Question

因此，Apple的文档说CIImage符合Equatable。我认为这意味着以下单元测试将通过。但事实并非如此。我对原因感兴趣。

func test_CIImageEqualityShouldWork() {
    let bundle = NSBundle(forClass: PrototypeTests.self)
    guard let path = bundle.pathForResource("testImage", ofType: "png") else { return }
    guard let image = UIImage(contentsOfFile: path) else { return }

    let thingy1 = CIImage(image: image)
    let thingy2 = CIImage(image: image)
    XCTAssert(thingy1 == thingy2)
}

图像存在，guard语句都通过，但断言失败，它们不相等。

出于兴趣，我尝试了两次创建UIImage并进行比较。那也失败了。

Answer 1

所有NSObject子类都符合Equatable和==函数在对象上调用isEqual:方法。 isEqual:方法 NSObject只是比较对象指针，即o1 == o2 如果o1和o2引用相同的对象实例，则成立。

参见例如 Interacting with Objective-C APIs：

Swift提供了==和===运算符的默认实现   对于派生自的对象采用Equatable协议   NSObject类。 ==运算符的默认实现调用   isEqual：方法，以及===的默认实现   运算符检查指针相等。你不应该重写相等   或从Objective-C导入的类型的标识运算符。

isEqual的基本实现：由NSObject类提供   等同于通过指针相等的身份检查。

许多NSObject个子类覆盖了isEqual:方法（例如NSString， NSArray，NSDate，...）但不是CIImage：

let thingy1 = CIImage(image: image)
let thingy2 = CIImage(image: image)

创建两个不同的CIImage个实例，这些实例比较为“不相等”。

Answer 2

FlexMonkey的ImageCompareDemo是一个不完整的端口，来自Facebook的C {+ {3}}到Swift。它省略了每像素像素比较的最后部分。我的是Swift 4，这是整个功能：

static func compareWithImage(reference:CGImage, target:CGImage, tolerance:CGFloat) -> Bool {
    guard reference.width == target.width && reference.height == target.height else { return false }
    let referenceImageSize = CGSize(width:CGFloat(reference.width), height:CGFloat(reference.height))
    let targetImageSize = CGSize(width:CGFloat(target.width), height:CGFloat(target.height))
    let minBytesPerRow = min(reference.bytesPerRow, target.bytesPerRow)
    let referenceImageSizeBytes = Int(referenceImageSize.height) * minBytesPerRow
    let referenceImagePixels = calloc(1, referenceImageSizeBytes)
    let targetImagePixels = calloc(1, referenceImageSizeBytes)
    let referenceImageCtx = CGContext(data: referenceImagePixels,
                                      width: Int(referenceImageSize.width),
                                      height: Int(referenceImageSize.height),
                                      bitsPerComponent: reference.bitsPerComponent,
                                      bytesPerRow: minBytesPerRow,
                                      space: reference.colorSpace!,
                                      bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)
    let targetImageCtx = CGContext(data: targetImagePixels,
                                   width: Int(targetImageSize.width),
                                   height: Int(targetImageSize.height),
                                   bitsPerComponent: target.bitsPerComponent,
                                   bytesPerRow: minBytesPerRow,
                                   space: target.colorSpace!,
                                   bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)
    guard let referenceImageContext = referenceImageCtx, let targetImageContext = targetImageCtx else {
        return false
    }
    referenceImageContext.draw(reference, in:CGRect(x:0, y:0, width:referenceImageSize.width, height:referenceImageSize.height))
    targetImageContext.draw(target, in:CGRect(x:0, y:0, width:targetImageSize.width, height:targetImageSize.height))
    var imageEqual = true
    if(tolerance == 0) {
        imageEqual =  (memcmp(referenceImagePixels, targetImagePixels, referenceImageSizeBytes) == 0)
    } else {
        let pixelCount = Int(referenceImageSize.width * referenceImageSize.height)

        let p1 = convertUMRPtoUInt32Array(pointer:referenceImagePixels!, length:referenceImageSizeBytes)
        let p2 = convertUMRPtoUInt32Array(pointer:targetImagePixels!, length:referenceImageSizeBytes)
        var percent:CGFloat = 0
        var numDiffPixels = 0
        for n in 0..<pixelCount {
            if(p1[n] != p2[n]) {
                numDiffPixels += 1
                percent = CGFloat(numDiffPixels) / CGFloat(pixelCount)
                if (percent > tolerance) {
                    imageEqual = false;
                    break;
                }
            }
        }
        //print(percent)
    }
    referenceImagePixels?.deallocate(bytes:referenceImageSizeBytes, alignedTo:1)
    targetImagePixels?.deallocate(bytes: referenceImageSizeBytes, alignedTo: 1)
    return imageEqual
}

CIImage应该是等同的吗？

2 个答案: