Sublassing NSImageRep to support new file format (pgm)

Question 1

It's actually much easier than you're thinking.

First of all, I would recommend making CTPWTPGMImageRep a subclass of NSBitmapImageRep rather than NSImageRep. That will take care of the "hardest" problem, since there will be no need to implement a custom draw method, as NSBitmapImageReps already know how to draw themselves. (In OS X 10.5 and later, NSBitmapImageRep is basically a direct wrapper around CoreGraphics CGImageRefs).

I'm not that familiar with the PGM format, but what you'll basically be doing is creating a representation of the image in the closest destination format that matches the source format. To use a specific example, we'll take the PGM example FEEP image from Wikipedia.

P2
# Shows the word "FEEP" (example from Netpbm main page on PGM)
24 7
15
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0

enter image description here

The closest native image to be able to portray the values in the example image would be a single channel grayscale image, 8 bits per pixel, no alpha. The plan then is to create a CGImageRef with the specified settings, and then use NSBitmapImageRep's initWithCGImage: method to initialize the custom subclass.

I would first override the following two methods to both rely on the singular +imageRepWithData::

+ (NSArray *)imageRepsWithData:(NSData *)data {
    id imageRep = [[self class] imageRepWithData:data];
    return [NSArray arrayWithObject:imageRep];
}

- (id)initWithData:(NSData *)data {
    CTPWTPGMImageRep *imageRep = [[self class] imageRepWithData:data];
    if (imageRep == nil) {
        [self release];
        return nil;
    }
    self = [imageRep retain];
    return self;
}

For me, it was necessary to implement the +imageRepsWithData: method to call the singular method before the image was able to be loaded properly.

I would then change the singular +imageRepWithData: method be as follows:

+ (id)imageRepWithData:(NSData *)data {
    if (data.length < 2) return nil;
    NSString *magicNumberP5 = @"P5";
    const unsigned char *mnP5 = (const unsigned char *)[magicNumberP5 UTF8String];
    unsigned char headerBuffer[20];
    [data getBytes:headerBuffer length:2];

    if (memcmp(mnP5, headerBuffer, 2) != 0) {
        NSLog(@"It is not supported pgm file format.");
        return nil;
    }
    NSArray *pgmParameters = [self calculatePgmParameters:data beginingByte:3];

    NSUInteger width = [[pgmParameters objectAtIndex:0] integerValue];
    NSUInteger height = [[pgmParameters objectAtIndex:1] integerValue];
    NSUInteger maxValue = [[pgmParameters objectAtIndex:2] integerValue];

    NSUInteger imageLength = width * height * 1;

    NSData *imageData = [data subdataWithRange:
                        NSMakeRange(data.length - imageLength, imageLength)];
    const UInt8 *imageDataBytes = [imageData bytes];
    UInt8 *expandedImageDataBytes = malloc(imageLength);

    for (NSUInteger i = 0; i < imageLength; i++) {
        expandedImageDataBytes[i] = 255 * (imageDataBytes[i] / (CGFloat)maxValue);
    }

    NSData *expandedImageData = [NSData dataWithBytes:expandedImageDataBytes
                                               length:imageLength];
    free(expandedImageDataBytes);
    CGDataProviderRef provider = CGDataProviderCreateWithCFData(
                                          (CFDataRef)expandedImageData);
    CGColorSpaceRef colorSpace =
             CGColorSpaceCreateWithName(kCGColorSpaceGenericGrayGamma2_2);
    CGImageRef imageRef = CGImageCreate(width,
                                        height,
                                        8,
                                        8,
                                        width * 1,
                                        colorSpace,
                                        kCGImageAlphaNone,
                                        provider,
                                        NULL,
                                        false,
                                        kCGRenderingIntentDefault);
    CGColorSpaceRelease(colorSpace);
    CGDataProviderRelease(provider);
    if (imageRef == NULL) {
        NSLog(@"CGImageCreate() failed!");
    }
    CTPWTPGMImageRep *imageRep = [[[CTPWTPGMImageRep alloc]
                                initWithCGImage:imageRef] autorelease];
    CGImageRelease(imageRef);
    return imageRep;
}

As you can see, we need to loop through the bytes in the original image and create a second copy of these bytes with the full expanded range between 0 and 255.

To make use of this image rep, you can call it like the following (being sure to use NSImage's initWithData: method):

// if it hasn't been done already:
[NSImageRep registerImageRepClass:[CTPWTPGMImageRep class]];

NSString *path = [[NSBundle mainBundle] pathForResource:@"feep" ofType:@"pgm"];
NSData *data = [NSData dataWithContentsOfFile:path];
NSImage *image = [[[NSImage alloc] initWithData:data] autorelease];
[self.imageView setImage:image];

One more note about your +imageUnfilteredFileTypes method: filename extensions are case insensitive, so there's no need to specify both lowercase and uppercase @"PGM", you can just do lowercase:

+ (NSArray *)imageUnfilteredFileTypes {
    static NSArray *types = nil;
    if (!types) types = [[NSArray alloc] initWithObjects:@"pgm", nil];
    return types;
}

Question 2

It might be easier to simply read the pixel data into memory and just create an NSBitmapImageRep from the pixel data instead of trying to create an image rep specifically for .pgm files.