Objective-C: Display an Array of Floats as an Image - objective-c

Question

In a Cocoa App I would like to display a 2d array of floats in an NSImageView. To make the code as simple as possible, start off by converting the data from float to NSData:
// dataArray: an Nx by Ny array of floats
NSMutableData *nsdata = [NSMutableData dataWithCapacity:0];
long numPixels = Nx*Ny;
for (int i = 0; i < numPixels; i++) {
[nsdata appendBytes:&dataArray[i] length:sizeof(float)];
}
and now try to display the data (the display is left blank):
[theNSImageView setImage:[[NSImage alloc] initWithData:nsdata]];
Is this the correct approach? Is a CGContext needed first? I was hoping to accomplish this with NSData.
I have noted the earlier Stack posts: 32 bit data, close but in reverse, almost worked but no NSData, color image data here, but not much luck getting variations on these working. Thanks for any suggestions.

Answer 1

You can use an NSBitmapImageRep to build up an NSImage float-by-float.
Interestingly, one of its initialisers has the longest method name in all of Cocoa:
- (id)initWithBitmapDataPlanes:(unsigned char **)planes
pixelsWide:(NSInteger)width
pixelsHigh:(NSInteger)height
bitsPerSample:(NSInteger)bps
samplesPerPixel:(NSInteger)spp
hasAlpha:(BOOL)alpha
isPlanar:(BOOL)isPlanar
colorSpaceName:(NSString *)colorSpaceName
bitmapFormat:(NSBitmapFormat)bitmapFormat
bytesPerRow:(NSInteger)rowBytes
bitsPerPixel:(NSInteger)
It's well documented at least. Once you've built it up by supplying float arrays in planes you can then get the NSImage to put in your view:
NSImage *image = [[NSImage alloc] initWithCGImage:[bitmapImageRep CGImage] size:NSMakeSize(width,height)];
Or, slightly cleaner
NSImage *image = [[[NSImage alloc] init] autorelease];
[im addRepresentation:bitmapImageRep];
There is an initialiser which just uses an NSData container:
+ (id)imageRepWithData:(NSData *)bitmapData
although that depends on your bitmapData containing one of the correct bitmap formats.

Answer 2

Ok got it to work. I had tried the NSBitmapImageRep before (thanks Tim) but the part I was missing was in properly converting my floating point data to a byte array. NSData doesn't do that and returns nil. So the solution was not so much in needing to build up an NSImage float-by-float. In fact, one can similarly build up a bitmapContext (using CGBitmapContextCreate (mentioned by HotLicks above)) and that works too, once the floating point data has been represented properly.