Swift从图像文件获取像素数组时索引超1000后值为0问题求助

核心错误是像素坐标转一维数组索引的计算公式错误：
你当前代码中给红、绿、蓝、alpha数组赋值时使用的索引是x + y，对于512×512的图像来说，x和y的取值范围都是0~511，二者相加的最大值仅为 511 + 511 = 1022，也就是说你只给数组前1023个索引的位置赋了有效值，索引≥1023的位置一直保留初始化时的默认值0，这就是你看到索引超过1000后数值全为0的直接原因。

1. 替换为正确的一维索引计算公式：对于宽度为width的图像，坐标(x,y)对应的一维数组索引应为 y * width + x
2. 补充手动分配的像素指针内存释放逻辑，避免内存泄漏
3. 调整循环顺序为外层遍历行y、内层遍历列x，更符合常规图像存储的顺序逻辑

修复后的代码如下：

func forLoopWay() -> [[UInt8]] {
    let startTime = CFAbsoluteTimeGetCurrent()
    print("for loop way")
    let Image = UIImage(named: imageName)
    let image: CGImage = Image!.cgImage!
    
    let width = image.width
    let height = image.height
    let colorspace = CGColorSpaceCreateDeviceRGB()
    let bytesPerRow = (4 * width);
    let bitsPerComponent = 8
    // 保存初始指针地址，后续用于释放内存
    let pixelsStart = UnsafeMutablePointer<UInt8>.allocate(capacity: width * height * 4 )
    var pixels = pixelsStart

    let context = CGContext.init(data: pixels,
                                 width: width,
                                 height: height,
                                 bitsPerComponent: bitsPerComponent,
                                 bytesPerRow: bytesPerRow,
                                 space: colorspace,
                                 bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)

    context?.draw(image, in: CGRect(x: 0, y: 0, width: CGFloat(width), height: CGFloat(height)))
    
    var alpha = [UInt8].init(repeating: 0, count: width * height)
    var red = [UInt8].init(repeating: 0, count: width * height)
    var green = [UInt8].init(repeating: 0, count: width * height)
    var blue = [UInt8].init(repeating: 0, count: width * height)

    let test_val = pixels.pointee
    print(test_val)
    print(pixels.pointee)
    // 调整循环顺序，外层遍历行y，内层遍历列x
    for y in 0..<height {
        for x in 0..<width {
            // 使用正确的索引计算公式
            let index = y * width + x
            red[index] = pixels.pointee
            pixels += 1
            green[index] = pixels.pointee
            pixels += 1
            blue[index] = pixels.pointee
            pixels += 1
            alpha[index] = pixels.pointee
            pixels += 1
        }
    }
    
    // 释放手动分配的内存
    pixelsStart.deallocate()
    
    let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime
    print("Time elapsed \(String(format : "%0.5f", timeElapsed)) seconds")
    print()
    return [red, green, blue, alpha]
}

内容的提问来源于stack exchange，提问作者KleinerWarden