NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/graphics/14163

Internet Message Format | 1993-01-24 | 5.5 KB

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Path: sparky!uunet!paladin.american.edu!gatech!swrinde!emory!sol.ctr.columbia.edu!The-Star.honeywell.com!umn.edu!csus.edu!netcom.com!levine
From: [e-mail] (Ron Levine)
Subject: Re: Row major or column major matrices?
Message-ID: <[e-mail]>
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
References: <[e-mail]>
Date: Sun, 24 Jan 1993 22:55:47 GMT
Lines: 102

In article <[e-mail]> [e-mail] (Eric Haines) writes:
>I feel vaguely like I'm in a twilight zone episode.  You know, like the ones
>where some guy wakes up and finds that all the lettering is backwards and
>everyone is left-handed.
>
>Someone was telling me that OpenGL allows only post-concatenation of matrices.
>This seemed bass-ackwards, and after a few minutes reconfirming in my slow
>turtle brain that you'd normally want pre-concatenation of matrices, I read
>over the OpenGL manual page carefully.  Aha, they store matrices in
>column-major form:
>
>    a0   a4   a8   a12    a typical    1  0  0 -2
>    a1   a5   a9   a13    translation    0  1  0  5
>    a2   a6   a10  a14    matrix -->    0  0  1  3
>    a3   a7   a11  a15            0  0  0  1
>
>Me, I learned on my pappy's knee (well, actually, through Newman & Sproull) to
>use row major matrices.  Up until today using column major matrices never
>crossed my mind.  Anywell, ideas like "we only post-concatenate column major
>matrices" translates into "we only pre-concatenate row major matrices".
>Lovely, a new way to confuse computer graphics people (as if left vs.
>right-handed coordinate systems weren't bad enough).  This is not a slam of
>OpenGL - Phigs uses column major matrices, too!
>
>It's admittedly a minor point, and really just a notational difference - the
>matrices are just the same, of course.  As long as you get the matrix elements
>in the right order it doesn't matter to the computer.  It's just writing it
>down on paper and presenting it is where the difference appears.
>
>Just out of curiosity, I looked at a few computer graphics books and whatnot
>and how they presented 3D matrices.
>
>Row Major users:
>
>    Foley & VD (1st edition), Newman & Sproull, Graphics Gems series,
>    Rogers books, Watt, Burger & Gillies, the Thalmanns, HP Starbase
>
>Column Major users:
>
>    Foley & VD & F & H (2nd edition), OpenGL, Phigs (and PEX, I assume)
>
>So Foley et al is the only book I could find that uses column-major matrices.
>Weirder yet, the first edition of Foley & VD uses the row-major form, but then
>the second edition goes to column-major form!  Evidently this book's second
>edition made quite an impression (or maybe just reflects some new teaching
>methods present at the time), since both OpenGL and PEX adopted it, in
>contrast with the rest of the research and academic community who (seem to)
>use row-major matrices.
>
>If you made it this far, take a minute and hit that "r" key and let me know
>which form (if either) you prefer, where you learned it, and any other
>insights you might have.  I'm interested to see if there's any correlation
>with school, or age, or whatever.  I'll summarize to the net.
>
>Thanks, and not that I'm biased or anything, but dismemberment and death to all
>column-major users,
>
>Eric Haines

Eric, I believe that your note is confusing two different
conventional distinctions.

"Row-major" and "column-major", as I have understood the terms for
years, refer to how the matrices are stored in memory, not to how they
are used to represent transformations. That is, the terms distinguish
between the Fortran convention, in which the first subscript varies
most rapidly as memory address increases, and the C convention, in
which the second subscript varies most rapidly as memory address
increases.  Although your note refers to how matrices are stored, I
think you are really concerned with quite a different distinction,
namely, whether the matrix represents a linear transformation as a
left multiplier of column vectors or as a right multiplier of row
vectors.  Mathematicians sometimes distinguish these two conventions
as the "pre-fix" and "post-fix" conventions.

Mathematicians have long been split into two camps, between pre-fix
and post-fix preferences.  Geometers and analysts have tended more to
the pre-fix convention--thus you learned in calculus that f(x) denotes
the value of a function f for a domain element x. Then, if you compose
two functions, (gf)(x) means g(f(x)), that is, first apply f then
apply g.  This corresponds to interpreting a matrix as a left
multiplier of column vectors.

But some schools of abstract algebraists logicians, and some authors
of computer graphics books, have insisted on the post-fix (x)f or xf.
I always work in the pre-fix convention when I have a choice, but this
is only because in the past most of my math professors and books have
used it and so it is a little more natural to me.  I especially prefer
it in computer graphics, which is inspired more by geometry than by
abstract algebra.

But the two conventions are completely equivalent, and one should have
no difficulty in using either.  Of course it is important to know
which convention is used by any particular system with which you are
working.  And of course, the matrices may be stored in row-major or
column-major order for either convention of writing functional
expressions; that distinction tends to depend on the programming
language.

Ron