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Neither PNG nor JPEG provides a way to say "compress this image to exactly X bytes."

Just because your image software doesn't have a slider for this, doesn't mean that isn't exactly what you are doing. When you prepare an JPG for the web, you select a compression ratio. If the final image isn't small enough, you select a higher compression ratio and try again, till you hit the desired size.

There's no ratio dial for PNG images though. The only way to make the file smaller is to resize it. Again, there's no dial or slider for "desired file size". You shrink the image until it is the desired file size.

Both these processes degrade the image.

merlyns view is only arse-forwards if you are facing the wrong way (arse-backwards makes no sense, your arse is always backwards) . merlyn has thought about the situation and noticed that both resizing something and lossily compressing something result in less of an intangible thing, which usually gets called 'information'.

Yes, he made a silly statement with "jpeg is not lossless", although it can work in lossless mode if you want it too. But he then made the good point that thanks to the design of JPG's algorithm, JPG usually gives better. more accurate pictures than a PNG of the same size.

And regarding the satellite imagery - the same situation as the web occurs, but because you are focussing on the word 'lossless', you are missing the bigger picture, where the trade-off is made.

When the Martian lander transmitted its pictures back, the engineers had to make a choice. They had a fixed (quite small) bandwidth. That's like the file size merlyn was talking about. To fit a picture into this bandwidth, they could either: reduce the picture size or lossily compress it.

____________________
Jeremy
I didn't believe in evil until I dated it.


In reply to Re: Re: •Re: GIF patent by jepri
in thread GIF patent by didier

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