(correction)

Okay, so the first time I posted this I was definitely not thinking straight at all. Here's a correction.

So let's take a look at the math involved:

In a scale, pixels (x',y') are given by the following:

x' = x * x_scale_factor

y' = y * y_scale_factor

In a skew, pixels (x",y") are given by the following:

x" = y * x_skew_factor

y" = x * y_skew_factor

If you remember our image is partly scaled and partly skewed. Remember that the images are divided along the line f(x)=x meaning along this line pixels of the skewed picture approach the scaled picture.

So let's pull out that high school math and we'll find that means that:

x * x_scale_factor = y * x_skew_factor

y * y_scale_factor = x * y_skew_factor

By substituting in the line y=x this tells us along the line y=x pixels converge when x_scale_factor = x_skew_factor and y_scale_factor = y_skew_factor.