Strange behive NSBitmapImageRep - macos

Question

I try to build image. Pixel by pixel. So first I build some class which can draw by loops different color at each pixel. But it works nice only if alpha is set to 255. Changing alpha makes colors darker and changes picture. Size and place is OK.
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 set to 255
alpha set to 196
alpha equals 127 this time.
And 64.
Where i'm wrong?

Answer 1

The most likely problem is that you're not premultiplying the alpha with the color components.
From the 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 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.
By the way, I strongly recommend reading the sections about NSImage and NSBitmapImageRep in the 10.6 AppKit release notes. Search for "NSImage, CGImage, and CoreGraphics impedance matching" and start reading there through the section "NSBitmapImageRep: CoreGraphics impedance matching and performance notes", which is most relevant here. That last section, in particular, has important information about working directly with pixels.