ios - CGImage 没有属性/元数据 (CGImageProperties)

Question

假设我有一个 CGImage从一些 URL 加载, 我想通过 CGImageSourceCopyPropertiesAtIndex 提取它的属性:

// Playground

import SwiftUI

func printPropertiesOf(_ image: CGImage) {
    guard let dataProvider = image.dataProvider else {
        print("Couldn't get the data provider.")
        return
    }
    guard let data = dataProvider.data else {
        print("Couldn't get the data.")
        return
    }
    guard let source = CGImageSourceCreateWithData(data, nil) else {
        print("Couldn't get the source.")
        return
    }
    guard let properties = CGImageSourceCopyPropertiesAtIndex(source, 0, nil) else {
        print("Couldn't get the properties.")
        return
    }
    print(properties)
}

let url = Bundle.main.url(forResource: "Landscape/Landscape_0", withExtension: "jpg")!
let source = CGImageSourceCreateWithURL(url as CFURL, nil)!
let cgImage = CGImageSourceCreateImageAtIndex(source, 0, nil)!

printPropertiesOf(cgImage)

输出:

Couldn't get the properties.

但是如果我使用 URL图像所在的位置，而不是 CGImage :

// Playground

import SwiftUI

func printPropertiesOfImageIn(_ url: URL) {
    guard let data = try? Data(contentsOf: url) else {
        print("Couldn't get the data.")
        return
    }
    guard let source = CGImageSourceCreateWithData(data as CFData, nil) else {
        print("Couldn't get the source.")
        return
    }
    guard let properties = CGImageSourceCopyPropertiesAtIndex(source, 0, nil) else {
        print("Couldn't get the properties.")
        return
    }
    print(properties)
}

let url = Bundle.main.url(forResource: "Landscape/Landscape_0", withExtension: "jpg")!
let source = CGImageSourceCreateWithURL(url as CFURL, nil)!
let cgImage = CGImageSourceCreateImageAtIndex(source, 0, nil)!

printPropertiesOfImageIn(url)

输出:

{
    ColorModel = RGB;
    DPIHeight = 72;
    DPIWidth = 72;
    Depth = 8;
    PixelHeight = 1200;
    PixelWidth = 1800;
    "{JFIF}" =     {
        DensityUnit = 1;
        JFIFVersion =         (
            1,
            0,
            1
        );
        XDensity = 72;
        YDensity = 72;
    };
    "{TIFF}" =     {
        Orientation = 0;
        ResolutionUnit = 2;
        XResolution = 72;
        YResolution = 72;
    };
}

Is there a way to retrieve the metadata from a CGImage itself, without having to rely on its source URL?

If not, is there a way to find out the source URL of a given CGImage?

(注意:以上示例中使用的图像 can be found here 。)

Answer 1

CGImage应该是完全原始的位图数据。一组最小的未压缩数据，甚至可以直观地呈现图像。来自 docs “位图图像或图像蒙版”。

我真的很惊讶您能够使用 CGImageSourceCreateWithData 构建源代码。以两种不同的方式:

在第一种情况下，您直接从原始未压缩数据创建它，即 CGImage .预计它绝对没有标题或关于如何显示的附加信息。

在第二种情况下，您从 JPEG 数据创建它，该数据是带有可能包含许多不同信息的 header 的压缩数据。某些信息可能完全不相关，例如拍摄图像的坐标或拍摄日期。

因此，您在第二种情况下显示的附加信息可能会被系统用于构造 CGImage对象(例如编码)。但这不是需要附加到 CGImage 的信息。为了显示它，因为它已经准备好(解码)以进行演示。

在您的标题中，您说“CGImage 没有属性/元数据(CGImageProperties)”。没有“CGImageProperties”之类的东西，只有“CGImageSourceProperties”。所以它是具有属性的源。

所以我相信这些属性不会被复制，也没有办法从 CGImage 获取它们。独自的。您可以直接从 CGImage 获得其他属性尽管:

cgImage.width

cgImage.height

cgImage.alphaInfo

cgImage.bitmapInfo

cgImage.bitsPerComponent

cgImage.colorSpace

...

您可以查看更多 here .