opencv从文件中读取图片imread()函数以及Mat对象的复制方法

比眉伴天荒 2022-12-30 14:52 13阅读 0赞

> @brief Loads an image from a file.
> 
> @anchor imread
> 
> The function imread loads an image from the specified file and returns it. If the image cannot be  
> read (because of missing file, improper permissions, unsupported or invalid format), the function  
> returns an empty matrix ( Mat::data==NULL ).
> 
> 函数imread从指定的文件中加载一个图像并返回它。如果图像不能被读取(因为文件丢失、权限不当、不支持或无效格式)，函数返回一个空矩阵(Mat::data==NULL)。

目前支持的格式：

> Currently, the following file formats are supported:
> 
> \- Windows bitmaps - \\\*.bmp, \\\*.dib (always supported)  
> \- JPEG files - \\\*.jpeg, \\\*.jpg, \\\*.jpe (see the \*Note\* section)  
> \- JPEG 2000 files - \\\*.jp2 (see the \*Note\* section)  
> \- Portable Network Graphics - \\\*.png (see the \*Note\* section)  
> \- WebP - \\\*.webp (see the \*Note\* section)  
> \- Portable image format - \\\*.pbm, \\\*.pgm, \\\*.ppm \\\*.pxm, \\\*.pnm (always supported)  
> \- PFM files - \\\*.pfm (see the \*Note\* section)  
> \- Sun rasters - \\\*.sr, \\\*.ras (always supported)  
> \- TIFF files - \\\*.tiff, \\\*.tif (see the \*Note\* section)  
> \- OpenEXR Image files - \\\*.exr (see the \*Note\* section)  
> \- Radiance HDR - \\\*.hdr, \\\*.pic (always supported)  
> \- Raster and Vector geospatial data supported by GDAL (see the \*Note\* section)

> @note**注意事项  
> \-** The function determines the type of an image by the content, not by the file extension**.该函数根据内容而不是文件扩展名确定图像的类型。**  
> \- In the case of color images, the decoded images will have the channels stored in \*\*B G R\*\* order.**在彩色图像的情况下，解码后的图像将具有按\*\* B G R \*\*顺序存储的通道。**  
> \- When using IMREAD\_GRAYSCALE, the codec's internal grayscale conversion will be used, if available.Results may differ to the output of cvtColor()。使用IMREAD\_GRAYSCALE时，将使用编解码器的内部灰度转换（如果可用）。结果可能与cvtColor（）的输出有所不同
> 
> \- On Microsoft WindowsOS and MacOSX, the codecs shipped with an OpenCV image (libjpeg,  
> libpng, libtiff, and libjasper) are used by default. So, OpenCV can always read JPEGs, PNGs,  
> and TIFFs. On MacOSX, there is also an option to use native MacOSX image readers. But beware  
> that currently these native image loaders give images with different pixel values because of  
> the color management embedded into MacOSX.  
> \- On Linux\\\*, BSD flavors and other Unix-like open-source operating systems, OpenCV looks for  
> codecs supplied with an OS image. Install the relevant packages (do not forget the development  
> files, for example, "libjpeg-dev", in Debian\\\* and Ubuntu\\\*) to get the codec support or turn  
> on the OPENCV\_BUILD\_3RDPARTY\_LIBS flag in CMake.  
> \- In the case you set \*WITH\_GDAL\* flag to true in CMake and @ref IMREAD\_LOAD\_GDAL to load the image,  
> then the \[GDAL\](http://www.gdal.org) driver will be used in order to decode the image, supporting  
> the following formats: \[Raster\](http://www.gdal.org/formats\_list.html),  
> \[Vector\](http://www.gdal.org/ogr\_formats.html).  
> \- If EXIF information is embedded in the image file, the EXIF orientation will be taken into account  
> and thus the image will be rotated accordingly except if the flags @ref IMREAD\_IGNORE\_ORIENTATION  
> or @ref IMREAD\_UNCHANGED are passed.  
> \- Use the IMREAD\_UNCHANGED flag to keep the floating point values from PFM image.  
> \- By default number of pixels must be less than 2^30. Limit can be set using system  
> variable OPENCV\_IO\_MAX\_IMAGE\_PIXELS
> 
> @param filename Name of file to be loaded.  
> @param flags Flag that can take values of cv::ImreadModes  
> \*/  
> CV\_EXPORTS\_W Mat imread( const String& filename, int flags = IMREAD\_COLOR );
> 
>  *  返回值,Mat 类型， 即返回读取的图像，读取图像失败时返回一个空的矩阵对象（Mat::data == NULL)
>  *  参数1 filename, 读取的图片文件名，可以使用相对路径或者绝对路径，但必须带完整的文件扩展名（图片格式后缀）
>  *  参数2 flags, 一个读取标记，用于选择读取图片的方式，默认值为IMREAD\_COLOR，flag值的设定与用什么颜色格式读取图片有关，传0是灰度图。

