Vertical Shearing

[pigubaoza]

Right now the affine shear function does a horizontal shear. Are there plans to add shearing in the vertical direction? Or is there another way of doing it?

[aleju]

That is currently not in the library.
You could hack it by subclassing imgaug.augmenters.geometric.Affine. Assuming that you use the cv2 backend in Affine, you would have to overwrite the method Affine._warp_cv2(...). Most of the code would be completely identical with the current code of the method, but the generation of the affine transformation matrix

matrix_to_topleft = tf.SimilarityTransform(translation=[-shift_x, -shift_y])
matrix_transforms = tf.AffineTransform(
    scale=(scale_x, scale_y),
    translation=(translate_x_px, translate_y_px),
    rotation=math.radians(rotate),
    shear=math.radians(shear)
)
matrix_to_center = tf.SimilarityTransform(translation=[shift_x, shift_y])
matrix = (matrix_to_topleft + matrix_transforms + matrix_to_center)

would have to be changed.

scikit-image's transform.AffineTransform (documentation) only supports shearing in horizontal direction, but I think you could get vertical direction by computing

1. the affine transformation matrix to rotate by -90°
2. the affine transformation matrix to apply the desired shearing in degrees (in horizontal direction)
3. the affine transformation matrix to rotate by +90°
4. the affine transformation matrix to apply the remaining parameters (same as in the code block above, but without shearing)

These matrices could then be multiplied to get the final affine transformation matrix.