Redimension thumbnail crop without creating new images

The answer is to not generate more image dimensions (and your theme overriding user settings is a big no no), but to select the best fitting image bt size and apply css based stretching and cropping by wrapping the img element with a span.

The to the img you use whatever is the best fit, and then by using a combination of relative positioning in the span and changing the image size you can get whatever effect you want.

This is really just a “view port” technique that is as old as CSS, the only wordpress specific part is to pass to the sizes that you want to the_post_thumbnail as the first parameter.

The gotcha with this is that obviously if wordpress gives you a cropped image you can’t “uncrop” it, and you will have to add your own image size setting for that.