Affine transforms - matrix algebra

Pavel Safrata pavel.safrata at
Wed Jul 11 13:17:40 PDT 2012

Hi Kirill,
what is the use-case? I suspect that if you want the raw matrix to do 
some math with it we are rather missing some operations doing the math 

On 11.7.2012 21:35, Kirill.Prazdnikov wrote:
> Hi Pavel,
>   I`m not sure, but many time times I found the following conversions 
> as very useful:
> public float [] asArray(float data[12]);
> public Affine(float data[]);
> along with 12-arguments ctor.
> Thanks
>   -Kirill
> On 7/11/2012 6:57 PM, Pavel Safrata wrote:
>> Hi Martin,
>> thank you for your input.
>> On 10.7.2012 18:36, Martin Desruisseaux wrote:
>>> Hello Pavel
>>> Many thanks for taking care of this task!
>>> Le 10/07/12 17:00, Pavel Safrata a écrit :
>>>> On Transform class:
>>>>     public Transform getConcatenation(Transform transform) // 
>>>> multiplication
>>>>     public Transform getInverse() throws 
>>>> NoninvertibleTransformException // negation
>>>>     public Transform copy()
>>> Sound fine to me, while I'm not sure why a 'copy' method instead 
>>> than overriding the 'clone()' method?
>> Good point, we can use clone().
>>>> Constructors:
>>>>     public Affine(Transform transform)
>>>>     public Affine(double mxx, double mxy, double mxz, double tx,
>>>>             double myx, double myy, double myz, double ty,
>>>>             double mzx, double mzy, double mzz, double tz)
>>> Look fine.
>>>> Setters of the entire matrix:
>>>>     (...snip...)
>>> I don't know for JavaFX, but in my experience with Java2D, I wasn't 
>>> using the setter methods often, except 'setToIdentity' and 
>>> 'setToTransform'. For example rather than invoking 
>>> 'setToTranslation', I strongly push our developers to use 
>>> 'translate' (or 'concatWithTranslation' in this proposal) instead. 
>>> If a developer really wants the functionality of 'setToTranslation', 
>>> he can get it by invoking 'setToIdentity()' followed by 
>>> 'concatWithTranslation'. Or yet better, 'setToTransform(...)' 
>>> instead than 'setToIdentity' with the coefficients of some previous 
>>> state that the user saved.
>> I don't insist on having the setters, anybody wants them?
>>> The rational is that in many cases, the affine transform is already 
>>> initialized to some important value. For example in Java2D, 
>>> AffineTransform is initialized to the transform from 'dot' to 
>>> whatever units the underlying device uses. When rendering on screen, 
>>> this is the identity transform. But when printing, this is something 
>>> different that depends on the printer resolution. In GIS 
>>> applications, it depends on the zoom level. Other applications may 
>>> use magnifier glass over some areas. Because the initial transform 
>>> is often (but not always) the identity one, developers with limited 
>>> experience with affine transforms often use 'setTranslate' or 
>>> 'setScale' in situations where they should really use 'translate' or 
>>> 'scale', and do not realize their bug before late in the development 
>>> process. For this reason, I would be inclined to discourage every 
>>> setter methods except 'setToIdentity()' and 'setToTransform'. 
>>> Keeping in mind that it is often easier to fill a hole later than to 
>>> fix something broken, I think it would be safer to leave out all 
>>> other setter methods for now, and revisit later if experience show 
>>> that they are really needed.
>> While I don't think our Affine class will ever have varying initial 
>> values I'm ok with keeping only setToIdentity() and setToTransform() 
>> if there is no demand for the other setters right now.
>>>> Operations on the matrix (modifying it in place):
>>>>     (...snip...)
>>> Sound good, minus the unfortunate 'concatWith*' naming :-(.
>>>> Instead of "concatWithTranslation" it would be more natural to use 
>>>> just "translate" (and similarly for the others), but unfortunately 
>>>> these method names are already taken by the static factory methods 
>>>> on Transform class. This is unpleasant but we need to be backward 
>>>> compatible so we have to introduce different names. We'll be happy 
>>>> to hear better naming suggestions than the concatWith* (which is 
>>>> pretty descriptive I think but not really nice).
>>> Hard to say... A consistency with "preTranslate" would be nice, but 
>>> "postTranslate" doesn't look very nice... What about the following?
>>> * Rename "preTranslate" as "appendTranslation"
>>> * Rename "concatWithTranslation" as "prependTranslation"
>>> The "preConcatenate(Tx)" name in Java2D was actually misleading to 
>>> some developers, because it works as if points were first 
>>> transformed by the original transform, then transformed by 'Tx'. 
>>> Maybe "appendTranslation(Tx)" would make clear that the translation 
>>> is applied after the original transform. This would also make 
>>> operation order clearer. The following code using Java2D API:
>>> tr.translate(...)
>>> tr.scale(...)
>>> tr.rotate(...)
>>> must be read from bottom to up: it is as if points were rotated 
>>> first, then scaled, then translated. So maybe the above proposition 
>>> would make that more obvious:
>>> tr.prependTranslation(....)
>>> tr.prependScale(...)
>>> tr.prependRotate(...)
>>> tr.appendTranslation(...) // Just for fun.
>> These names indeed sound way better.
>> If we are going this way, shouldn't we do also this?
>> * Rename 'concatenate' as 'prepend'
>> * Rename 'preConcatenate' as 'append'
>> * Rename 'getConcatenation' as .. well .. 'getPrependage' :-)
>> As a black-box transformation composing this makes sense. As a matrix 
>> algebra, it may be also confusing: tA.append(tB) means matrix 
>> multiplication 'B x A', does it sound good? Maybe yes, I'm not sure. 
>> Anyway, so far the best proposal I think.
>>>> Would you want static factory methods on Affine (creating Affine 
>>>> instances with the simple transforms)?
>>> I don't think it is necessary. I found the static factory methods of 
>>> Java2D AffineTransform rarely used.
>>>> Would it be important to you whether or not matrix changes are 
>>>> atomic? If you call one of the methods that modify the entire 
>>>> matrix, can be listeners for each member called immediately as the 
>>>> members are set, or do they need to be called after all the members 
>>>> are updated?
>>> I don't have experience in this area. But naively, it seems to we 
>>> that it would be better to be notified only after the full matrix 
>>> has been updated...
>> Yes, I would also pick this option. It's just that it would mean 
>> writing much more complicated (and a bit less effective) code so the 
>> question is whether or not you think it is important.
>> Thanks,
>> Pavel
>>>     Regards,
>>>         Martin

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