Image Resizing Techniques_ iOS图像分辨率

本文探讨了多种iOS平台上的图像缩放方法及其性能特点,包括UIKit、CoreGraphics、Image I/O、Core Image及Accelerate框架等,并通过实测对比不同方法的优劣。

Since time immemorial, iOS developers have been perplexed by a singular question: “How do you resize an image?”. It is a question of beguiling clarity, spurred on by a mutual mistrust of developer and platform. A thousand code samples litter web search results, each claiming to be the One True Solution, and all the others false prophets.


It’s embarrassing, really.

This week’s article endeavors to provide a clear explanation of the various approaches to image resizing on iOS (and OS X, making the appropriate UIImage → NSImage conversions), using empirical evidence to offer insights into the performance characteristics of each approach, rather than simply prescribing any one way for all situations.

Before reading any further, please note the following:

When setting a UIImage on a UIImageView, manual resizing is unnecessary for the vast majority of use cases. Instead, one can simply set the contentMode property to either .ScaleAspectFit to ensure that the entire image is visible within the frame of the image view, or .ScaleAspectFill to have the entire image view filled by the image, cropping as necessary from the center.

imageView.contentMode = .ScaleAspectFit
imageView.image = image
Determining Scaled Size

Before doing any image resizing, one must first determine the target size to scale to.

Scaling by Factor

The simplest way to scale an image is by a constant factor. Generally, this involves dividing by a whole number to reduce the original size (rather than multiplying by a whole number to magnify).

A new CGSize can be computed by scaling the width and height components individually:

let size = CGSize(width: image.size.width / 2, height: image.size.height / 2)
…or by applying a CGAffineTransform:

let size = CGSizeApplyAffineTransform(image.size, CGAffineTransformMakeScale(0.5, 0.5))
Scaling by Aspect Ratio

It’s often useful to scale the original size in such a way that fits within a rectangle without changing the original aspect ratio. AVMakeRectWithAspectRatioInsideRect is a useful function found in the AVFoundation framework that takes care of that calculation for you:

import AVFoundation
let rect = AVMakeRectWithAspectRatioInsideRect(image.size, imageView.bounds)
Resizing Images

There are a number of different approaches to resizing an image, each with different capabilities and performance characteristics.

UIGraphicsBeginImageContextWithOptions & UIImage -drawInRect:

The highest-level APIs for image resizing can be found in the UIKit framework. Given a UIImage, a temporary graphics context can be used to render a scaled version, using UIGraphicsBeginImageContextWithOptions() and UIGraphicsGetImageFromCurrentImageContext():

let image = UIImage(contentsOfFile: self.URL.absoluteString!)

let size = CGSizeApplyAffineTransform(image.size, CGAffineTransformMakeScale(0.5, 0.5))
let hasAlpha = false
let scale: CGFloat = 0.0 // Automatically use scale factor of main screen

UIGraphicsBeginImageContextWithOptions(size, !hasAlpha, scale)
image.drawInRect(CGRect(origin: CGPointZero, size: size))

let scaledImage = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
UIGraphicsBeginImageContextWithOptions() creates a temporary rendering context into which the original is drawn. The first argument, size, is the target size of the scaled image. The second argument, isOpaque is used to determine whether an alpha channel is rendered. Setting this to false for images without transparency (i.e. an alpha channel) may result in an image with a pink hue. The third argument scale is the display scale factor. When set to 0.0, the scale factor of the main screen is used, which for Retina displays is 2.0 or higher (3.0 on the iPhone 6 Plus).

CGBitmapContextCreate & CGContextDrawImage

Core Graphics / Quartz 2D offers a lower-level set of APIs that allow for more advanced configuration. Given a CGImage, a temporary bitmap context is used to render the scaled image, using CGBitmapContextCreate() and CGBitmapContextCreateImage():

let cgImage = UIImage(contentsOfFile: self.URL.absoluteString!).CGImage

let width = CGImageGetWidth(cgImage) / 2
let height = CGImageGetHeight(cgImage) / 2
let bitsPerComponent = CGImageGetBitsPerComponent(cgImage)
let bytesPerRow = CGImageGetBytesPerRow(cgImage)
let colorSpace = CGImageGetColorSpace(cgImage)
let bitmapInfo = CGImageGetBitmapInfo(cgImage)

let context = CGBitmapContextCreate(nil, width, height, bitsPerComponent, bytesPerRow, colorSpace, bitmapInfo.rawValue)

