filling NSImage with binary data from NSData

Hi,
I'm working on a cocoa app that visualizes part of a 3D dataset (MRI
images) as a discrete 2D "slice". I am using NSImage to show the
image. The user chooses a slice of the array (a 256 x 256 grid) to
show in an NSImageView.

The hard part here is the data file format, which is a binary file
consisting of an 8-bit header followed by a 256*256*256 array of
bytes. Each byte specifies a grayscale value.

I have hucked together some code based on other postings and replies
on this list, but I've hit a dead end. Here's where I am at:

I load this data file into an NSData object (declared with global
scope) as follows

vmrData = [NSData dataWithCFile:fileName];

>From what I can tell, this seems to do the right thing.

Then I trigger the following procedure that actually loads the desired
portion of the data into the NSImage using an NSBitmapImageRep:
- (void)drawXImage:(int)xSlice
{
NSBitmapImageRep* imageRep;
// a pointer to the portion of vmrData containing the desired slice
unsigned char *plane;

// pointer to the part of the dataset we want to visualize
// note vmrData has global scope
plane = (unsigned char*)[vmrData bytes]+8+(256 * 256 * xSlice);

// init the NSBitmapImageRep
imageRep=[[[NSBitmapImageRep alloc]

pixelsWide:256
pixelsHigh:256
bitsPerSample:8
samplesPerPixel:1
hasAlpha:N
isPlanar:YES

bytesPerRow:0
bitsPerPixel:0] autorelease];

// create an NSImage and show it in the window
//n.b. imageView is my delegate for the NSImageView I want to
// show the image in.
NSImage *producedNSImage;
producedNSImage = [NSImage alloc];
[producedNSImage addRepresentation:imageRep];
[imageView setImage:producedNSImage];

}

The procedure produces a segfault when I run it, but not till after it
returns. I suspect the problem ahs to do with how I am setting the
"plane" pointer or how I'm telling the NSBitmapImageRep about it.

As a sidenote: I am also unclear whether I should be releasing
producedNSImage and imageRep at the end of the procedure, or whether
the view needs this to be retained.

Thanks for whatever help you can give me on this,
-Marc-

major problem is that the plane parameter has to be a pointer an array
of pointers to the image data - and not a pointer to the image data itself.
Each pointer in this array points to a different plane of the image. But
being a grayscale image, there should be only one pointer present in this
array.

Alternately, the program could pass NULL for the plane parameter. In that
case the NSBitmageImageRep object would allocate enough memory to hold the
image data based on the image's other measurements. The program would then
copy the image data into the address returned by calling bitmapData on the
bitmap image rep object.

Greg

Message
From: "marc joanisse" <marc.joanisse (AT) gmail (DOT) com>
To: <cocoa-dev (AT) lists (DOT) apple.com>
Sent: Sunday, May 07, 2006 12:37 PM
Subject: filling NSImage with binary data from NSData

Hi,
I'm working on a cocoa app that visualizes part of a 3D dataset (MRI
images) as a discrete 2D "slice". I am using NSImage to show the
image. The user chooses a slice of the array (a 256 x 256 grid) to
show in an NSImageView.

The hard part here is the data file format, which is a binary file
consisting of an 8-bit header followed by a 256*256*256 array of
bytes. Each byte specifies a grayscale value.

I have hucked together some code based on other postings and replies
on this list, but I've hit a dead end. Here's where I am at:

I load this data file into an NSData object (declared with global
scope) as follows

vmrData = [NSData dataWithCFile:fileName];

>From what I can tell, this seems to do the right thing.

Then I trigger the following procedure that actually loads the desired
portion of the data into the NSImage using an NSBitmapImageRep:
- (void)drawXImage:(int)xSlice
{
NSBitmapImageRep* imageRep;
// a pointer to the portion of vmrData containing the desired slice
unsigned char *plane;

// pointer to the part of the dataset we want to visualize
// note vmrData has global scope
plane = (unsigned char*)[vmrData bytes]+8+(256 * 256 * xSlice);

// init the NSBitmapImageRep
imageRep=[[[NSBitmapImageRep alloc]

pixelsWide:256
pixelsHigh:256
bitsPerSample:8
samplesPerPixel:1
hasAlpha:N
isPlanar:YES

bytesPerRow:0
bitsPerPixel:0] autorelease];

// create an NSImage and show it in the window
//n.b. imageView is my delegate for the NSImageView I want to
// show the image in.
NSImage *producedNSImage;
producedNSImage = [NSImage alloc];
[producedNSImage addRepresentation:imageRep];
[imageView setImage:producedNSImage];

}

The procedure produces a segfault when I run it, but not till after it
returns. I suspect the problem ahs to do with how I am setting the
"plane" pointer or how I'm telling the NSBitmapImageRep about it.

As a sidenote: I am also unclear whether I should be releasing
producedNSImage and imageRep at the end of the procedure, or whether
the view needs this to be retained.

Thanks for whatever help you can give me on this,
-Marc-

Hi,
5/7/06, Greg Herlihy <greghe (AT) pacbell (DOT) netwrote:
Alternately, the program could pass NULL for the plane parameter. In that
case the NSBitmageImageRep object would allocate enough memory to hold the
image data based on the image's other measurements. The program would then
copy the image data into the address returned by calling bitmapData on the
bitmap image rep object.

Aha! Yes, this does work well (sort of) - I can copy the bytes I want
to the pointer to bitmapData and it draws it perfectly. *Except*:
when I call the procedure a second time, (e.g., to set a different
xSlice, thus changing the data to be displayed) it segfaults.

Here's the code so far. It works once but fails when called a second time.
- (void)drawXImage:(int)xSlice
{
NSBitmapImageRep* imageRep;
unsigned char *plane;
int i,j;

imageRep=[[[NSBitmapImageRep alloc] initWithBitmapDataPlanes:nil
pixelsWide:256
pixelsHigh:256
bitsPerSample:8
samplesPerPixel:1
hasAlpha:N
isPlanar:YES

bytesPerRow:256

bitsPerPixel:0] autorelease];

NSImage* producedNSImage = [[[NSImage alloc]
initWithSize:NSMakeSize(256, 256)] autorelease];

[producedNSImage addRepresentation:imageRep];
// pointer to the bitmapData
unsigned char *data = [imageRep bitmapData];
// pointer to the portion of vmrData (an NSData object) I want to show
// segfault happens here!
plane = (unsigned char*) [vmrData bytes]+8+(256*256*xSlice);

// copy bytes from *data to *plane
for (i=0;i<256;i++){
for (j=0;j<256;j++){
*dataplane++;
}
}

[imageView setImage:producedNSImage];
}

Any hints are appreciated.

5/8/06, Lester Dowling <lesterdowling (AT) bigpond (DOT) comwrote:

That's what I used to think, too -- it's just what I found to work
reliably in my applications. In this case, you're right though because
I found the seg fault doesn't happen because of the planar setting.

I've tried it both ways and apparently it doesn't matter, at least in my app.

object when it has already been auto-released. The problem is fixed by
substituting this line:

vmrData = [[NSData dataWithCFile:fileName] retain];

Yes, of course. Thanks for the help everyone. It works swimmingly now.
Thanks to everyone for their help,
-Marc-