如何将CVPixelBuffer适应一个正方形图像，并保持长宽比？

可以使用Core Graphics（CoreGraphics.framework）库来操作CVPixelBuffer，并将其转换为UIImage。然后，可以将UIImage转换为正方形，同时保持原始宽高比。

以下是Swift版本的示例：

func pixelBufferToSquareImage(pixelBuffer: CVPixelBuffer) -> UIImage? {
    // Get the width and height of the pixel buffer
    let width = CVPixelBufferGetWidth(pixelBuffer)
    let height = CVPixelBufferGetHeight(pixelBuffer)
    
    // Create CGImage from pixel buffer
    let ciImage = CIImage(cvPixelBuffer: pixelBuffer)
    let ciContext = CIContext()
    guard let cgImage = ciContext.createCGImage(ciImage, from: CGRect(x: 0, y: 0, width: width, height: height)) else {
        return nil
    }
    
    // Determine the size of the square image that will contain the original image
    let sideLength = min(width, height)
    let newSize = CGSize(width: sideLength, height: sideLength)
    
    // Create an UIImage from the CGImage and resize it to the new size
    let uiImage = UIImage(cgImage: cgImage)
    let scaledImage = uiImage.resizeTo(size: newSize)
    
    return scaledImage
}

extension UIImage {
    func resizeTo(size: CGSize) -> UIImage? {
        let scale = UIScreen.main.scale
        let newSize = CGSize(width: size.width * scale, height: size.height * scale)
        UIGraphicsBeginImageContextWithOptions(newSize, false, scale)
        draw(in: CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage
    }
}

要使用此方法，请调用pixelBufferToSquareImage()函数并将您的CVPixelBuffer作为输入参数传递。此方法将返回一个UIImage对象，其中包含您的CVPixelBuffer转换为正方形后的图像。