Difference bewteen TTML2 Shear Calculations and actual imscJS implementation

[dtatut]

In TTML2, 10.4.5.3 Shear Calculations use the trigonometric function tangent to generate the shear matrix. imscJS uses arc sinus. Any reason for this difference? The visual result is obviously not the same depending on what function you use.

[palemieux]

In TTML2, shear is specified as a percentage, but the (normative) mathematical transform is expressed as a 2D matrix transformation which uses the tangent of the shear angle.

As it turns out, TTML2 does not specify how to convert from the shear percentage to the shear angle. An issue has been filed.

imscJS currently uses shear_percentage = sin(shear_angle) although it is not clear why -- probably some historical reason.

It is likely that the correct conversion is shear_percentage = shear_angle * 100 / 90

The (relatively) good news is that sin(x) is approximately equal to x for small x.

Thanks for the catch!