Follow-up Comment #6, bug #21735 (project mypaint):

More info.  It appears this has to do with GTK reporting values as -1 to 1
instead of degrees or radians.  Krita on the other hand, being QT, does it
differently and uses degrees.  So really the -1 to 1 values are sort-of a
percentage of 60 degrees.  I wonder if we can just convert that number into
radians based on the 60 degree assumption, and THEN do the declination
calculation and everything would be fixed...?

http://sourceforge.net/p/linuxwacom/mailman/linuxwacom-devel/?viewmonth=201203

"GIMP, Inkscape, and MyPaint all treat the values they get from GTK as
the normalized length of the tool's projection along the corresponding
axis (they take the arctangent of the values to get an azimuth). While
GTK doesn't explicitly state that this is what applications receive,
its a reasonable reading of their documentation: "tilt attributes
range from -1.0 to 1.0 ... representing the maximum tilt to the left
or up ... [and] to the right or down". It looks like in practice GTK
actually only scales the tilt values it gets from X based on the
minimum and maximum. Its not what applications expect, but appears to
work well enough that nobody's bothered to file a bug."

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