CGContextSetInterpolationQuality(context, kCGInterpolationHigh)

CGContextDrawImage(context, CGRect(origin: CGPointZero, size: CGSize(width: CGFloat(width), height: CGFloat(height))), cgImage)

let scaledImage = CGBitmapContextCreateImage(context).flatMap { UIImage(CGImage: $0) }
CGBitmapContextCreate takes several arguments to construct a context with desired dimensions and amount of memory for each channel within a given colorspace. In the example, these values are fetched from the CGImage. Next, CGContextSetInterpolationQuality allows for the context to interpolate pixels at various levels of fidelity. In this case, kCGInterpolationHigh is passed for best results. CGContextDrawImage allows for the image to be drawn at a given size and position, allowing for the image to be cropped on a particular edge or to fit a set of image features, such as faces. Finally, CGBitmapContextCreateImage creates a CGImage from the context.

CGImageSourceCreateThumbnailAtIndex

Image I/O is a powerful, yet lesser-known framework for working with images. Independent of Core Graphics, it can read and write between many different formats, access photo metadata, and perform common image processing operations. The framework offers the fastest image encoders and decoders on the platform, with advanced caching mechanisms and even the ability to load images incrementally.

CGImageSourceCreateThumbnailAtIndex offers a concise API with different options than found in equivalent Core Graphics calls:

import ImageIO

if let imageSource = CGImageSourceCreateWithURL(self.URL, nil) {
    let options: [NSString: NSObject] = [
        kCGImageSourceThumbnailMaxPixelSize: max(size.width, size.height) / 2.0,
        kCGImageSourceCreateThumbnailFromImageAlways: true
    ]

    let scaledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options).flatMap { UIImage(CGImage: $0) }
}
Given a CGImageSource and set of options, CGImageSourceCreateThumbnailAtIndex creates a thumbnail image. Resizing is accomplished by the kCGImageSourceThumbnailMaxPixelSize. Specifying the maximum dimension divided by a constant factor scales the image while maintaining the original aspect ratio. By specifying either kCGImageSourceCreateThumbnailFromImageIfAbsent or kCGImageSourceCreateThumbnailFromImageAlways, Image I/O will automatically cache the scaled result for subsequent calls.

Lanczos Resampling with Core Image

Core Image provides a built-in Lanczos resampling functionality with the CILanczosScaleTransform filter. Although arguably a higher-level API than UIKit, the pervasive use of key-value coding in Core Image makes it unwieldy.

That said, at least the pattern is consistent. The process of creating a transform filter, configuring it, and rendering an output image is just like any other Core Image workflow:

let image = CIImage(contentsOfURL: self.URL)

let filter = CIFilter(name: "CILanczosScaleTransform")!
filter.setValue(image, forKey: "inputImage")
filter.setValue(0.5, forKey: "inputScale")
filter.setValue(1.0, forKey: "inputAspectRatio")
let outputImage = filter.valueForKey("outputImage") as! CIImage

let context = CIContext(options: [kCIContextUseSoftwareRenderer: false])
let scaledImage = UIImage(CGImage: self.context.createCGImage(outputImage, fromRect: outputImage.extent()))
CILanczosScaleTransform accepts an inputImage, inputScale, and inputAspectRatio, all of which are pretty self-explanatory. A CIContext is used to create a UIImage by way of a CGImageRef intermediary representation, since UIImage(CIImage:) doesn’t often work as expected.

Creating a CIContext is an expensive operation, so a cached context should always be used for repeated resizing. A CIContext can be created using either the GPU or the CPU (much slower) for rendering—use the kCIContextUseSoftwareRenderer key in the options dictionary to specify which.

vImage in Accelerate

The Accelerate framework includes a suite of vImage image-processing functions, with a set of functions that scale an image buffer. These lower-level APIs promise high performance with low power consumption, but at the cost of managing the buffers yourself. The following is a Swift version of a method suggested by Nyx0uf on GitHub:

let cgImage = UIImage(contentsOfFile: self.URL.absoluteString!).CGImage

// create a source buffer
var format = vImage_CGImageFormat(bitsPerComponent: 8, bitsPerPixel: 32, colorSpace: nil,
    bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.First.rawValue),
    version: 0, decode: nil, renderingIntent: CGColorRenderingIntent.RenderingIntentDefault)
var sourceBuffer = vImage_Buffer()
defer {
    sourceBuffer.data.dealloc(Int(sourceBuffer.height) * Int(sourceBuffer.height) * 4)
}

var error = vImageBuffer_InitWithCGImage(&sourceBuffer, &format, nil, cgImage, numericCast(kvImageNoFlags))
guard error == kvImageNoError else { return nil }

