NSBitmapImageRep generated BMP can't be read on Windows

Question

I have an NSBitmapImageRep that I am creating the following way:

+ (NSBitmapImageRep *)bitmapRepOfImage:(NSURL *)imageURL {
    CIImage *anImage = [CIImage imageWithContentsOfURL:imageURL];
    CGRect outputExtent = [anImage extent];

    NSBitmapImageRep *theBitMapToBeSaved = [[NSBitmapImageRep alloc]  
                                        initWithBitmapDataPlanes:NULL pixelsWide:outputExtent.size.width  
                                        pixelsHigh:outputExtent.size.height  bitsPerSample:8 samplesPerPixel:4  
                                        hasAlpha:YES isPlanar:NO colorSpaceName:NSDeviceRGBColorSpace  
                                        bytesPerRow:0 bitsPerPixel:0];

    NSGraphicsContext *nsContext = [NSGraphicsContext graphicsContextWithBitmapImageRep:theBitMapToBeSaved];

    [NSGraphicsContext saveGraphicsState];
    [NSGraphicsContext setCurrentContext: nsContext];
    CGPoint p = CGPointMake(0.0, 0.0);

    [[nsContext CIContext] drawImage:anImage atPoint:p fromRect:outputExtent];

    [NSGraphicsContext restoreGraphicsState];

    return [[theBitMapToBeSaved retain] autorelease];
}

And being saved as BMP this way:

NSBitmapImageRep *original = [imageTools bitmapRepOfImage:fileURL];
NSData *converted = [original representationUsingType:NSBMPFileType properties:nil];
[converted writeToFile:filePath atomically:YES];

The thing here is that the BMP file can be read and manipulated correctly under Mac OSX, but under Windows, it just fails to load, just like in this screenshot:

screenshot http://dl.dropbox.com/u/1661304/Grab/74a6dadb770654213cdd9290f0131880.png

If the file is opened with MS Paint (yes, MS Paint can open it) and then resaved, though, it will work.

Would appreciate a hand here. :)

Thanks in advance.

Answer 1

I think the main reason your code is failing is that you are creating your NSBitmapImageRep with 0 bits per pixel. That means your image rep will have precisely zero information in it. You almost certainly want 32 bits per pixel.

However, your code is an unbelievably convoluted way to obtain an NSBitmapImageRep from an image file on disk. Why on earth are you using a CIImage? That is a Core Image object designed for use with Core Image filters and makes no sense here at all. You should be using an NSImage or CGImageRef.

Your method is also poorly named. It should instead be named something like +bitmapRepForImageFileAtURL: to better indicate what it is doing.

Also, this code makes no sense:

[[theBitMapToBeSaved retain] autorelease]

Calling retain and then autorelease does nothing, because all it does in increment the retain count and then decrement it again immediately.

You are responsible for releasing theBitMapToBeSaved because you created it using alloc. Since it is being returned, you should call autorelease on it. Your additional retain call just causes a leak for no reason.

Try this:

+ (NSBitmapImageRep*)bitmapRepForImageFileAtURL:(NSURL*)imageURL
{
    NSImage* image = [[[NSImage alloc] initWithContentsOfURL:imageURL] autorelease];
    return [NSBitmapImageRep imageRepWithData:[image TIFFRepresentation]];
}

+ (NSData*)BMPDataForImageFileAtURL:(NSURL*)imageURL
{
    NSBitmapImageRep* bitmap = [self bitmapRepForImageFileAtURL:imageURL];
    return [bitmap representationUsingType:NSBMPFileType properties:nil];
}

You really need to review the Cocoa Drawing Guide and the Memory Management Guidelines, because it appears that you are having trouble with some basic concepts.