我尝试构建图像。逐个像素。所以首先我构建了一些类,它可以在每个像素上循环绘制不同的颜色。但它仅在将 alpha 设置为 255 时才有效。更改 alpha 会使颜色变暗并改变图片。大小和位置都可以。
var rep:NSBitmapImageRep = NSBitmapImageRep(
bitmapDataPlanes: nil,
pixelsWide: width,
pixelsHigh: height,
bitsPerSample: 8,
samplesPerPixel: 4,
hasAlpha: true,
isPlanar: false,
colorSpaceName: NSDeviceRGBColorSpace,
bytesPerRow: 0,
bitsPerPixel: 0)!
for posX in 0-offset...width+offset*2-1 {
for posY in 0-offset...height+offset*2-1 {
var R = Int(Float(posX)/Float(width)*255)
var G = Int(Float(posY)/Float(height)*255)
var B = Int(rand() % 256)
pixel = [R,G,B,alpha]
rep.setPixel(&pixel, atX: posX, y: posY)
}
}
rep.drawInRect(bounds)
alpha 设置为 255
alpha 设置为 196
这次 alpha 等于 127。
还有 64 个
我哪里错了?
最佳答案
最可能的问题是您没有将 alpha 与颜色分量预乘。
来自NSBitmapImageRep
class reference :
Alpha Premultiplication and Bitmap Formats
When creating a bitmap using a premultiplied format, if a coverage (alpha) plane exists, the bitmap’s color components are premultiplied with it. In this case, if you modify the contents of the bitmap, you are therefore responsible for premultiplying the data. Note that premultiplying generally has negligible effect on output quality. For floating-point image data, premultiplying color components is a lossless operation, but for fixed-point image data, premultiplication can introduce small rounding errors. In either case, more rounding errors may appear when compositing many premultiplied images; however, such errors are generally not readily visible.
For this reason, you should not use an
NSBitmapImageRep
object if you want to manipulate image data. To work with data that is not premultiplied, use the Core Graphics framework instead. (Specifically, create images using theCGImageCreate
function andkCGImageAlphaLast
parameter.) Alternatively, include theNSAlphaNonpremultipliedBitmapFormat
flag when creating the bitmap.Note
Use the
bitmapFormat
parameter to theinitWithBitmapDataPlanes:pixelsWide:pixelsHigh:bitsPerSample:samplesPerPixel:hasAlpha:isPlanar:colorSpaceName:bitmapFormat:bytesPerRow:bitsPerPixel:
method to specify the format for creating a bitmap. When creating or retrieving a bitmap with other methods, the bitmap format depends on the original source of the image data. Check thebitmapFormat
property before working with image data.
您使用了 -init...
方法,没有 bitmapFormat
参数。在这种情况下,您需要查询结果对象的 bitmapFormat
并确保构建像素值以匹配该格式。请注意,格式规定了 alpha 在组件顺序中的位置、颜色组件是否预乘了 alpha 以及组件是 float 还是整数。
您可以切换到使用具有 bitmapFormat
参数的 -init...
方法,并指定 NSAlphaNonpremultipliedBitmapFormat
与您的选择混合其他标志(第一个或最后一个,整数或 float ,字节顺序)。但请注意,并非所有可能的格式都支持绘图。
顺便说一下,我强烈建议阅读 10.6 AppKit release notes 中有关 NSImage
和 NSBitmapImageRep
的部分。 .搜索“NSImage、CGImage 和 CoreGraphics 阻抗匹配”,然后通过“NSBitmapImageRep:CoreGraphics 阻抗匹配和性能说明”部分开始阅读,这与此处最相关。特别是最后一节包含有关直接使用像素的重要信息。
关于macos - 奇怪的行为 NSBitmapImageRep,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/32416828/