OpenCV學習筆記（八）：OpenCV使用imwrite儲存4通道圖片（僅對png有效）
轉載OpenCV官方文件中imwrite函式的說明，如何儲存4通道的影象。
轉載地址
https://www.docs.opencv.org/2.4.13/modules/highgui/doc/reading_and_writing_images_and_video.html?highlight=imwrite#imwrite
內容
Saves an image to a specified file.
C++: bool imwrite(const string& filename, InputArray img, const vector<int>& params
Python: cv2.imwrite(filename, img[, params]) → retval
C: int cvSaveImage(const char* filename, const CvArr* image, const int* params=0 )
Python: cv.SaveImage(filename, image) → None
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The function imwrite saves the image to the specified file. The image format is chosen based on the filenameextension (see imread() for the list of extensions). Only 8-bit (or 16-bit unsigned (CV_16U) in case of PNG, JPEG 2000, and TIFF) single-channel or 3-channel (with ‘BGR’ channel order) images can be saved using this function. If the format, depth or channel order is different, use Mat::convertTo() , and cvtColor() to convert it before saving. Or, use the universal FileStorage I/O functions to save the image to XML or YAML format.
It is possible to store PNG images with an alpha channel using this function. To do this, create 8-bit (or 16-bit) 4-channel image BGRA, where the alpha channel goes last. Fully transparent pixels should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535. The sample below shows how to create such a BGRA image and store to PNG file. It also demonstrates how to set custom compression parameters
大致含義
imwrite函式只支援儲存8位（或16位無符號）的單通道或者三通道（BGR）影象為PNG、JPEG 2000、TIFF格式的影象檔案。其他格式的影象要麼轉化為以上的格式，要麼使用FileStorage提供的方法儲存為XML或YAML檔案。
程式碼
#include <vector>
#include <stdio.h>
#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(CV_IMWRITE_PNG_COMPRESSION);
compression_params.push_back(9);
try {
imwrite("alpha.png", mat, compression_params);
}
catch (runtime_error& 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;
}