OT: .BMP or .JPG?

[Richard Russell]

I don't think you and Dennis are really saying different things. Yes, you're correct in saying than an image is "just" data, but equally Dennis is correct in saying it's a special kind of organised data that facilitates (lossless) compression using algorithms specifically designed for the purpose. It does "make a difference" whether the data represents an image or not.

For example the lossless profile of the JPEG2000 standard is quite specifically designed for compressing 2D images; it will typically give much greater amounts of lossless compression (i.e. a smaller file size) than if you used, say, ZIP or LZW on the same data. Equally, the JPEG2000 compression algorithm will work very poorly, if at all, if applied to data that doesn't represent an image.

Richard.

[Man at B&Q]

whether the algorithm is suited to the nature of the data.

The only point I take issue with is Dennis's assertion that using LZW to compress an image is not image compression.

MBQ

[Richard Russell]

This is just semantics. It's 'compressing an image' but it's not 'compression intended for images'. So in one sense it *is* image compression and in the other sense it *isn't* image compression. Not worth arguing about, I'd say.

Richard.

[dennis]

Lets get this straight, data compression is *not* image compression! You can claim it is as long as you like it will not make it fact.

[dennis]

I agree, I don't like him telling me I know nothing about compression just because he doesn't. Anyway its pointless arguing with him so I will end it here.

[Appin]

The message from John Rumm contains these words:

Had a very demanding Dutch broadsheet newspaper editor wanting to use one of my pix for a half-page broadsheet pic!

It was published, in full colour. And yes, it was a 35mm neg.

[The Natural Philosopher]

Well dennis has got a tiny bit of te picture, and is arguing over it/.

LZW compressions is a general purpose 'lossless' compressor that doesn't actually do a lot for colour images, because the data doesn't fit what its looking for. It likes black and white drawings. With lots of white space. LZW tiffs of colour images are still bloody vast.

[The Natural Philosopher]

No, but image compression IS data compression!"

A specialised subset, but data compression, none the less.

>

[The Natural Philosopher]

35mm is about, with good quality slide film, something like 8Mpx of resolution.

A really good medium format with a good lens stopped riht is capbale of about 4-8 times that.

if you tajke a state of the art DSLR, wich is about like what kodachrome

25 was, at 12 Mpxl, that's about 4000x3000 pixels roughly.put across 10 inches of page that's 400 dpi. Printers prefer 600dpi for real class work. The glossies. A broadsheet is tat anyway.

[dennis]

So what? That has nowt to do with what has been said and nobody has said it isn't, yet!

[dennis]

IME the films might but not the lenses. Most camera lenses are actually quite poor optically and certainly aren't finished to 1/8 wavelength or better.

Which printing process can actual manage anywhere near 600 dpi full colour on paper? Other than dye sub which I already have and produces photo quality at around 150-200 dpi but needs special paper.

>

[Man at B&Q]

[e-mail]...

If you're so clever then tell us what IS image compression?

MBQ

[Man at B&Q]

Where did I say that?

Lost the argument, you mean?

MBQ

[dennis]

You can believe that if you want to. I am not going to get into a technical argument with someone who can't grasp the difference between compressing picture elements and bits in a data file. There is a big difference between substituting short bit patterns for long bit patterns and analyzing picture elements and removing those that don't matter. One is subjective, the other isn't. I will let you think about which is which.

[John Rumm]

Its actually pretty good on colour images as well if they are the right sort - where it fails is continuous tone images. Things like colour maps, line drawings, or anything with solid colours no graduated tones, and no dithers etc will LZW compress very well.

[Man at B&Q]

[e-mail]...

"One is subjective". Indeed. Remove bits that DO matter and the rendering will suffer, to some people more than others. That's why images can also be compressed using LOSSLESS algorithms that do not remove any information. The images are still being compressed, whether you like it or not.

MBQ