The shear specification takes an angle and a ratio. The angle is the
direction along which the shear moves things; the line through the
origin in this direction is the fixed point of the transformation.
The ratio tells how much to slide things along this direction, as
a function of distance from the fixed point. A ratio of 1 means
that the shear produces a 45-degree angle.

ilk -i init.ppm -0 u:0.5,0.5 -t shear:0,1 -b pad | tojpg > shx.jpg

ilk -i init.ppm -0 u:0.5,0.5 -t shear:90,0.5 -b pad | tojpg > shy.jpg