On Wed, Jun 29, 2005 at 11:40:08AM +0200, Alex S wrote:

> Dot is defined to be x*x + y*y, but in this file the author defined

> cross to be x*x + y*y.

> And also cross should be x*y - y*x, but the author defined Dot to be x*y

> - y*x.

Thankfully, neither that Dot nor Cross macro is used in current code,

so I've simply removed both macros.

Incidentally, libnr/nr-point-fns.h has a dot function with the correct

definition, and a cross function that arguably gives the wrong sign,

depending on what you expect, e.g. whether you think y increases upwards

or downwards, and whether you think of cross in terms of dot product

with rotation in a certain direction or in terms of the basic scalar

operations. (I'm assuming that the conventional definition of (a cross b)

for two-dimensional vectors is ax * by - ay * bx.)

By Inkscape's angle conventions & its rationale (see below) (and with the

above assumption as to the traditional mathematical definition), the cross

function should be changed:

# Angle direction in Inkscape code: If you use the traditional

# mathematics convention that y increases upwards, then positive angles

# are anticlockwise as per the mathematics convention. If you take the

# common non-mathematical convention that y increases downwards, then

# positive angles are clockwise, as is common outside of mathematics.

This convention is intended to reduce problems with interfacing with

libraries & humans that use the mathematical convention.

Now that fred (the author of that cross function and most or all of its

callers) is no longer working on Inkscape, I might get around to doing a

mass change some time after 0.42 is released; though it's not very high

priority of course.

pjrm.

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