[Development] matrix math help needed - https://bugreports.qt.io/browse/QTBUG-84441

Matthew Woehlke mwoehlke.floss at gmail.com
Wed May 27 15:58:39 CEST 2020

On 27/05/2020 04.34, Edward Welbourne wrote:
> Matthew Woehlke (26 May 2020 18:15) wrote:
>> The documentation is not clear if the scale, rotate, etc. methods of
>> QTransform apply *before* or *after* whatever the QTransform is already
>> doing. The bug report indicates that they are applied *first*.
>> Given the potential for breaking existing code which expects the current
>> behavior, my inclination would be to clarify the documentation to
>> clearly state the existing behavior.
> Yes, the docs do need updated; they do correctly say what QTransform does

Really? Where?

Here is, for example, the documentation of QTransform::scale:

   Scales the coordinate system by sx horizontally and sy vertically, and
   returns a reference to the matrix.

*Nothing* there clearly states, at least to my reading, whether the 
"new" transform happens *before* or *after* any existing transforms that 
the QTransform is already doing.

IMO, changing this to clarify that would help significantly.

I would argue this should be done *even if* the class description 
explains this. However, I didn't find any clarification there, either. 
(Admittedly, I did not rigorously read the entire description, but 
that's still making my point; if it's hard to find, users are going to 
be confused.)


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