// create a destination buffer
let scale = UIScreen.mainScreen().scale
let destWidth = Int(image.size.width * CGFloat(scalingFactor) * scale)
let destHeight = Int(image.size.height * CGFloat(scalingFactor) * scale)
let bytesPerPixel = CGImageGetBitsPerPixel(image.CGImage) / 8
let destBytesPerRow = destWidth * bytesPerPixel
let destData = UnsafeMutablePointer<UInt8>.alloc(destHeight * destBytesPerRow)
defer {
    destData.dealloc(destHeight * destBytesPerRow)
}
var destBuffer = vImage_Buffer(data: destData, height: vImagePixelCount(destHeight), width: vImagePixelCount(destWidth), rowBytes: destBytesPerRow)

// scale the image
error = vImageScale_ARGB8888(&sourceBuffer, &destBuffer, nil, numericCast(kvImageHighQualityResampling))
guard error == kvImageNoError else { return nil }

// create a CGImage from vImage_Buffer
let destCGImage = vImageCreateCGImageFromBuffer(&destBuffer, &format, nil, nil, numericCast(kvImageNoFlags), &error)?.takeRetainedValue()
guard error == kvImageNoError else { return nil }

// create a UIImage
let scaledImage = destCGImage.flatMap { UIImage(CGImage: $0, scale: 0.0, orientation: image.imageOrientation) }
The Accelerate APIs used here clearly operate at a lower-level than the other resizing methods. To use this method, you first create a source buffer from your CGImage using a vImage_CGImageFormat with vImageBuffer_InitWithCGImage(). The destination buffer is allocated at the desired image resolution, then vImageScale_ARGB8888 does the actual work of resizing the image. Managing your own buffers when operating on images larger than your app’s memory limit is left as an exercise for the reader.

Performance Benchmarks

So how do these various approaches stack up to one another?

Here are the results of a set of performance benchmarks done on an iPhone 6 running iOS 8.4, via this project:

JPEG

Loading, scaling, and displaying a large, high-resolution (12000 ⨉ 12000 px 20 MB JPEG) source image from NASA Visible Earth at 1/10th the size:

Operation    Time (sec)    σ
UIKit    0.612    14%
Core Graphics 1    0.266    3%
Image I/O    0.255    2%
Core Image 2    3.703    33%
vImage 3    –    –
PNG

Loading, scaling, and displaying a reasonably large (1024 ⨉ 1024 px 1MB PNG) rendering of the Postgres.app Icon at 1/10th the size:

Operation    Time (sec)    σ
UIKit    0.044    30%
Core Graphics 4    0.036    10%
Image I/O    0.038    11%
Core Image 5    0.053    68%
vImage    0.050    25%
1, 4 Results were consistent across different values of CGInterpolationQuality, with negligible differences in performance benchmarks.

3 The size of the NASA Visible Earth image was larger than could be processed in a single pass on the device.

2, 5 Setting kCIContextUseSoftwareRenderer to true on the options passed on CIContext creation yielded results an order of magnitude slower than base results.

Conclusions

UIKit, Core Graphics, and Image I/O all perform well for scaling operations on most images.
Core Image is outperformed for image scaling operations. In fact, it is specifically recommended in the Performance Best Practices section of the Core Image Programming Guide to use Core Graphics or Image I/O functions to crop or downsample images beforehand.
For general image scaling without any additional functionality, UIGraphicsBeginImageContextWithOptions is probably the best option.
If image quality is a consideration, consider using CGBitmapContextCreate in combination with CGContextSetInterpolationQuality.
When scaling images with the intent purpose of displaying thumbnails, CGImageSourceCreateThumbnailAtIndex offers a compelling solution for both rendering and caching.

Unless you’re already working with vImage, the extra work to use the low-level Accelerate framework for resizing doesn’t pay off.


