Affine transforms - matrix algebra

Pavel Safrata pavel.safrata at
Thu Jul 12 04:23:26 PDT 2012

I like the get(row, col) method, I think we can introduce it in addition 
to the array conversions. Similarly we can add set(row, col, value). 
Both just convenience methods internally calling the actual property 


On 12.7.2012 11:05, Martin Desruisseaux wrote:
> The array methods may also be useful for working on matrix 
> coefficients using indices rather than hard-coded access to the mxx, 
> mxy, etc. properties. For example: when the affine may contain scale, 
> rotation, flip and translation but no shear, at least in the 2D case 
> (I think it works in 3D too), we can compute the scale factors "as if 
> there were no flip or rotation" by computing the square root of the 
> sum of the square of all values (except translation) on a row or a 
> column. Whatever we compute on rows or on columns depend on whatever 
> we want the scale factors to be expressed relative to the axes of the 
> source coordinate system or the target coordinate system (this is not 
> the same if there is a rotation). For this kind of computation, 
> looping over an array using indices - or alternatively providing a 
> Matrix.get(row, column) method - is convenient.
>     Martin
> Le 12/07/12 08:35, Pavel Safrata a écrit :
>> Hi Kirill,
>> you are not right with the performance, the elements are (possibly 
>> invalid) properties so we need to call all the getters anyway, plus 
>> construct the array, so the performance is actually worse. But the 
>> use-case seems to be valid, especially if the other libraries support 
>> such conversion. I just wouldn't call it 'asArray' which suggests 
>> that the array will keep updating with the matrix changes but rather 
>> 'toArray'. To be consistent we need to use doubles instead of floats 
>> and we should also add a similar setter. So I propose:
>> public double[] toArray()
>> public double[] toArray(double[] a)
>> public Affine(double[] matrix)
>> public void setTransform(double[] matrix)
>> Where the first two will behave similarly as if it was a List of 12 
>> doubles, the latter two will throw IllegalArgumentException if length 
>> of the array is not 12.
>> Regards,
>> Pavel

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