[R] solving for a 2D transformation matrix

[Prof Brian Ripley]

The best least-squares fit maps centroid to centroid.  So find the mean of
the observed and target, and shift the observed to match the target. That
just leaves a rotation, and I would directly maximize the sum of squared
errors over that using optimize().

On 2 Jul 2004, Russell Senior wrote:

> 
> We have recently digitized a set of points from some scanned
> engineering drawings (in the form of PDFs).  The digitization resulted
> in x,y page coordinates for each point.  The scans were not aligned
> perfectly so there is a small rotation, and furthermore each
> projection (e.g. the yz-plane) on the drawing has a different offset
> from the page origin to the projection origin.  From the dimensions
> indicated on the drawing, I know the intended "world" coordinates of a
> subset of the points.  I want to use this subset of points to compute
> a best-fit transformation matrix so that the remaining points can be
> converted to world coordinates.
> 
> The transformation matrix is (I think) of the form:
> 
>  [ x' ]   [ a11 a12 a13 ] [ x ]
>  | y' | = | a21 a22 a23 | | y |
>  [ w' ]   [ a31 a32 a33 ] [ w ]
> 
>  where:
>  
>     x,y = page coordinates
>     x',y' = world coordinates
> 
>     a13 = translation of x
>     a23 = translation of y
> 
>     a11 = scale * cos(theta)
>     a12 = sin(theta)
>     a21 = -sin(theta)
>     a22 = scale * cos(theta)
> 
>     a31 = 0
>     a31 = 0
>     a33 = 1
>     w' = 1
>     w = 1
> 
> Can anyone give me a pointer on how to go about solving for the
> transformation matrix given a set of points, where x,y and x',y' are
> available?  I sense the presence a solution lingering in the murky
> mists, (some kind of least squares?) but I am not sure what it is or
> how to go about it exactly.
> 
> Thanks for your help!
> 
> 

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595