Click and drag the white plus-shaped selectors to adjust the cropping rectangle. Click on the cropped image to change it.
I managed to make a variable width and height cropper by just using a costume and a clone! The 4 selection points determine the cropping rectangle. If you want to crop a specific rectangle, type the x and y coordinates of its 4 corners in the 'cornerx' and 'cornery' lists respectively, in clockwise or anti-clockwise direction (after deleting the selection sprite.) NOTE: If you want a smooth boundary cropping rectangle, convert the black rectangle costume in the cropper to vector mode. It might become smaller or bigger due to the conversion, so after that change every mention of '32.5' in the cropper scripts to a close value to it, bigger if the crop is smaller than required, and vice versa if the crop is bigger than required.
