为什么这个受支持的CGBitmapContextCreate参数组合被拒绝为不受支持？

Question

我有以下Swift代码来制作一个 CGBitmapContext

let imageDirectoryPath:String = "/Users/blah/blah/"
let imageFileName:String = "flower.tif"
let imageNS = NSImage(contentsOfFile: imageDirectoryPath + imageFileName)!
let imageCG = imageNS.CGImageForProposedRect(nil, context: nil, hints: nil)
var rawDataIn:[UInt8] = [UInt8](count: Int(imageNS.size.width) * Int(imageNS.size.height) * 4, repeatedValue: 0xff)
let context = CGBitmapContextCreate(
    &rawDataIn,
    Int(imageNS.size.width),
    Int(imageNS.size.height),
    8,
    Int(imageNS.size.width * 4),
    CGColorSpaceCreateDeviceRGB(),
    CGImageAlphaInfo.PremultipliedLast.rawValue)

我从这里得到一个错误,并使用Xcode的方案为项目设置环境变量CGBITMAP_CONTEXT_LOG_ERRORS我得到有关错误的详细信息:

Nov 10 10:12:16  SwiftConsoleGrabcut[826] : 
    CGBitmapContextCreate: unsupported parameter combination:
        8 integer bits/component;
        32 bits/pixel;
        RGB color space model; kCGImageAlphaPremultipliedLast;
        19789 bytes/row.
    Valid parameters for RGB color space model are:
        16  bits per pixel,      5  bits per component,      kCGImageAlphaNoneSkipFirst
        32  bits per pixel,      8  bits per component,      kCGImageAlphaNoneSkipFirst
        32  bits per pixel,      8  bits per component,      kCGImageAlphaNoneSkipLast
        32  bits per pixel,      8  bits per component,      kCGImageAlphaPremultipliedFirst
        32  bits per pixel,      8  bits per component,      kCGImageAlphaPremultipliedLast
        64  bits per pixel,      16 bits per component,      kCGImageAlphaPremultipliedLast
        64  bits per pixel,      16 bits per component,      kCGImageAlphaNoneSkipLast
        128 bits per pixel,      32 bits per component,      kCGImageAlphaNoneSkipLast |kCGBitmapFloatComponents
        128 bits per pixel,      32 bits per component,      kCGImageAlphaPremultipliedLast |kCGBitmapFloatComponents
    See Quartz 2D Programming Guide (available online) for more information.

但是我使用的参数组合,每像素32位,每个组件8位,并且kCGImageAlphaPremultipliedLast 是列出的支持组合之一.

为什么被拒绝？

这可能是一个问题size.width.我加载的tif图像imageNS是481像素宽,但imageNS.size.width报告为4947.4285714285716,这可以解释错误消息中给出的每行字节数的值.