From: Dr. Olaf Hoffmann <Dr.O.Hoffmann@gmx.de>

Date: Sat, 21 Mar 2009 18:42:36 +0100

To: www-svg@w3.org

Message-Id: <200903211842.37242.Dr.O.Hoffmann@gmx.de>

Date: Sat, 21 Mar 2009 18:42:36 +0100

To: www-svg@w3.org

Message-Id: <200903211842.37242.Dr.O.Hoffmann@gmx.de>

Hello svg-wg, especially because it is mentioned, that this is not only intended for SVG, I think it is a good idea to provide some more explanations and graphics about the used coordinate system in the introduction and how the matrices, vectors and scalars are related to the user coordinate system. One graphical representation could be a previously not transformed user coordinate system and its relation to the viewport. Especially the directions of x, y, and z (respectively d) should be interesting for authors. Hopefully the direction of the z axes is consistent with that of the SVG filter chapter, the SVG rotation direction etc... Could be helpful for authors too to see how a simple, but total asymmetric object is presented after each basic transformation and the projection with an example graphics. In detail: About 1.1: I assume x, y, d in [x y d] are scalars (coordinates) representing a vector (what is an object with direction and length (euclidian) - is this correct? How are the matrix elements in the projection matrix related to the vector and the user coordinate system? I assume, that the <px>, <py> and <pd> of 2.2 property are related to the vector [x y d] - correct? If so, what happens with pd = d = 0 with the matrix element noted to be '1/-d'? About 1.2: Similar question, how is the matrix related to user coordinate, respectively how transforms a vector in the user coordinate system with this matrix? (could be similar explained as done in the SVG specifications) It is noted, that [a b c d e f g h i j k m] is a vector - what is the meaning of direction and the length (euclidian) for this vector? If it has no meaning, it is maybe more a list of scalars? Is the d matrix element here related to that with the same name in the projection matrix? I assume not, if this is true, maybe the name for the scalar in the projection matrix should be changed to another unused letter to avoid confusion. About 1.2.1.-1.2.6.: I think, it would be simpler to reuse the 'a b c d e f g h i j k m' from 1.2 in the matrices provided here, if it is a simple scalar. If these are more complex terms write something like cos(alpha) is ok, but it should be noted, what alpha is (an angle in degree I assume as usual in SVG). Note that the 'a' symbol appears with different meanings in section 1.2 - first as a matrix element, then as an (undefined) angle, as I assume from the position within the cos - to avoid confusion the angle should be replaced with another symbol, for example a greek letter. In 1.2.3. it is not obvious, what nx, ny, nz or something like 't.nx.nx' might be - is '.' here a symbol for multiplication or some DOM/ecma-script-style notation separator or something else? I assume multiplication, because I know those formulars already, but I think this is not obvious for every reader. Best wishesReceived on Saturday, 21 March 2009 17:50:33 GMT

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