奇怪的蜂巢NSBitmapImageRep

最可能的问题是您没有将alpha与颜色分量预乘。

从NSBitmapImageRep类参考：

Alpha预乘法和位图格式

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 the CGImageCreate function and kCGImageAlphaLast parameter.) Alternatively, include the NSAlphaNonpremultipliedBitmapFormat flag when creating the bitmap.

Note

Use the bitmapFormat parameter to the initWithBitmapDataPlanes: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 the bitmapFormat property before working with image data.

You have used the -init... method without the bitmapFormat parameter. In that case, you need to query the bitmapFormat of the resulting object and make sure you build your pixel values to match that format. Note that the format dictates where the alpha appears in the component order, whether the color components are premultiplied by the alpha, and whether the components are floating point or integer.

You can switch to using the -init... method that does have the bitmapFormat parameter and specify NSAlphaNonpremultipliedBitmapFormat mixed in with your choice of other flags (first or last, integer or floating point, endianness). Note, though, that not all possible formats are supported for drawing.

顺便说一句，我强烈建议阅读的章节NSImage，并NSBitmapImageRep在10.6了AppKit发行说明。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，此处最相关。尤其是最后一部分，提供了有关直接使用像素的重要信息。