CGContext.init（）-不再允许使用NULL颜色空间 [英] CGContext.init() -- NULL color space no longer allowed

问题描述

TL; DR：在旧式Obj-C代码中，颜色空间参数值为 NULL 。 Swift等效项中不允许这样做。使用什么值？

TL;DR: In legacy Obj-C code, the color space param value was NULL. That is not allowed in the Swift equivalent. What value to use?

我继承了以下代码：

unsigned char pixel[1] = {0};
CGContextRef context = CGBitmapContextCreate(
    pixel,1, 1, 8, 1, NULL, (CGBitmapInfo)kCGImageAlphaOnly
);

Swift 4的端口 CGContext 很简单，除了 NULL 颜色空间值。使用一个合理的值，我从 CGContext.init？（）得到了 nil 。我的翻译是：

The port to Swift 4 CGContext is straightforward, except for that NULL color space value. Using a plausible value, I am getting nil back from CGContext.init?(). My translation is:

var pixelValue = UInt8(0)
var pixel = Data(buffer: UnsafeBufferPointer(start:&pixelValue, count:1))
let context = CGContext(
    data            : &pixel,
    width           : 1,
    height          : 1,
    bitsPerComponent: 8,
    bytesPerRow     : 1,
    space           : CGColorSpace(name:CGColorSpace.genericRGBLinear)!,
    bitmapInfo      : CGImageAlphaInfo.alphaOnly.rawValue
)! // Returns nil; unwrapping crashes

Q ：<$ c $的合适值是多少c>空格？ （我提供的值没有返回 nil ；它是 CGContext（）本身。

Q: What is the appropriate value for space? (The value I provide is not returning nil; it's the CGContext() call itself.

设置环境变量 CGBITMAP_CONTEXT_LOG_ERRORS 会产生如下错误日志：

Setting the environment variable CGBITMAP_CONTEXT_LOG_ERRORS yields an error log like this:

Assertion failed: (0), function get_color_model_name, 
file /BuildRoot/Library/Caches/com.apple.xbs/Sources/Quartz2D_Sim/
Quartz2D-1129.2.1/CoreGraphics/API/CGBitmapContextInfo.c, line 210.

对于某些背景故事，上下文用于通过以下方式在 UIImage 中找到单个像素的alpha值：

For some more backstory, the context was used to find the alpha value of a single pixel in a UIImage in the following way:

unsigned char pixel[1] = {0};
CGContextRef context = CGBitmapContextCreate(pixel,1, 1, 8, 1, NULL, (CGBitmapInfo)kCGImageAlphaOnly);
UIGraphicsPushContext(context);
[image drawAtPoint:CGPointMake(-point.x, -point.y)];
UIGraphicsPopContext();
CGContextRelease(context);
CGFloat alpha = pixel[0]/255.0;

（我确实有找到alpha的替代方法，但是为了不遗留遗留代码，希望保持这种方式。）

(I do have possible alternatives for finding alpha, but in the interest of leaving legacy code alone, would like to keep it this way.)

推荐答案

我最近处理过类似的主题，也许此代码示例将帮助某人：

I recently worked with similar topic, maybe this code sample will help someone:

let image = UIImage(named: "2.png")
guard let cgImage = image?.cgImage else {
    fatalError()
}

let width = cgImage.width
let height = cgImage.height
//CGColorSpaceCreateDeviceGray - 1 component, 8 bits
//i.e. 1px = 1byte
let bytesPerRow = width
let bitmapByteCount = width * height

let bitmapData: UnsafeMutablePointer<UInt8> = .allocate(capacity: bitmapByteCount)
defer {
    bitmapData.deallocate()
}
bitmapData.initialize(repeating: 0, count: bitmapByteCount)

guard let context = CGContext(data: bitmapData, width: width, height: height,
                              bitsPerComponent: 8, bytesPerRow: bytesPerRow,
                              space: CGColorSpaceCreateDeviceGray(), bitmapInfo: CGImageAlphaInfo.alphaOnly.rawValue) else {
                                fatalError()
}
//draw image to context
var rect = CGRect(x: 0, y: 0, width: width, height: height)
context.draw(cgImage, in: rect)

// Enumerate through all pixels
for row in 0..<height {
    for col in 0..<width {
        let alphaValue = bitmapData[row * width + col]
        if alphaValue != 0 {
            //visible pixel
        }
    }
}

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