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

Tomasz Olszak olszak.tomasz at gmail.com
Mon Jul 11 12:53:51 CEST 2016

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();
    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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