Displaying an image with alpha channel on OSX - image

Question

I am trying to display a 32bit, 4 channeled image in my window. The fourth channel in the image is an alpha channel. As an experiment, I create a completely Red image, where each pixel has the RGB value: 255,0,0. For each pixel I also add an alpha value of 204.
What I expected to see was a completely Red image with some transparency, but what I see instead is a completely opaque image with the values altered.
Expected Output:
Current output:
The code I am using:
NSRect windowRect = {0,0,200,200};
m_NSWindow = [[NSWindow alloc] initWithContentRect:windowRect styleMask:NSBorderlessWindowMask backing:NSBackingStoreBuffered defer:NO];
[m_NSWindow setTitle:#"overlayWindow"];
[m_NSWindow makeKeyAndOrderFront:nil];
g_imageView = [[NSImageView alloc] initWithFrame:NSMakeRect(0,0,200,200)];
[m_NSWindow.contentView addSubview:g_imageView];
[m_NSWindow setOpaque:NO];
[m_NSWindow setAlphaValue:1.0];
NSBitmapImageRep* imageRep = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:nil
pixelsWide:200
pixelsHigh:200
bitsPerSample:8
samplesPerPixel:4
hasAlpha:YES
isPlanar:NO
colorSpaceName:NSDeviceRGBColorSpace
bitmapFormat:NSAlphaNonpremultipliedBitmapFormat
bytesPerRow:(200*4)
bitsPerPixel:32];
memcpy(imageRep.bitmapData,m_paintBuffer.data,160000);
NSSize imageSize = NSMakeSize(200,200);
NSImage* myImage = [[NSImage alloc] initWithSize: imageSize];
[myImage addRepresentation:imageRep];
[g_imageView setImage:myImage];
where m_paintBuffer.data points to the raw pixel data for the image.
Not sure if it is relevant to the question, but m_paintBuffer is of type Mat from OpenCV

Answer 1

The window's content view is not the entirety of the window's content, as it were. Even a borderless window has (private) theme views surrounding and containing the content view.
In particular, the window has a background color of a light gray. This is drawn "behind" your content view and is opaque. However, you can change it by setting the window's backgroundColor property to, for example, [NSColor clearColor]. And that's what you should do to achieve your desired effect.