参考 http://nshipster.com/image-resizing/


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<think>嗯,用户遇到了V4L2设备无法打开和摄像头初始化失败的问题。首先,我需要回忆一下V4L2的相关知识。根据引用[2],V4L2设备通常位于/dev/video*这样的设备文件,用户可能需要检查这些文件的权限和是否存在。比如,用户是否拥有访问/dev/video0的权限?或者设备文件是否正确创建? 然后,考虑设备节点的问题。如果设备文件不存在,可能需要使用modprobe加载驱动模块,比如uvcvideo。此外,用户可能使用了错误的设备节点,比如视频设备可能是video1而不是video0,尤其是在有多个摄像头的情况下。这时候可以用v4l2-ctl --list-devices来列出设备。 接下来是驱动兼容性问题。有些摄像头可能需要特定的内核驱动,如果驱动未正确加载或版本不兼容,会导致初始化失败。检查dmesg日志可能会有帮助,看看是否有硬件识别的错误信息。 然后是应用程序权限的问题。用户可能需要以root权限运行程序,或者将用户加入video组。比如,使用sudo或者修改组权限。 另外,格式设置的问题也很重要。引用[3]提到需要正确使用vidioc_s_fmt_vid_cap等调用来设置格式。如果格式不支持,摄像头可能无法初始化。可以先用v4l2-ctl --list-formats-ext来查看支持的格式,并确保应用程序设置的格式匹配。 硬件连接问题也不容忽视,比如USB接触不良或供电不足。尝试更换接口或使用带电源的集线器可能会有帮助。 最后,用户提供的错误信息可能不够详细,建议他们检查具体的错误代码和日志,比如errno值或dmesg输出,以便更精准地定位问题。</think>### 解决V4L2设备无法打开及摄像头初始化失败的步骤 #### 1. 检查设备节点权限与存在性 - **设备路径**:V4L2设备默认位于 `/dev/video0`、`/dev/video1` 等路径[^2]。使用以下命令确认设备存在: ```bash ls -l /dev/video* ``` 若设备未显示,可能是驱动未加载或硬件未识别。 - **权限问题**:确保当前用户对设备文件有读写权限。可通过以下命令修改: ```bash sudo chmod 666 /dev/video0 # 临时解决 sudo usermod -aG video $USER # 永久加入video用户组 ``` #### 2. 验证驱动加载与兼容性 - 加载摄像头驱动模块(如UVC摄像头): ```bash sudo modprobe uvcvideo ``` - 检查内核日志以确认硬件识别: ```bash dmesg | grep -i "camera\|uvc\|video" ``` 若出现 `Failed to submit URB` 或 `Timeout` 等错误,可能是硬件兼容性问题。 #### 3. 使用工具调试设备能力 - 通过 `v4l2-ctl` 工具查询设备信息: ```bash v4l2-ctl --list-devices # 列出可用设备 v4l2-ctl --all --device /dev/video0 # 查看详细参数 v4l2-ctl --list-formats-ext # 检查支持的格式[^3] ``` 若输出显示 `VIDIOC_QUERYCAP: EINVAL`,则设备可能未正确初始化。 #### 4. 应用程序代码调整 - **打开设备时**,确保使用正确的标志(如 `O_RDWR`)和非阻塞模式: ```c int fd = open("/dev/video0", O_RDWR | O_NONBLOCK); if (fd == -1) { perror("Device open failed"); // 输出具体错误代码 } ``` - **初始化格式**时,需通过 `VIDIOC_S_FMT` 设置与硬件匹配的分辨率和像素格式: ```c struct v4l2_format fmt = {0}; fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; fmt.fmt.pix.width = 640; fmt.fmt.pix.height = 480; fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_MJPEG; // 根据设备支持调整 if (ioctl(fd, VIDIOC_S_FMT, &fmt) == -1) { perror("Format setting failed"); } ``` #### 5. 排查硬件问题 - 尝试更换USB接口或使用带电源的USB集线器。 - 通过其他软件(如 `cheese` 或 `ffplay`)验证摄像头是否正常工作: ```bash ffplay -f v4l2 -input_format mjpeg -video_size 640x480 /dev/video0 ``` #### 6. 内核与用户空间兼容性 - 若内核版本较旧(如引用[1]提到的Linux 2.6.14),部分V4L2特性可能缺失。建议升级到较新的LTS内核(如5.15.x)。
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