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Shift a face-region mask by a number of pixels

Source: R/shift_mask.R
shift_mask.Rd

Slides a logical mask by down and right pixels and returns the shifted mask in the same shape as the input. Convenience wrapper around the manual recipe shown in vignette section 4.5 for tuning an elliptical make_face_mask() region ("full", "nose", "mouth", "upper_face", "lower_face") on a base image whose features sit a few pixels off the default geometry.

For the rectangle regions ("eyes", "left_eye", "right_eye"), prefer the region_bounds argument of make_face_mask(): passing region_bounds = c(row_min, row_max, col_min, col_max) in 0-1 image fractions is more precise than counting pixels and is the canonical tuning route for those regions.

Pixels shifted off the image are dropped, and the newly exposed edge is filled with FALSE (i.e., outside the mask).

Usage

shift_mask(mask, down = 0L, right = 0L, img_dims = NULL)

Arguments

mask

Logical vector of length prod(img_dims) (column-major, as returned by make_face_mask()) or a logical matrix.

down, right

Integer pixel offsets. Positive down moves the mask towards the bottom of the image; positive right moves it towards the right edge. Negative values move up / left. Defaults are 0 (no shift).

img_dims

Integer c(nrow, ncol) (or a single integer for a square image). Required when mask is a vector; ignored when mask is already a matrix.

Value

Logical mask of the same shape as the input (matrix-in, matrix-out; vector-in, vector-out).

See also

make_face_mask() (the region_bounds route for the rectangle regions), plot_mask_overlay(), plot_face_mask().

Examples

# Reshape the default mouth mask, slide it down 20 pixels on a
# 256-pixel image (about 8% of the height), and confirm it
# still has the right number of TRUE pixels.
m <- make_face_mask(c(256L, 256L), region = "mouth")
shifted <- shift_mask(m, down = 20L, img_dims = c(256L, 256L))
identical(sum(m), sum(shifted))
#> [1] TRUE

# Matrix-in, matrix-out.
mat <- matrix(m, 256L, 256L)
shifted_mat <- shift_mask(mat, down = 20L, right = 8L)
dim(shifted_mat)
#> [1] 256 256