[cairo] Adding cairo_shear and family
M Joonas Pihlaja
jpihlaja at cc.helsinki.fi
Sat Jan 10 14:42:53 PST 2009
Hi,
I think calling a 2x2 multiplication by
[ 1 sx]
[ sy 1]
a shear is slightly misleading since shears don't commute.
While it's true that shears along the coordinate axes are given
respectively by
[1 sx], [1 0],
[0 1] [sy 1]
it matters whether one does an x-shear first or a y-shear first,
and [1 sx; sy 1] isn't a simple combination of x- and y-shears
alone. Also, the inverse of the [1 sx; sy 1] matrix is
1 [1 -sx]
----------- [-sy 1].
(1 - sx sy)
Finally, I think the docs shouldn't give the equations in matrix
form but rather in words and scalar equations like the others do.
None of the other cairo_t or cairo_matrix_t functions use matrix
form.
Cheers,
Joonas
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