# Affine transforms - matrix algebra

Pavel Safrata pavel.safrata at oracle.com
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
getters/setters.

Pavel

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
>

```