NSImage from a 1D pixel array?

Question

I have a large 1D dynamic array in my program that represents a FITS image on disk i.e. it holds all the pixel values of the image. The type of the array is double

Answer 1

initWithData only works for image types that the system already knows about. For unknown types -- and raw pixel data -- you need to construct the image representation yourself. You can do this via Core Graphics as suggested in the answer that Kirby links to. Alternatively, you can use NSImage by creating and adding an NSBitmapImageRep.

The exact details will depend on the format of your pixel data, but here's an example of the process for a greyscale image where the source data (the samples array) is represented as double in the range [0,1]:

/* generate a greyscale image representation */
NSBitmapImageRep *greyRep =
    [[NSBitmapImageRep alloc]
        initWithBitmapDataPlanes: nil  // allocate the pixel buffer for us
                      pixelsWide: xDim 
                      pixelsHigh: yDim
                   bitsPerSample: 8
                 samplesPerPixel: 1  
                        hasAlpha: NO
                        isPlanar: NO 
                  colorSpaceName: NSCalibratedWhiteColorSpace // 0 = black, 1 = white in this color space
                     bytesPerRow: 0     // passing 0 means "you figure it out"
                    bitsPerPixel: 8];   // this must agree with bitsPerSample and samplesPerPixel

NSInteger rowBytes = [greyRep bytesPerRow];

unsigned char* pix = [greyRep bitmapData];
for ( i = 0; i < yDim; ++i )
{
    for ( j = 0; j < xDim; ++j )
    {
        pix[i * rowBytes + j] = (unsigned char)(255 * (samples[i * xDim + j]));
    }
}

NSImage* greyscale = [[NSImage alloc] initWithSize:NSMakeSize(xDim,yDim)];
[greyscale addRepresentation:greyRep];
[greyRep release];

---

EDIT (in response to comment)

I didn't know for sure whether 16 bit samples were supported, but you seem to have confirmed that they are.

What you're seeing stems from still treating the pixels as unsigned char, which is 8 bits. So you're only setting half of each row, and you're setting each of those pixels, one byte at a time, to the two byte value 0xFF00 -- not quite true white, but very close. The other half of the image is not touched, but would have been initialised to 0, so it stays black.

You need instead to work in 16 bit, by first casting the value you get back from the rep:

unsigned short * pix = (unsigned short*) [greyRep bitmapData];

And then assigning 16 bit values to the pixels:

if ( j % 2 )
{
    pix[i * rowBytes + j] = 0xFFFF;
}
else
{
    pix[i * rowBytes + j] = 0;
}

Scratch that, rowBytes is in bytes so we need to stick with unsigned char for pix and cast when assigning, which is a bit uglier:

if ( j % 2 )
{
    *((unsigned short*) (pix + i * rowBytes + j * 2)) = 0xFFFF;
}
else
{
    *((unsigned short*) (pix + i * rowBytes + j * 2)) = 0;
}

(I've switched the order of clauses because the == 0 seemed redundant. Actually for something like this it would be much neater to use ?: syntax, but enough of this C futzing.)