Digital Image cropping in Python

Question

Got this question from a professor, a physicist.

I am a beginner in Python programming. I am not a computer professional I am a physicist. I was trying to write a code in python for my own research which involves a little image processing.

All I need to do is to display an image and then select a region of interest using my mouse and finally crop out the selected region. I can do this in Matlab using the ginput() function.

I tried using PIL. But I find that after I issue the command Image.show(), the image is displayed but then the program halts there unless I exit from the image window. Is there any way to implement what I was planning. Do I need to download any other module? Please advise.

Answer 1

Image.show() just calls whatever simple picture viewer it can find on the current platform, one that may or may not have a crop-and-save facility.

If you are on a Windows box and you just need to make it work on your machine, set the ‘Open with...’ association to make it so running an image loads it into an editor of your choice. On OS X and *nix you'd want to hack the _showxv() method at the bottom of Image.py to change the command used to open the image.

If you do actually need to provide a portable solution, you'll need to use a UI framework to power your cropping application. The choices boil down to Tkinter (ImageTk.py gives you a wrapper for displaying PIL images in Tk), PyQT4 (ImageQt in PIL 1.1.6 gives you a wrapper for displaying images in QT4) or wxPython (a higher-level application authoring toolkit using wxWidgets). It'll be quite a bit of work to get the hang of a full UI kit, but you'll be able to completely customise how your application's interface will work.

Answer 2

For what it's worth (coming from another physicist), I would just do this in an image processing program like the GIMP. The main benefit of doing this task in Python (or any language) would be to save time by automating the process, but unless you - well, the professor - can somehow develop an algorithm to automatically figure out what part of the image to crop, there doesn't seem to be much time to be saved by automation.

If I remember correctly, GIMP is actually scriptable, possibly with Python, so it might be possible to write a time-saving GIMP script to do what your professor friend wants.

Answer 3

While I agree with David that you should probably just use GIMP or some other image manipulation program, here is a script (as I took it to be an exercise to the reader) using pygame that does what you want. You will need to install pygame as well as the PIL, usage would be:

scriptname.py <input_path> <output_path>

Actual script:

import pygame, sys
from PIL import Image
pygame.init()

def displayImage( screen, px, topleft):
    screen.blit(px, px.get_rect())
    if topleft:
        pygame.draw.rect( screen, (128,128,128), pygame.Rect(topleft[0], topleft[1], pygame.mouse.get_pos()[0] - topleft[0], pygame.mouse.get_pos()[1] - topleft[1]))
    pygame.display.flip()

def setup(path):
    px = pygame.image.load(path)
    screen = pygame.display.set_mode( px.get_rect()[2:] )
    screen.blit(px, px.get_rect())
    pygame.display.flip()
    return screen, px

def mainLoop(screen, px):
    topleft = None
    bottomright = None
    runProgram = True
    while runProgram:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                runProgram = False
            elif event.type == pygame.MOUSEBUTTONUP:
                if not topleft:
                    topleft = event.pos
                else:
                    bottomright = event.pos
                    runProgram = False
        displayImage(screen, px, topleft)
    return ( topleft + bottomright )


if __name__ == "__main__":
    screen, px = setup(sys.argv[1])
    left, upper, right, lower = mainLoop(screen, px)
    im = Image.open(sys.argv[1])
    im = im.crop(( left, upper, right, lower))
    im.save(sys.argv[2])

Hope this helps :)