Image resizing

[ppppenguin]

The question about not cascading codecs goes back to the earliest days of analogue to digital conversion. You shouldn't do it but it's amazing what you can get away with.

I would crop before compress because you get rid of unwanted parts of an image before you attempt data compression. There's a notorious test pattern that was (still is?) used to break coding systems. It's an ordinary picture in a circle with the rest of the screen as random colour noise. You can't compress random noise So get rid of most of the random-ish background before you compress.

One thing that doesn't work well is saving photos as PNG or GIF. It will happen, and it will be lossless, but the files are BIG.

The purists may be offended by this but I'm astonished at how effective JPEG coding is for everyday use. Used with care you can get anything between high quality with fairly modest file size and usable quality with amazingly small files. Even used casually it's usually good enough. If you're doing images for display at huge sizes, to look good when greatly magnified or high quality print publication that's different. You then need to know and understand the techniques to get the best out of them.