Re: Converting CGImageRefs to NSImages, how?

FROM : Scott Thompson
DATE : Wed Apr 27 21:16:53 2005

>>    NSImage* image = [[NSImage alloc] initWithSize:rect.size];
>>    [image lockFocus];
>>    CGContextDrawImage([[NSGraphicsContext currentContext]
>> graphicsPort], *(CGRect*)&rect, imageRef);
>>    [image unlockFocus];
>>

>
> Looks good, but it's not exactly what I'd call 'conversion' but 
> more of a
> 'duplication'.  As I understand it, the CGImage and NSImage will not
> share the same pixel memory, so depending on your uses this may be 
> a big
> problem.


Actually that concept really doesn't mesh with the idea of a CGImage 
to begin with.  A CGImage represents a rectangular array of pixels, 
but there is no reason to expect that it has an explicit memory 
buffer that it might share with an NSImage.

For example, if you create a CGImage using 
CGImageCreateWithJPEGDataProvider the resulting CGImage may, or may 
not have a pixel buffer associated with it.  That is an 
implementation detail of CGImage.

Pure speculation on my part, but I can imagine that in more up-to-
date uses of CGImage, there is also the possibility that, if it has 
one, the CGImage has moved it's pixel buffer to the video card.  If 
this is the case then having an NSImage pointing at the place where 
the pixel buffer USED to be.

Finally, the image content of an NSImage is mutable (through 
lockFocus) while that of a CGImage is not.

These are pretty strong reasons that CGImage keeps the pointer to 
it's image buffer to itself.

> Actually, are the pixel values even guaranteed to be the same or
> does the process of 'drawing' (ie CGContextDrawImage()) do some colour
> conversion?


That certainly seems possible.