处理后 AVFoundation 相机图像质量下降

Question

我制作了一个 AVFoundation 相机来根据@fsaint 的答案裁剪方形图像：Cropping AVCaptureVideoPreviewLayer output to a square。照片的尺寸很棒，效果很好。但是图像质量明显下降（见下图：第一张图像是显示良好分辨率的预览层，第二张是捕获的退化图像）。这肯定与processImage: 中发生的事情有关，因为没有它，图像分辨率很好，只是纵横比不合适。关于图像处理的文档非常简单，非常感谢任何见解！

设置摄像头：

func setUpCamera() {

      captureSession = AVCaptureSession()
      captureSession!.sessionPreset = AVCaptureSessionPresetPhoto
      let backCamera = AVCaptureDevice.defaultDeviceWithMediaType(AVMediaTypeVideo)
      if ((backCamera?.hasFlash) != nil) {
         do { 
            try backCamera.lockForConfiguration()
            backCamera.flashMode = AVCaptureFlashMode.Auto
            backCamera.unlockForConfiguration()
         } catch {
            // error handling
         }
      }
      var error: NSError?
      var input: AVCaptureDeviceInput!
      do {
         input = try AVCaptureDeviceInput(device: backCamera)
      } catch let error1 as NSError {
         error = error1
         input = nil
      }
      if error == nil && captureSession!.canAddInput(input) {
         captureSession!.addInput(input)
         stillImageOutput = AVCaptureStillImageOutput()
         stillImageOutput!.outputSettings = [AVVideoCodecKey: AVVideoCodecJPEG]
         if captureSession!.canAddOutput(stillImageOutput) {
            captureSession!.sessionPreset = AVCaptureSessionPresetHigh;
            captureSession!.addOutput(stillImageOutput)
            previewLayer = AVCaptureVideoPreviewLayer(session: captureSession)
            previewLayer!.videoGravity = AVLayerVideoGravityResizeAspectFill
            previewLayer!.connection?.videoOrientation = AVCaptureVideoOrientation.Portrait
            previewVideoView.layer.addSublayer(previewLayer!)
            captureSession!.startRunning()
         }
      }
   }

拍照：

@IBAction func onSnapPhotoButtonPressed(sender: UIButton) {

      if let videoConnection = self.stillImageOutput!.connectionWithMediaType(AVMediaTypeVideo) {
         videoConnection.videoOrientation = AVCaptureVideoOrientation.Portrait
self.stillImageOutput?.captureStillImageAsynchronouslyFromConnection(videoConnection, completionHandler: {(sampleBuffer, error) in

            if (sampleBuffer != nil) {

               let imageData = AVCaptureStillImageOutput.jpegStillImageNSDataRepresentation(sampleBuffer)
               let dataProvider = CGDataProviderCreateWithCFData(imageData)
               let cgImageRef = CGImageCreateWithJPEGDataProvider(dataProvider, nil, true, CGColorRenderingIntent.RenderingIntentDefault)

               let image = UIImage(CGImage: cgImageRef!, scale: 1.0, orientation: UIImageOrientation.Right)   
               self.processImage(image)
               self.clearPhotoButton.hidden = false
               self.nextButton.hidden = false
               self.view.bringSubviewToFront(self.imageView)
            }
         })
      }
   }

将图像处理成正方形：

  func processImage(image:UIImage) {

      let deviceScreen = previewLayer?.bounds
      let width:CGFloat = (deviceScreen?.size.width)!
      UIGraphicsBeginImageContext(CGSizeMake(width, width))
      let aspectRatio:CGFloat = image.size.height * width / image.size.width
      image.drawInRect(CGRectMake(0, -(aspectRatio - width) / 2.0, width, aspectRatio))
      let smallImage = UIGraphicsGetImageFromCurrentImageContext()
      UIGraphicsEndImageContext()
      let cropRect = CGRectMake(0, 0, width, width)
      let imageRef:CGImageRef = CGImageCreateWithImageInRect(smallImage.CGImage, cropRect)!
      imageView.image = UIImage(CGImage: imageRef)
   }

Answer 1

您的 processImage() 函数有一些问题。

首先，您正在使用UIGraphicsBeginImageContext() 创建一个新的图形上下文。

According to the Apple docs on this function:

此函数相当于调用 UIGraphicsBeginImageContextWithOptions 函数，其中 opaque 参数设置为 NO，比例因子为 1.0。

因为比例因子是1.0，所以在屏幕上显示时会看起来像素化，因为屏幕的分辨率（很可能）更高。

您想使用 UIGraphicsBeginImageContextWithOptions() 函数，并传递 0.0 作为比例因子。根据有关此函数的文档，对于 scale 参数：

如果您指定值 0.0，则比例因子设置为设备主屏幕的比例因子。

例如：

UIGraphicsBeginImageContextWithOptions(CGSizeMake(width, width), NO, 0.0);

您的输出现在应该看起来漂亮而清晰，因为它以与屏幕相同的比例进行渲染。

---

其次，你传入的宽度有问题。

let width:CGFloat = (deviceScreen?.size.width)!
UIGraphicsBeginImageContext(CGSizeMake(width, width))

你不应该在这里传递屏幕的宽度，它应该是图像的宽度。例如：

let width:CGFloat = image.size.width

然后您必须更改 aspectRatio 变量以获取屏幕宽度，例如：

let aspectRatio:CGFloat = image.size.height * (deviceScreen?.size.width)! / image.size.width

---

第三，您可以显着简化裁剪功能。

func processImage(image:UIImage) {

    let screenWidth = UIScreen.mainScreen().bounds.size.width

    let width:CGFloat = image.size.width
    let height:CGFloat = image.size.height

    let aspectRatio = screenWidth/width;

    UIGraphicsBeginImageContextWithOptions(CGSizeMake(screenWidth, screenWidth), false, 0.0) // create context
    let ctx = UIGraphicsGetCurrentContext()

    CGContextTranslateCTM(ctx, 0, (screenWidth-(aspectRatio*height))*0.5) // shift context up, to create a sqaured 'frame' for your image to be drawn in

    image.drawInRect(CGRect(origin:CGPointZero, size: CGSize(width:screenWidth, height:height*aspectRatio))) // draw image

    let img = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    imageView.image = img
}

不需要绘制两次图像，只需将上下文向上翻译，然后绘制图像。