R – Computing a UIImage to be saved to the photo album

bitmapcglayeriphoneobjective cuiimage

I basically want to automatically create a tiled image from a bunch of source images, and then save that to the user's photo album. I'm not having any success drawing a bunch of small UIImage's into one big UIImage. What's the best way to accomplish this? Currently I'm using UIGraphicsBeginImageContext() and [UIImage drawAtPoint], etc. All I ever end up with is a 512×512 black square. How should I be doing this? I'm looking to CGLayer's, etc. seems there are a lot of options but none that work particularly easily.

Let me actually put my code in:

CGSize size = CGSizeMake(512, 512);
UIGraphicsBeginImageContext(size);
UIGraphicsPushContext(UIGraphicsGetCurrentContext());
for (int i = 0; i < 4; i++)
{
    for (int j = 0; j < 4; j++)
    {
        UIImage *image = [self getImageAt:i:j];
        [image drawAtPoint:CGPointMake(i*128,j*128)];
    }
}
UIGraphicsPopContext();
UIImage *result = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
UIImageWriteToSavedPhotosAlbum(result, nil, nil, nil);

I should note that actually the above is not what happens in my code exactly. What really happens is that I call every line before and including to UIGraphicsPushContext, then in an animation timer I slowly increment the drawing and draw to the context. Then after it's all done I call everything from UIGraphicsPopContext onward.

Best Answer

Oh, then you can just save the onscreen view after it has been rendered on screen:

 UIGraphicsBeginImageContext(myBigView.bounds.size);
    [view drawRect:myBigView.bounds];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

are you storing it back to an image?

UIImage *myBigImage = UIGraphicsGetImageFromCurrentImageContext();

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The last two are identical; "atomic" is the default behavior (~~note that it is not actually a keyword; it is specified only by the absence of nonatomic~~ -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

R – Objective-C Sprite Class (iPhone SDK)

When you say "size" is a CGSize object, I think you have a misunderstanding. CGSize is a regular C structure, and CGSizeMake returns a "CGSize" not a "CGSize *". My guess is that "size" is declared as "CGSize *" because you thought it was an object. Try removing the *.

Best Answer

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