ios - CGContext.init() -- NULL 颜色空间不再允许
<p><p>TL;DR:在旧版 Obj-C 代码中，颜色空间参数值为 <code>NULL</code>。这在 Swift 等效项中是不允许的。使用什么值？</p>
<p>我继承了如下代码:</p>
<pre><code>unsigned char pixel = {0};
CGContextRef context = CGBitmapContextCreate(
pixel,1, 1, 8, 1, NULL, (CGBitmapInfo)kCGImageAlphaOnly
);
</code></pre>
<p>到 Swift 4 <code>CGContext</code> 的移植很简单，除了那个 <code>NULL</code> 颜色空间值。使用一个合理的值，我从 <code>CGContext.init?()</code> 得到 <code>nil</code>。我的翻译是:</p>
<pre><code>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
</code></pre>
<p><strong>问</strong>:<code>space</code> 的合适值是多少？ (我提供的值不返回 <code>nil</code>；它是 <code>CGContext()</code> 调用本身。</p>
<p>设置环境变量 <code>CGBITMAP_CONTEXT_LOG_ERRORS</code> 会产生如下错误日志:</p>
<pre><code>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.
</code></pre>
<p>对于更多背景故事，上下文用于通过以下方式查找 <code>UIImage</code> 中单个像素的 alpha 值:</p>
<pre><code>unsigned char pixel = {0};
CGContextRef context = CGBitmapContextCreate(pixel,1, 1, 8, 1, NULL, (CGBitmapInfo)kCGImageAlphaOnly);
UIGraphicsPushContext(context);
;
UIGraphicsPopContext();
CGContextRelease(context);
CGFloat alpha = pixel/255.0;
</code></pre>
<p>(我确实有可能的替代方法来寻找 alpha，但为了不影响遗留代码，我希望保持这种方式。)</p></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>我最近研究过类似的主题，也许这个代码示例会对某人有所帮助:</p>
<pre><code>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
if alphaValue != 0 {
//visible pixel
}
}
}
</code></pre></p>
<p style="font-size: 20px;">关于ios - CGContext.init() -- NULL 颜色空间不再允许，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/48195801/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/48195801/
</a>
</p>
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