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

[Matthew Woehlke]

On 27/05/2020 11.09, Giuseppe D'Angelo via Development wrote:
> Sure, augmenting the docs would help. But <devil's advocate> the whole 
> point of the API is for its usage to be straightforward. If you do
> 
>> QTransform t;
>> t.translate();
>> t.rotate();
>> t.scale();
>> auto result = t.map(foo);
> 
> the "obvious" meaning should be that foo is getting first translated, 
> then rotated, then scaled; not the other way around.

While that may be true, changing it now is going to break *every* user 
that uses these methods to generate compound transformations... and 
it'll be a silent break. I would be *very* surprised if that doesn't 
generate more bug reports.

I think the *absolute best* we could do would be to add an optional 
parameter specifying in what order to apply the change, defaulted to the 
old value, with a macro to instead default it to the new value. Maybe we 
can then, eventually, make defaulting to the old behavior deprecated 
(either opt-in to the new, or explicitly specify), and eventually remove 
it and make the new behavior default. But we're probably talking 2-3 
release cycles.

Is it really worth it?

-- 
Matthew