为什么当我减少图片中的像素数量时，文件的大小会变大？

Question

我正在尝试使用 swift 调整图像大小。我一直在尝试将像素宽度设置为 1080p。我使用的原始照片宽度为 1200p，文件大小为 760 KB。使用以下函数将图像调整为 1080p 后，结果为 833 KB。

    func resizeImage(image: UIImage, newWidth: CGFloat) -> UIImage {
    let scale = newWidth / image.size.width
    let newHeight = image.size.height * scale
    UIGraphicsBeginImageContext(CGSize(width: newWidth, height: newHeight))
    image.draw(in: CGRect(x: 0, y: 0, width: newWidth, height: newHeight))
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    return newImage! 
}

下面是完整的代码

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!
    var image:UIImage!

    override func viewDidLoad() {
        super.viewDidLoad()
        image = imageView.image
        let bcf = ByteCountFormatter()
        bcf.allowedUnits = [.useKB] // optional: restricts the units to MB only
        bcf.countStyle = .file
        var string = bcf.string(fromByteCount: Int64(image.jpegData(compressionQuality: 1.0)!.count))
        print("formatted result: \(string)")
        compressionLabel.text = string

        print("Image Pixels: \(CGSize(width: image.size.width*image.scale, height: image.size.height*image.scale))")

        image = resizeImage(image: image, newWidth: 1080)

        string = bcf.string(fromByteCount: Int64(image.jpegData(compressionQuality: 1.0)!.count))
        print("formatted result: \(string)")
        compressionLabel.text = string

        print("Image Pixels: \(CGSize(width: image.size.width*image.scale, height: image.size.height*image.scale))")
    }


    func resizeImage(image: UIImage, newWidth: CGFloat) -> UIImage {
        let scale = newWidth / image.size.width
        let newHeight = image.size.height * scale
        UIGraphicsBeginImageContext(CGSize(width: newWidth, height: newHeight))
        image.draw(in: CGRect(x: 0, y: 0, width: newWidth, height: newHeight))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage!

    }

}

Answer 1

无法确定，但您在转换后的图像上指定了 compressionQuality: 1.0，所以原件可能是以不同的 quality:size 比率保存的？（即，由于压缩算法的性质，以低质量保存的较大图像很容易导致比以高质量保存的较小图像更小的文件大小）

为什么当我减少图片中的像素数量时，文件的大小会变大？

why is it that when I decrease the number of pixels in a picture the size of the file gets larger?

ios

swift

nsdata

pixel

photo