Cropping, Resizing and Rescaling

This demonstration shows how to use cropping, resizing and rescaling operations on an image in Julia using ImageTransformations.jl

using Images, TestImages, OffsetArrays
# ImageTransformations is reexported by Images
# load an example image
img_source = testimage("lighthouse")

Cropping Operation

Cropping is one of the most basic photo manipulation processes, and it is carried out to remove an unwanted object or irrelevant noise from the periphery of a photograph, to change its aspect ratio, or to improve the overall composition.

Let's first check the size of the image

img_size = size(img_source)

(512, 768)

Output is (512,768) which stands means img_source is 512 in height and 768 in width. In Julia, images as multidimensional arrays are stored in column-major order, which means that this first index corresponds to the vertical axis (column) and the second to the horizontal axis (row).

Let's crop the image from sides by 1/8 of img_source each side and leave it as it is from top to bottom.

Easiest way to do this is indexing: img_source[y1:y2, x1:x2]

Region of Interest: [y1, y2] sets the range for y-axis and [x1, x2] sets the range for x-axis of source image.

img_cropped = @view img_source[ :,floor(Int, 1/8*img_size[2]) : floor(Int, 7/8*img_size[2])]

Let's see the size of the cropped image:

size(img_cropped)

(512, 577)

We can also do size-preserved cropping of the image by replacing the contents of image to white pixels or transparent pixels using PaddedView:

img_padded = PaddedView(
    zero(eltype(img_source)),
    OffsetArray(img_cropped, OffsetArrays.Origin(1, floor(Int, 1/8*img_size[2]))),
    axes(img_source))

Resizing Operation

Resizing is a method to resize an image to a given specific output image shape. It is different from rescaling as in rescaling we use a scaling factor to manipulate the image.

img_square = imresize(img_source, (400, 400));
img_small = imresize(img_source, ratio=1/4);
img_medium = imresize(img_small, size(img_small).*2);
mosaicview(img_source, img_square, img_small, img_medium; nrow=1)

Rescaling

Rescale operation resizes an image by a given scaling factor. The scaling factor can either be a single floating point value, or multiple values - one along each axis. Image scaling is the process of changing the size of an image while preserving the original aspect ratio.

Rescaling by percentage

percentage_scale = 0.6
new_size = trunc.(Int, size(img_source) .* percentage_scale)
img_rescaled = imresize(img_source, new_size);
mosaicview(img_source, img_rescaled; nrow=1)

We calculated new size by estimating the size of frame by multiplying size by scale and then truncated it to Int format.

Rescaling to a specific dimension

new_width = 200
percentage_scale = new_width / size(img_source,2);
new_size = trunc.(Int, size(img_source) .* percentage_scale);
img_rescaled = imresize(img_source, new_size);
mosaicview(img_source, img_rescaled; nrow=1)

We have updated our scale by percentage solution to calculate scale-percentage dynamically based on a change in one of the dimensions. Remember: size(sourceimage) == (height, width)

Rescaling by two-fold using restrict function

rescaled_both = restrict(img_source); # both side
rescaled_height = restrict(img_source, 1); # dims=1, i.e., height dimension
rescaled_width = restrict(img_source, 2); # dims=2, i.e., width dimension
mosaicview(img_source, rescaled_both, rescaled_height, rescaled_width; nrow=1)