# [Transforms] Feedback about "5. Converting a 4x4 to a 3x3 matrix"

From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>
Date: Sun, 22 Mar 2009 20:40:59 +0100

Message-Id: <200903222040.59339.Dr.O.Hoffmann@gmx.de>
```Hello SVG WG,

just minor notes about
http://www.w3.org/TR/2009/WD-SVG-Transforms-20090320/#_4x4-to-3x3-conversion

It starts with:
"A rectangle ABCD is given on plane X-Y. When a 3D affine transform
and perspective projection are applied, a quadrangle A'B'C'D' will
appear on the X-Y plane. Note the X-Y plane is the projection plane.
Generally, this mapping is expressed as a 4x4 matrix."

As far as I understand this, this rectangle ABCD in not used in the
later part of the section. What is the purpose of 'ABCD'?
For authors it should be sufficient, to explain, how a vector transforms
and is projected, what is done with the point K.
For an implementor it is maybe interesting to know, whether planes are
projected to planes, lines or points; straight line are projected to
straight lines or points and cubic curves are projected to other
cubic curves, that only the points and control points have to be
recalculated - or whether these are more complex computations.

There are some expressions like
"M = P.T"

If '.' is meant here to be an operator for multiplication (Just guessing,
I have never seen this before, typically something like * or &middot; or
something is used), this should be defined ;o)
In PHP the '.' is used to join/jam strings together for example...

Typos (?):

"An affine 3D Transform Matrix T is given as M = ..."
->
"An affine 3D Transform Matrix T is given as T = ..." ?

"The combined matrix Mcan be expressed as ..."
->
"The combined matrix M can be expressed as ..." ?

"If matrix F can be used to map point K to point K' as shown below ..."
I'm missing the 'then' case here or is it a typo? 'If' instead of 'The'
or 'A'?

"Therefore, the combination of An affine 3D Transform Matrix
and a Perspective Projection Matrix"
->
"Therefore, the combination of an affine 3D Transform Matrix
and a Perspective Projection Matrix"?

Best wishes

Olaf
```
Received on Sunday, 22 March 2009 19:43:52 UTC

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