Resizing and cropping¶
Creating thumbnails (by resizing images) and cropping are most frequent works about images. This guide explains ways to deal with sizes of images.
Above all, to get the current size of the image check
width
and height
properties:
>>> from urllib.request import urlopen
>>> from wand.image import Image
>>> f = urlopen('http://pbs.twimg.com/profile_images/712673855341367296/WY6aLbBV_normal.jpg')
>>> with Image(file=f) as img:
... width = img.width
... height = img.height
...
>>> f.close()
>>> width
48
>>> height
48
If you want the pair of (width
,
height
), check size
property also.
Note
These three properties are all readonly.
Resize images¶
It scales an image into a desired size even if the desired size is larger
than the original size. ImageMagick provides so many algorithms for resizing.
The constant FILTER_TYPES
contains names of filtering
algorithms.
See also
- ImageMagick Resize Filters
Demonstrates the results of resampling three images using the various resize filters and blur settings available in ImageMagick, and the file size of the resulting thumbnail images.
Image.resize()
method takes width
and height
of a desired size, optional filter
('undefined'
by
default which means IM will try to guess best one to use) and optional
blur
(default is 1). It returns nothing but resizes itself in-place.
>>> img.size
(500, 600)
>>> img.resize(50, 60)
>>> img.size
(50, 60)
Sample images¶
Although Image.resize()
provides
many filter
options, it’s relatively slow. If speed is important for
the job, you’d better use Image.sample()
instead. It works in similar way to Image.resize()
except it doesn’t provide filter
and
blur
options:
>>> img.size
(500, 600)
>>> img.sample(50, 60)
>>> img.size
(50, 60)
Crop images¶
To extract a sub-rectangle from an image, use the
crop()
method. It crops the image in-place.
Its parameters are left
, top
, right
, bottom
in order.
>>> img.size
(200, 300)
>>> img.crop(10, 20, 50, 100)
>>> img.size
(40, 80)
It can also take keyword arguments width
and height
. These parameters
replace right
and bottom
.
>>> img.size
(200, 300)
>>> img.crop(10, 20, width=40, height=80)
>>> img.size
(40, 80)
There is an another way to crop images: slicing operator. You can crop
an image by [left:right, top:bottom]
with maintaining the original:
>>> img.size
(300, 300)
>>> with img[10:50, 20:100] as cropped:
... print(cropped.size)
...
(40, 80)
>>> img.size
(300, 300)
Specifying gravity
along with width
and height
keyword
arguments allows a simplified cropping alternative.
>>> img.size
(300, 300)
>>> img.crop(width=40, height=80, gravity='center')
>>> img.size
(40, 80)
Transform images¶
Use this function to crop and resize an image at the same time, using ImageMagick geometry strings. Cropping is performed first, followed by resizing.
For example, if you want to crop your image to 300x300 pixels and then scale it by 2x for a final size of 600x600 pixels, you can call:
img.transform('300x300', '200%')
Other example calls:
# crop top left corner
img.transform('50%')
# scale height to 100px and preserve aspect ratio
img.transform(resize='x100')
# if larger than 640x480, fit within box, preserving aspect ratio
img.transform(resize='640x480>')
# crop a 320x320 square starting at 160x160 from the top left
img.transform(crop='320+160+160')
See also
- ImageMagick Geometry Specifications
Cropping and resizing geometry for the
transform
method are specified according to ImageMagick’s geometry string format. The ImageMagick documentation provides more information about geometry strings.
Seam carving (also known as content-aware resizing)¶
New in version 0.3.0.
Seam carving is an algorithm for image resizing that functions by establishing a number of seams (paths of least importance) in an image and automatically removes seams to reduce image size or inserts seams to extend it.
In short: you can magickally resize images without distortion! See the following examples:
Original |
Resized |
![]() |
![]() |
Cropped |
Seam carving |
![]() |
![]() |
You can easily rescale images with seam carving using Wand:
use Image.liquid_rescale()
method:
>>> image = Image(filename='seam.jpg')
>>> image.size
(320, 234)
>>> with image.clone() as resize:
... resize.resize(234, 234)
... resize.save(filename='seam-resize.jpg')
... resize.size
...
(234, 234)
>>> with image[:234, :] as crop:
... crop.save(filename='seam-crop.jpg')
... crop.size
...
(234, 234)
>>> with image.clone() as liquid:
... liquid.liquid_rescale(234, 234)
... liquid.save(filename='seam-liquid.jpg')
... liquid.size
...
(234, 234)
Note
It may raise MissingDelegateError
if your
ImageMagick is configured --without-lqr
option. In this case
you should recompile ImageMagick.
See also
- Seam carving — Wikipedia
The article which explains what seam carving is on Wikipedia.
Note
The image seam.jpg
used in the above example is taken by
D. Sharon Pruitt and licensed under CC-BY-2.0.
It can be found the original photography from Flickr.