transform() vs pos() for QGraphicsItem

[bootchk]

It is not just setPos(), but also setScale() and setRotation(), and setTransformations(), that do not affect the separate transform() matrix. (e.g. transform() returns the same matrix before and after a call to setScale().)

I won't argue which is the best approach for the API, but I will say that the Qt approach is not conventional: most graphics programming texts approach use a single transform matrix that a rotation operation would change.

And the Qt documentation must be read VERY carefully. For example: "The scale is combined with the item's rotation(), transform() and transformations() to map the item's coordinate system to the parent item." Why doesn't it mention pos()? Then you should follow the link to "Transformations" to learn what "combine" means. And "scale a transformation" is QTransform.scale(), not the same thing as calling "setScale()" on a graphic item. QTransform.scale() is a verb setter, QGraphicsItem.scale() is a noun getter. QTransform is separate from QGraphicsTransform. transform() is not the same as transformations(), even though both are noun getters with too similar names.

This discussion is related to how to serialize a QGraphicsItem. Do you need to serialize (pos(), scale(), rotation(), transform(), transformations()) or can you somehow get a single matrix representing all of those?

[wysota]

It is not just setPos(), but also setScale() and setRotation(), and setTransformations(), that do not affect the separate transform() matrix. (e.g. transform() returns the same matrix before and after a call to setScale().)

The methods you mention were introduced in Qt 4.6 thus the discussion in this thread does not take them into consideration.

I won't argue which is the best approach for the API, but I will say that the Qt approach is not conventional: most graphics programming texts approach use a single transform matrix that a rotation operation would change.

Those methods were added to QGraphicsItem as a convenience. Therefore both "old" and "new" ways exist to operate on the item.

Why doesn't it mention pos()?

Because pos() is not part of the "transformation".

Then you should follow the link to "Transformations" to learn what "combine" means. And "scale a transformation" is QTransform.scale(), not the same thing as calling "setScale()" on a graphic item. QTransform.scale() is a verb setter, QGraphicsItem.scale() is a noun getter. QTransform is separate from QGraphicsTransform. transform() is not the same as transformations(), even though both are noun getters with too similar names.

Again, some classes and methods are a later addition to Qt. Since you can't remove something that's already defined in an earlier release, some functionality is "doubled".

This discussion is related to how to serialize a QGraphicsItem. Do you need to serialize (pos(), scale(), rotation(), transform(), transformations()) or can you somehow get a single matrix representing all of those?

The attributes you mention are not part of the object definition thus this is not about serializing the item but rather the scene (as in getting a snapshot of current mappings of items in the scene). There is no built-in mechanism for that, you need to query all the attributes you are interested in on your own.