROGRESSIVE    = 2,  //!< Enable JPEG features, 0 or 1, default is False.
       IMWRITE_JPEG_OPTIMIZE       = 3,  //!< Enable JPEG features, 0 or 1, default is False.
       IMWRITE_JPEG_RST_INTERVAL   = 4,  //!< JPEG restart interval, 0 - 65535, default is 0 - no restart.
       IMWRITE_JPEG_LUMA_QUALITY   = 5,  //!< Separate luma quality level, 0 - 100, default is 0 - don't use.
       IMWRITE_JPEG_CHROMA_QUALITY = 6,  //!< Separate chroma quality level, 0 - 100, default is 0 - don't use.
       IMWRITE_PNG_COMPRESSION     = 16, //!< For PNG, it can be the compression level from 0 to 9. A higher value means a smaller size and longer compression time. If specified, strategy is changed to IMWRITE_PNG_STRATEGY_DEFAULT (Z_DEFAULT_STRATEGY). Default value is 1 (best speed setting).
       IMWRITE_PNG_STRATEGY        = 17, //!< One of cv::ImwritePNGFlags, default is IMWRITE_PNG_STRATEGY_RLE.
       IMWRITE_PNG_BILEVEL         = 18, //!< Binary level PNG, 0 or 1, default is 0.
       IMWRITE_PXM_BINARY          = 32, //!< For PPM, PGM, or PBM, it can be a binary format flag, 0 or 1. Default value is 1.
       IMWRITE_WEBP_QUALITY        = 64, //!< For WEBP, it can be a quality from 1 to 100 (the higher is the better). By default (without any parameter) and for quality above 100 the lossless compression is used.
       IMWRITE_PAM_TUPLETYPE       = 128,//!< For PAM, sets the TUPLETYPE field to the corresponding string value that is defined for the format
     };

//! Imwrite PNG specific flags used to tune the compression algorithm.
/** These flags will be modify the way of PNG image compression and will be passed to the underlying zlib processing stage.

-   The effect of IMWRITE_PNG_STRATEGY_FILTERED is to force more Huffman coding and less string matching; it is somewhat intermediate between IMWRITE_PNG_STRATEGY_DEFAULT and IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY.
-   IMWRITE_PNG_STRATEGY_RLE is designed to be almost as fast as IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY, but give better compression for PNG image data.
-   The strategy parameter only affects the compression ratio but not the correctness of the compressed output even if it is not set appropriately.
-   IMWRITE_PNG_STRATEGY_FIXED prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.
*/
enum ImwritePNGFlags {
       IMWRITE_PNG_STRATEGY_DEFAULT      = 0, //!< Use this value for normal data.
       IMWRITE_PNG_STRATEGY_FILTERED     = 1, //!< Use this value for data produced by a filter (or predictor).Filtered data consists mostly of small values with a somewhat random distribution. In this case, the compression algorithm is tuned to compress them better.
       IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY = 2, //!< Use this value to force Huffman encoding only (no string match).
       IMWRITE_PNG_STRATEGY_RLE          = 3, //!< Use this value to limit match distances to one (run-length encoding).
       IMWRITE_PNG_STRATEGY_FIXED        = 4  //!< Using this value prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.
     };

//! Imwrite PAM specific tupletype flags used to define the 'TUPETYPE' field of a PAM file.
enum ImwritePAMFlags {
       IMWRITE_PAM_FORMAT_NULL = 0,
       IMWRITE_PAM_FORMAT_BLACKANDWHITE = 1,
       IMWRITE_PAM_FORMAT_GRAYSCALE = 2,
       IMWRITE_PAM_FORMAT_GRAYSCALE_ALPHA = 3,
       IMWRITE_PAM_FORMAT_RGB = 4,
       IMWRITE_PAM_FORMAT_RGB_ALPHA = 5,
     };

       注意使用imwire函數保存圖像時，一般只能保存8位（或16位無符號型<CV_16U>）的單通道或三通道圖像(通道順序位BGR)，保存的格式取決於所使用的擴展名。如果圖像格式、深度或通道順序不同，需要先使用ConvertTo或者CvtColor函數進行轉換，或者使用通用的FileStorage I/O函數將其保存爲XML或者YAML類型的文件。

使用此函數還可以保存帶有alpha通道的png圖片，需要首先創建一個8位或16位的4通道的圖像BGRA，其中alpha通道放在最後，對於全透明的圖像，其alpha通道值的應該設置爲0，對於完全不透明的圖像，其alpha通道的值應該設置爲255/65535。C++版本的示例代碼如下：

#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

void createAlphaMat(Mat &mat)
{
    CV_Assert(mat.channels() == 4);
    for (int i = 0; i < mat.rows; ++i) {
        for (int j = 0; j < mat.cols; ++j) {
            Vec4b& bgra = mat.at<Vec4b>(i, j);
            bgra[0] = UCHAR_MAX; // Blue
            bgra[1] = saturate_cast<uchar>((float (mat.cols - j)) / ((float)mat.cols) * UCHAR_MAX); // Green
            bgra[2] = saturate_cast<uchar>((float (mat.rows - i)) / ((float)mat.rows) * UCHAR_MAX); // Red
            bgra[3] = saturate_cast<uchar>(0.5 * (bgra[1] + bgra[2])); // Alpha
        }
    }
}

int main(int argv, char **argc)
{
    // Create mat with alpha channel
    Mat mat(480, 640, CV_8UC4);
    createAlphaMat(mat);

    vector<int> compression_params;
    compression_params.push_back(IMWRITE_PNG_COMPRESSION);
    compression_params.push_back(9);
    try{
        imwrite("alpha.png", mat, compression_params);
    }
    catch (cv::Exception& ex) {
        fprintf(stderr, "Exception converting image to PNG format: %s\n", ex.what());
        return 1;
    }

    fprintf(stdout, "Saved PNG file with alpha data.\n");
    return 0;
}

如果你想保存其他數據類型的圖像，可按照下列方式：

1.CV_16U格式的圖像可以保存爲：PNG、JPEG 2000、 TIFF 格式

2.CV_32F格式的圖像可以保存爲：TIFF、 OpenEXR、 Radiance HDR格式。