How to resize an image with OpenCV2.0 and Python2.6

Question

If you wish to use CV2, you need to use the resize function.

For example, this will resize both axes by half:

small = cv2.resize(image, (0,0), fx=0.5, fy=0.5) 

and this will resize the image to have 100 cols (width) and 50 rows (height):

resized_image = cv2.resize(image, (100, 50)) 

Another option is to use scipy module, by using:

small = scipy.misc.imresize(image, 0.5)

There are obviously more options you can read in the documentation of those functions (cv2.resize, scipy.misc.imresize).

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Update:
According to the SciPy documentation:

imresize is deprecated in SciPy 1.0.0, and will be removed in 1.2.0.
Use skimage.transform.resize instead.

Note that if you're looking to resize by a factor, you may actually want skimage.transform.rescale.

Example doubling the image size

There are two ways to resize an image. The new size can be specified:

1. Manually;

   height, width = src.shape[:2]

   dst = cv2.resize(src, (2*width, 2*height), interpolation = cv2.INTER_CUBIC)

2. By a scaling factor.

   dst = cv2.resize(src, None, fx = 2, fy = 2, interpolation = cv2.INTER_CUBIC), where fx is the scaling factor along the horizontal axis and fy along the vertical axis.

To shrink an image, it will generally look best with INTER_AREA interpolation, whereas to enlarge an image, it will generally look best with INTER_CUBIC (slow) or INTER_LINEAR (faster but still looks OK).

Example shrink image to fit a max height/width (keeping aspect ratio)

import cv2

img = cv2.imread('YOUR_PATH_TO_IMG')

height, width = img.shape[:2]
max_height = 300
max_width = 300

# only shrink if img is bigger than required
if max_height < height or max_width < width:
    # get scaling factor
    scaling_factor = max_height / float(height)
    if max_width/float(width) < scaling_factor:
        scaling_factor = max_width / float(width)
    # resize image
    img = cv2.resize(img, None, fx=scaling_factor, fy=scaling_factor, interpolation=cv2.INTER_AREA)

cv2.imshow("Shrinked image", img)
key = cv2.waitKey()

Using your code with cv2

import cv2 as cv

im = cv.imread(path)

height, width = im.shape[:2]

thumbnail = cv.resize(im, (round(width / 10), round(height / 10)), interpolation=cv.INTER_AREA)

cv.imshow('exampleshq', thumbnail)
cv.waitKey(0)
cv.destroyAllWindows()

You could use the GetSize function to get those information, cv.GetSize(im) would return a tuple with the width and height of the image. You can also use im.depth and img.nChan to get some more information.

And to resize an image, I would use a slightly different process, with another image instead of a matrix. It is better to try to work with the same type of data:

size = cv.GetSize(im)
thumbnail = cv.CreateImage( ( size[0] / 10, size[1] / 10), im.depth, im.nChannels)
cv.Resize(im, thumbnail)

Hope this helps ;)

Julien

Here's a function to upscale or downscale an image by desired width or height while maintaining aspect ratio

# Resizes a image and maintains aspect ratio
def maintain_aspect_ratio_resize(image, width=None, height=None, inter=cv2.INTER_AREA):
    # Grab the image size and initialize dimensions
    dim = None
    (h, w) = image.shape[:2]

    # Return original image if no need to resize
    if width is None and height is None:
        return image

    # We are resizing height if width is none
    if width is None:
        # Calculate the ratio of the height and construct the dimensions
        r = height / float(h)
        dim = (int(w * r), height)
    # We are resizing width if height is none
    else:
        # Calculate the ratio of the width and construct the dimensions
        r = width / float(w)
        dim = (width, int(h * r))

    # Return the resized image
    return cv2.resize(image, dim, interpolation=inter)

Usage

import cv2

image = cv2.imread('1.png')
cv2.imshow('width_100', maintain_aspect_ratio_resize(image, width=100))
cv2.imshow('width_300', maintain_aspect_ratio_resize(image, width=300))
cv2.waitKey()

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Using this example image

Simply downscale to width=100 (left) or upscale to width=300 (right)

def rescale_by_height(image, target_height, method=cv2.INTER_LANCZOS4):
    """Rescale `image` to `target_height` (preserving aspect ratio)."""
    w = int(round(target_height * image.shape[1] / image.shape[0]))
    return cv2.resize(image, (w, target_height), interpolation=method)

def rescale_by_width(image, target_width, method=cv2.INTER_LANCZOS4):
    """Rescale `image` to `target_width` (preserving aspect ratio)."""
    h = int(round(target_width * image.shape[0] / image.shape[1]))
    return cv2.resize(image, (target_width, h), interpolation=method)