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

Matthew Woehlke mwoehlke.floss at gmail.com
Tue May 26 18:15:00 CEST 2020

On 26/05/2020 12.06, Thiago Macieira wrote:
> On Tuesday, 26 May 2020 08:59:09 PDT Thiago Macieira wrote:
>> Neither QMatrix nor QTransform have seen any change since the Qt 5.0
>> repository import that was related to the underlying math. Only licensing
>> and C++ updates.
>> The task https://bugreports.qt.io/browse/QTBUG-84457 is claiming there's
>> some wrong math or at least unexpected behaviour. Can someone who
>> understands these classes take a look?
> Updated subject: https://bugreports.qt.io/browse/QTBUG-84441 is the right
> task.

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.

(Note: I didn't actually test this myself or look at the code, I am just 
going off of what the bug report says. In any case, however, the 
documentation should be fixed.)


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