使用OpenCV保存指定格式、指定类型的图像

最近使用OpenCV的函数保存图像，发现这里面存在诸多的坑，经过多次尝试，这才发现原来保存图像也是一门学问哦！之前已经写过一篇图像读写的博客：opencv读写图像问题！，这里再写一篇作为补充。

一、类型转换

1.Mat数据的类型

opencv中的mat数据类型共包含28种：

类型	C1	C2	C3	C4
CV_8U	0	8	16	24
CV_8S	1	9	17	25
CV_16U	2	10	18	26
CV_16S	3	11	19	27
CV_32S	4	12	20	28
CV_32F	5	13	21	29
CV_64F	6	14	22	30

opencv提供了获取类型的函数：type()
这里提供一个简单的获取类型名的代码：

#include <string>
#include <opencv2/opencv.hpp>
using namespace std;
using namesapce cv;
string getImageType(Mat img)
{
 int type = img.type();
 string channel = "";
 string depth = "";

 // 判断通道数
 if (type >= 0 && type <= 6)
 {
 	channel += "C1";
 }
 else if (type >= 8 && type <= 14)
 {
 	channel += "C2";
 }
 else if (type >= 16 && type <= 22)
 {
 	channel += "C3";
 }
 else if (type >= 24 && type <= 30)
 {
 	channel += "C4";
 }
 // 判断位深
 if (0 == type || 8 == type || 16 == type || 24 == type)
 {
 	depth += "CV_8U";
 }
 else if (1 == type || 9 == type || 17 == type || 25 == type)
 {
 	depth += "CV_8S";
 }
 else if (2 == type || 10 == type || 18 == type || 26 == type)
 {
 	depth += "CV_16U";
 }
 else if (3 == type || 11 == type || 19 == type || 27 == type)
 {
 	depth += "CV_16S";
 }
 else if (4 == type || 12 == type || 20 == type || 28 == type)
 {
 	depth += "CV_32S";
 }
 else if (5 == type || 13 == type || 21 == type || 29 == type)
 {
 	depth += "CV_32F";
 }
 else if (6 == type || 14 == type || 22 == type || 30 == type)
 {
 	depth += "CV_64F";
 }

 return depth + channel;
}

2.类型转换

opencv中提供了类型转换的函数convertTo()用于进行图像类型转换，函数简介如下：

 /** @brief Converts an array to another data type with optional scaling.

    The method converts source pixel values to the target data type. saturate_cast\<\> is applied at
    the end to avoid possible overflows:

    \f[m(x,y) = saturate \_ cast<rType>( \alpha (*this)(x,y) +  \beta )\f]
    @param m output matrix; if it does not have a proper size or type before the operation, it is
    reallocated.
    @param rtype desired output matrix type or, rather, the depth since the number of channels are the
    same as the input has; if rtype is negative, the output matrix will have the same type as the input.
    @param alpha optional scale factor.
    @param beta optional delta added to the scaled values.
     */
    void convertTo( OutputArray m, int rtype, double alpha=1, double beta=0 ) const;

示例如下：

    string path = "C:\\Users\\admin\\Desktop\\0.png";
	Mat img = imread(path, -1);
	cout << getImageType(img) << endl;

	Mat cv8u;
	cvtColor(img, cv8u, CV_BGRA2GRAY);
	cout << getImageType(cv8u) << endl;

	Mat cv8uc3;
	cvtColor(img, cv8uc3, CV_BGRA2BGR);
	cout << getImageType(cv8uc3) << endl;

	Mat cv16u;
	cv8u.convertTo(cv16u, CV_16UC1);
	cout << getImageType(cv16u) << endl;

	Mat cv16uc3;
	cv8uc3.convertTo(cv16uc3, CV_16UC3);
	cout << getImageType(cv16uc3) << endl;
    // 其他类型依此类推

注意：在转换过程中如果想要转换为3通道的图像，必须保证被转换的图像也为3通道的图像，否则最终转换过来仍然是单通道。

二、图像保存

1.保存类型

我在这里经过大量测试，总结成一个表格，每种格式的图像可保存的类型如下：

类型/格式	TIFF/TIF(*.tiff)	JPG(*.jpg)	PNG(*.png)	BMP(*.bmp)
CV_8U	√	√	√	√
CV_8UC3	√	√	√	√
CV_8S	×	×	×	×
CV_8SC3	×	×	×	×
CV_16U	√	×	√	×
CV_16UC3	√	×	√	×
CV_16S	×	×	×	×
CV_16SC3	×	×	×	×
CV_32S	×	×	×	×
CV_32SC3	×	×	×	×
CV_32F	√	×	×	×
CV_32FC3	√	×	×	×
CV_64F	√	×	×	×
CV_64FC3	×	×	×	×

可以看到，以上四种格式都无法保存所有类型的Mat，想要保存任何一种Mat，可以使用XML或YAML格式，opencv提供了FileStorage类用于将数据存储为xml或yaml格式的文件

2.图像保存的接口

CV_EXPORTS_W bool imwrite( const String& filename, InputArray img, const std::vector<int>& params = std::vector<int>());