enum ImreadModes {
           IMREAD_UNCHANGED            = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped). Ignore EXIF orientation.
           IMREAD_GRAYSCALE            = 0,  //!< If set, always convert image to the single channel grayscale image (codec internal conversion).
           IMREAD_COLOR                = 1,  //!< If set, always convert image to the 3 channel BGR color image.
           IMREAD_ANYDEPTH             = 2,  //!< If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
           IMREAD_ANYCOLOR             = 4,  //!< If set, the image is read in any possible color format.
           IMREAD_LOAD_GDAL            = 8,  //!< If set, use the gdal driver for loading the image.
           IMREAD_REDUCED_GRAYSCALE_2  = 16, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/2.
           IMREAD_REDUCED_COLOR_2      = 17, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.
           IMREAD_REDUCED_GRAYSCALE_4  = 32, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/4.
           IMREAD_REDUCED_COLOR_4      = 33, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
           IMREAD_REDUCED_GRAYSCALE_8  = 64, //!< If set, always convert image to the single channel grayscale image and the image size reduced 1/8.
           IMREAD_REDUCED_COLOR_8      = 65, //!< If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.
           IMREAD_IGNORE_ORIENTATION   = 128 //!< If set, do not rotate the image according to EXIF's orientation flag.
         };

<table> 
 <thead> 
  <tr> 
   <th>c++定义</th> 
   <th>python定义</th> 
   <th>说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>IMREAD_UNCHANGED</td> 
   <td>Python: cv.IMREAD_UNCHANGED</td> 
   <td>如果设置，则按原样返回加载的图像（使用Alpha通道，否则会被裁剪）</td> 
  </tr> 
  <tr> 
   <td>IMREAD_GRAYSCALE</td> 
   <td>Python: cv.IMREAD_GRAYSCALE</td> 
   <td>如果设置，则始终将图像转换为单通道灰度图像（编解码器内部转换）。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_COLOR</td> 
   <td>Python: cv.IMREAD_COLOR</td> 
   <td>如果设置，请始终将图像转换为3通道BGR彩色图像。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_ANYDEPTH</td> 
   <td>Python: cv.IMREAD_ANYDEPTH</td> 
   <td>如果设置，则在输入具有相应深度时返回16位/ 32位图像，否则将其转换为8位。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_ANYCOLOR</td> 
   <td>Python: cv.IMREAD_ANYCOLOR</td> 
   <td>如果设置，则以任何可能的颜色格式读取图像。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_LOAD_GDAL</td> 
   <td>Python: cv.IMREAD_LOAD_GDAL</td> 
   <td>如果设置，使用gdal驱动程序加载图像</td> 
  </tr> 
  <tr> 
   <td>IMREAD_REDUCED_GRAYSCALE_2</td> 
   <td>Python: cv.IMREAD_REDUCED_GRAYSCALE_2</td> 
   <td>如果设置，则始终将图像转换为单通道灰度图像，图像尺寸减小1/2。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_REDUCED_COLOR_2</td> 
   <td>Python: cv.IMREAD_REDUCED_COLOR_2</td> 
   <td>如果设置，则始终将图像转换为3通道BGR彩色图像，图像尺寸减小1/2。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_REDUCED_GRAYSCALE_4</td> 
   <td>Python: cv.IMREAD_REDUCED_GRAYSCALE_4</td> 
   <td>如果设置，则始终将图像转换为单通道灰度图像，图像尺寸减小1/4</td> 
  </tr> 
  <tr> 
   <td>IMREAD_REDUCED_COLOR_4</td> 
   <td>Python: cv.IMREAD_REDUCED_COLOR_4</td> 
   <td>如果设置，则始终将图像转换为3通道BGR彩色图像，图像尺寸减小1/4</td> 
  </tr> 
  <tr> 
   <td>IMREAD_REDUCED_GRAYSCALE_8</td> 
   <td>Python: cv.IMREAD_REDUCED_GRAYSCALE_8</td> 
   <td>如果设置，则始终将图像转换为单通道灰度图像，图像尺寸减小1/8。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_REDUCED_COLOR_8</td> 
   <td>Python: cv.IMREAD_REDUCED_COLOR_8</td> 
   <td>如果设置，则始终将图像转换为3通道BGR彩色图像，图像尺寸减小1/8。</td> 
  </tr> 
  <tr> 
   <td>IMREAD_IGNORE_ORIENTATION</td> 
   <td>Python: cv.IMREAD_IGNORE_ORIENTATION</td> 
   <td>如果设置，请不要根据EXIF的方向标志旋转图像。</td> 
  </tr> 
 </tbody> 
</table>

# Mat对象的复制 #

函数原型

*  Mat clone() const;
 *  void copyTo( OutputArray m ) const;
 *  void copyTo( OutputArray m, InputArray mask) const;

copyTo()有重载，

inline Mat Mat::clone() const
    
    {
    
       Mat m;
       copyTo(m);
       return m;
    }

opencv矩阵赋值函数**copyTo**、**clone**、**重载元算赋‘=’**之间实现的功能相似均是给不同的矩阵赋值功能。

copyTo和clone函数基本相同，被赋值的矩阵和赋值矩阵之间空间独立，不共享同一空间（深拷贝）。

但是重载元算赋‘=’，被赋值的矩阵和赋值矩阵之间空间共享，改变任一个矩阵的值，会同时影响到另一个矩阵（浅拷贝）。当矩阵作为函数的返回值时其功能和重载元算赋‘=’相同，赋值运算赋会给矩阵空间增加一次计数，所以函数变量返回后函数内部申请的变量空间并不会被撤销，在主函数中仍可以正常使用传递后的参数。