Re: FLASH: OT: Fireworks =>attn JD

[John Dowdell]

At 2:44 PM 11/5/0, stephan seifert wrote:

> we have noticed that fireworks compresses gifs and jpg's to

> a smaller file size than photoshop and other image manipulation

> programmes.  how does it do it?


There could be a couple of things that change the final size of a GIF file:


1)  How many colors are in the palette? If there are only 64 colors, for

instance, then each new run of color can be described in 6 bits... if the

image requires all 256 colors in its palette, though, then each new color

run must be described by 8 bits.


2)  How long are rows of identical color? A lot of this depends on the

particular image, for sure, but the "dither" option can improve appearance

by increasing file size, while the "lossy GIF" option makes rows a little

color, improving filesize while risking some streakiness. (Both dithering

and lossiness can be controlled by sliders so you can choose.)


3)  Does the GIF use a dictionary for repeating patterns? This is called

"LZW compression" and requires licensing... a regular GIF uses Run-Length

Encoding to describe how many pixels in a row share the same color. A LZW

dictionary can refer to a previous description, so it can save much

filesize on similar rows.


4)  How is the file actually written? Extra comments or headers in the file

can add bytes.



That said, GIF compression is pretty similar across tools these days...

Fireworks has historically been the best, but other tools now employ the

above techniques too. (Adobe's ImageReady was actually the first to

implement that "lossy GIF" slider... this was an actual Adobe advance.)

Photoshop can tend to add additional comments to some of its exports, and I

haven't seen the 6.0 trial yet so I'm not sure whether they've ported the

lossy/dithering sliders from the dead app yet... you still might be able to

get better filesize in ImageReady than Photoshop.



JPEG images use a very different compression method... the factors here include:

--  quality settings (the numbers are app-relative, and vary across tools!)

--  smoothing settings (removes details, and so reduces file size)

--  edge-enhancement options (helps with areas of stark contrast)

--  different compression settings for different areas of the image

--  the writing of comments or other extraneous detail into the file



Generally it's easier to benchmark GIF compression than JPG compression,

because GIF is usually a lossless format -- you can check whether the

content changed. JPG compression is more subjective to compare, though,

because this necessarily changes the content... the key thing here is to

look at the range of options offered.



jd






John Dowdell, Macromedia Tech Support, San Francisco CA US

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