ImageMagick: convert, crop, resize large images

Question

I have users who upload high resolution images which need to be cropped. However, when they crop a image, I would show a thumbnail or a resized image (smaller) on the browser and they select the crop on the resized image. I would like to know how i can use those dimensions from the resized image and apply it to the original high resolution image?

Answer 1

Take the coordinates returned for cropping the thumbnail and scale them:

full_crop = thumb_crop * full_size / thumb_size