Cropping, scaling,...MacMPEG2Decoder allows you to crop and scale the image, very handy when the source originates from a DVD, where often black borders surround the image.
Preparing for transformationMove the position slider (which you can find at the top of the main window) to somewhere in the middle, to a frame with no black stuff at the edges of the actual image (make sure the 'Preview' checkbox is checked to see what the frame looks like). Now click the 'Transform' button.
You get the following window:
Automatic cropping and scalingIn case your MPEG2 file originates from a DVD, MacMPEG2Decoder allows you to have the cropping and scaling set for you. In the Transformation window, choose the right aspect ratio from the popup menu (you can usually find this ratio at the back of the DVD box), and click the 'Ask the Wizard' button. If it complains, go back to the main window and find a nicer frame with the position slider. If the image in the preview looks allright (ignoring the artifacts; they're a known bug. They do not appear in the final movie), click the 'OK' button.
Manual cropping and scalingYou can also enter cropping and scaling values manually. Just enter them in the appropriate boxes. Click 'OK' to save the transformation settings.