[Development] QImage::transformed returns shallow copy for QTransform::TxNone matrix type.

[Tomasz Olszak]

The pointer comes from external object and it's lifetime is managed by it.
I need to ensure that I will not use it anymore after leaving certain
function. Hence it don't even need to be thread operation, So using clean
function won't help here.

The problem here is that you can't say if transformed returns shallow or
deep copy. Result depends on argument value. Of course the transform
returning shallow copy on identity matrix is nice but should be documented
somewhere. After reading docs I expected deep copy like from copy function.
But maybe it's only me :)

Thanks for profound explanation.


2016-07-11 12:10 GMT+01:00 Simon Hausmann <Simon.Hausmann at qt.io>:

> Hi,
>
>
> Thank you for the explanation of your use-case. What remains unclear to me
> is why it is necessary for you to ensure a deep copy?
>
>
> Is it related to the fact that your image is created from a raw data
> pointer and after your worker thread is done, you cannot guarantee
>
> for the life-time of the raw data anymore?
>
>
> Perhaps you could use the QImage constructor that takes a cleanup function
> (and cleanupInfo pointer). If your worker thread
>
> ends up making a copy of the image data as a result of your
> transformation, then your original copy can and will be deleted
>
> when you discard the source of the transform() call. That is something
> that you can keep track of. Vice-versa if it's not being called,
>
> then you know that transformed() (or generally speaking _any_ sequence of
> operations you may have done) still operate on a shallow
>
> copy. Then you can do an explicit deep copy.
>
>
> Generally speaking my impression - assuming I did understand your use-case
> correctly - is that your problem stems from the unknown
>
> life-cycle of your data. I don't think the behavior of
> QImage::transformed() should be made worse for the common case where shallow
>
> copying benefits performance and memory consumption.
>
>
> Simon
> ------------------------------
> *From:* Development <development-bounces+simon.hausmann=
> qt.io at qt-project.org> on behalf of Tomasz Olszak <olszak.tomasz at gmail.com>
> *Sent:* Monday, July 11, 2016 12:53:51 PM
> *To:* development at qt-project.org
> *Subject:* Re: [Development] QImage::transformed returns shallow copy for
> QTransform::TxNone matrix type.
>
> QImage::copy returns deep copy and has similar documentation so I assumed
> that when docs say copy it means deep copy.
>
> Let's consider common case:
>
> 1. QImage img created from raw data pointer got from e.g. driver.
> 2. img used in thread to perform some transforms
> 3. Save result as deep copy
>
> Ensuring step 3 is most efficient and does not perform unnecessary copying
> I need to write something like:
>
> QTransform t;
> if (m.type() == QTransform::TxNone)
>     return img.copy();
> else
>     return img.transformed(m);
>
> AFAIU img.transformed(m).copy() will copy twice when  m.type() !=
> QTransform::TxNone right?
>
> Don't you think that is seems strange and is not documented enough?
>
> 2016-07-11 10:59 GMT+01:00 Simon Hausmann <Simon.Hausmann at qt.io>:
>
>> Hi,
>>
>>
>> Could you elaborate on what you see as the discrepancy between docs and
>> implementation? The docs don't say whether
>>
>> it's a shallow or a deep copy, so it looks to me that the implementation
>> is within the bounds of the docs.
>>
>>
>> Plus it seems sensible to return a shallow copy, doesn't it?
>>
>>
>>
>> Simon
>> ------------------------------
>> *From:* Development <development-bounces+simon.hausmann=
>> qt.io at qt-project.org> on behalf of Tomasz Olszak <olszak.tomasz at gmail.com
>> >
>> *Sent:* Monday, July 11, 2016 11:38:06 AM
>> *To:* development at qt-project.org
>> *Subject:* [Development] QImage::transformed returns shallow copy for
>> QTransform::TxNone matrix type.
>>
>> Hello,
>>
>> QImage:: transformed(const QTransform &matrix, Qt::TransformationMode
>> mode = Qt::FastTransformation) docs:
>>
>> "Returns a copy of the image that is transformed using the given
>> transformation matrix and transformation mode."
>>
>> But if matrix.type() == QTransform::TxNone then shallow instead of deep
>> copy is returned.
>>
>> I'm happy to submit a fix but I don't know what is expected behaviour?
>> Should implementation follow docs or docs or docs follow implementation
>> here?
>>
>> T.
>>
>>
>>
>
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