从 C++ 读取 iPhone 上的图像

Question

I am working on a C++ library that shall do image processing. My approach now is to pass two strings to C++ from swift. One string is the path to the image and the second is the output directory.

All the time I get that the file does not exist. How can I get the correct path the asset? The image lies in a directory I have created on my own and is called "test.jpg". I also have a "test2.jpg" in Assets. Have not managed to find that either.

Swift Code:

func getGrayImage() -> Image {
    if var resourcePath = Bundle.main.url(forResource: "test", withExtension: "jpg") {
        let dir = resourcePath.deletingLastPathComponent()
        VideoProcessingWrapper().rgb2gray(resourcePath.absoluteString, dir.absoluteString)
    }
    return Image("test2")
}

C++ Code:

void VideoProcessing::rgb2gray(const std::string& image_path, const std::string& dir) {
  std::cout << "C++: " << std::endl;
  std::cout << image_path << std::endl;
  std::cout << std::endl;
  std::cout << dir << std::endl;
  std::cout << "In directory: " <<std::endl;
  for (const auto& entry : std::filesystem::directory_iterator(dir)) {
        std::cout << entry.path() << std::endl;
  }
  if (std::filesystem::exists(image_path)) {
    std::cout << "File exists" << std::endl;
  } else {
    std::cout << "File does NOT exist" << std::endl;
    return;
  }
  cv::imread(image_path, cv::IMREAD_COLOR);
}

It even crashes when trying to display all files in the directory, stating that the directory does not exist. But what have I then been given from Bundle.main.url -call?

This is the printout from the C++ function using a simulator:

C++: 
file:///Users/name/Library/Developer/CoreSimulator/Devices/7F438661-8BF5-4A60-B41F-1D4B7FEC6A8E/data/Containers/Bundle/Application/AB3EFF3E-79A4-469C-A3BF-ABDD31A09E61/TestVideoProcess.app/test.jpg

file:///Users/name/Library/Developer/CoreSimulator/Devices/7F438661-8BF5-4A60-B41F-1D4B7FEC6A8E/data/Containers/Bundle/Application/AB3EFF3E-79A4-469C-A3BF-ABDD31A09E61/TestVideoProcess.app/

I get the same behaviour when running on a real iPhone.

Answer 1

The problem here is that Foundation framework represent paths with URI 's, while C++ standard library filesystem relies on more file-path-specific format which may consist of only the following components :

1. root-name(optional) : identifies the root on a filesystem with multiple roots (such as "C:" or "//myserver" ). In case of ambiguity, the longest sequence of characters that forms a valid root-name is treated as the root-name. The standard library may define additional root-names besides the ones understood by the OS API.
2. root-directory(optional) : a directory separator that, if present, marks this path as absolute. If it is missing (and the first element other than the root name is a file name), then the path is relative and requires another path as the starting location to resolve to a file name.
3. Zero or more of the following:

• file-name : sequence of characters that aren't directory separators or preferred directory separators (additional limitations may be imposed by the OS or file system). This name may identify a file, a hard link, a symbolic link, or a directory. Two special file-names are recognized:
• dot : the file name consisting of a single dot character . is a directory name that refers to the current directory dot-dot: the file name consisting of two dot characters .. is a directory name that refers to the parent directory.
• directory-separators : the forward slash character / or the alternative character provided as path::preferred_separator. If this character is repeated, it is treated as a single directory separator: /usr///////lib is the same as /usr/lib

From the rules above I can conclude that the difference of a path from URI is at least that the latter contains scheme . One trick you can refer to is to build the path from pathComponents of the URL class:

if let resourcePath = Bundle.main.url(forResource: "test", withExtension: "jpg") {
    let dir = resourcePath.deletingLastPathComponent()
    VideoProcessingWrapper().rgb2gray(resourcePath.pathComponents.joined(separator: "/"),
                                      dir.pathComponents.joined(separator: "/"))

}

Alternatively, as suggested by mani in the comments of your question , you can use the path property of the same class, but for some reason it's now deprecated.

Answer 2

No matter what you do, the bundle directory is inside your application and you have no write access to it.