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copied!<p>I took a quick look and fixed a few other problems along the way. Essentially my changes do this:</p> <ul> <li>Draw the bounding box on a temporary image, set its alpha transparency, and then blit this over top of the main image.</li> <li>Avoid extraneous drawing cycles (when the mouse isn't moving, no sense in drawing the same image again).</li> <li>Ensure that width and height are always positive. If the rect is drawn by dragging the mouse left or up, your code would end up with a negative width and/or height, raising an exception when trying to write the final image.</li> </ul> <p>Here is a screenshot of running the fixed code:</p> <p><img src="https://i.stack.imgur.com/OLOO2.png" alt="enter image description here"></p> <p>I split the code into two parts to avoid the scrollbars:</p> <pre><code>import pygame, sys from PIL import Image pygame.init() def displayImage(screen, px, topleft, prior): # ensure that the rect always has positive width, height x, y = topleft width = pygame.mouse.get_pos()[0] - topleft[0] height = pygame.mouse.get_pos()[1] - topleft[1] if width < 0: x += width width = abs(width) if height < 0: y += height height = abs(height) # eliminate redundant drawing cycles (when mouse isn't moving) current = x, y, width, height if not (width and height): return current if current == prior: return current # draw transparent box and blit it onto canvas screen.blit(px, px.get_rect()) im = pygame.Surface((width, height)) im.fill((128, 128, 128)) pygame.draw.rect(im, (32, 32, 32), im.get_rect(), 1) im.set_alpha(128) screen.blit(im, (x, y)) pygame.display.flip() # return current box extents return (x, y, width, height) </code></pre> <p>And part 2 (concatenate to the above):</p> <pre><code>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 = bottomright = prior = None n=0 while n!=1: for event in pygame.event.get(): if event.type == pygame.MOUSEBUTTONUP: if not topleft: topleft = event.pos else: bottomright = event.pos n=1 if topleft: prior = displayImage(screen, px, topleft, prior) return ( topleft + bottomright ) if __name__ == "__main__": input_loc = 'stack.png' output_loc = 'out.png' screen, px = setup(input_loc) left, upper, right, lower = mainLoop(screen, px) # ensure output rect always has positive width, height if right < left: left, right = right, left if lower < upper: lower, upper = upper, lower im = Image.open(input_loc) im = im.crop(( left, upper, right, lower)) pygame.display.quit() im.save(output_loc) </code></pre>

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