CODEWITHSUNDEEP
geometrycomputer-visionaffinetransformscikit-image
Luca
Luca

Reputation: 10996

Shearing in 2D affine transformations

I was looking at some code in python skimage toolkit regarding representing affine transformations in 2D and there is an AffineTransform class which is defined as:

Parameters
----------
matrix : (3, 3) array, optional
    Homogeneous transformation matrix.
scale : (sx, sy) as array, list or tuple, optional
    Scale factors.
rotation : float, optional
    Rotation angle in counter-clockwise direction as radians.
shear : float, optional
    Shear angle in counter-clockwise direction as radians.
translation : (tx, ty) as array, list or tuple, optional
    Translation parameters.

I notice that the shearing only takes one parameter (shearing angle in counter-clockwise direction). However, why should this not be two parameters? I can shear in x AND in y direction. How come these two operations map into one free parameter in 2D?

Upvotes: 0

Views: 1444

Answers (1)

Stefan van der Walt
Stefan van der Walt

Reputation: 7253

Currently in the code we have, for the transformation matrix:

            [sx * math.cos(rotation), -sy * math.sin(rotation + shear), 0],
            [sx * math.sin(rotation),  sy * math.cos(rotation + shear), 0],
            [                      0,                                0, 1]

I wonder if this is indeed the most intuitive way to specify shear. I have opened an issue: https://github.com/scikit-image/scikit-image/issues/1779

Upvotes: 1

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