Question

So, I’m learning my self python by this tutorial and I’m stuck with exercise number 13 which says:

Write a function to uniformly shrink or enlarge an image. Your function should take an image along with a scaling factor. To shrink the image the scale factor should be between 0 and 1 to enlarge the image the scaling factor should be greater than 1.

This is not meant as a question about PIL, but to ask which algorithm to use so I can code it myself.

I’ve found some similar questions like this, but I dunno how to translate this into python.

Any help would be appreciated.

I’ve come to this:

import image

win = image.ImageWin()
img = image.Image("cy.png")

factor = 2

W = img.getWidth()
H = img.getHeight()

newW = int(W*factor)
newH = int(H*factor)

newImage = image.EmptyImage(newW, newH)

for col in range(newW):
    for row in range(newH):
        p = img.getPixel(col,row)
        newImage.setPixel(col*factor,row*factor,p)

newImage.draw(win)

win.exitonclick()

I should do this in a function, but this doesn’t matter right now. Arguments for function would be (image, factor). You can try it on OP tutorial in ActiveCode. It makes a stretched image with empty columns :.

Answer 1

Your code as shown is simple and effective for what’s known as a Nearest Neighbor resize, except for one little bug:

    p = img.getPixel(col/factor,row/factor)
    newImage.setPixel(col,row,p)

Edit: since you’re sending a floating point coordinate into getPixel you’re not limited to Nearest Neighbor – you can implement any interpolation algorithm you want inside. The simplest thing to do is simply truncate the coordinates to int which will cause pixels to be replicated when factor is greater than 1, or skipped when factor is less than 1.