Re: Transparancy

lilley (
Tue, 4 Apr 1995 17:46:20 +0500

Kjetil Torgrim Homme writes:

> [Christopher J. Osborne]
> | One thing Netscape information providors can do is to make the
> | transparent background colour of their gifs light grey (ie RGB
> | value of 195,195,195).

Anti-aliasing to a mid grey also looks reasonable against coloured
backgrounds of similar tone.

> | you will
> | greatly increse the chances of you transparent gifs still looking
> | good in other browsers (and I don't just mean Mosaic.

Ah, but one of the prime culprits for non-support of Gif89 transparency
is Mac Mosaic, whci of course has a white background as do many Mac

> If you for some reason choose, e.g., a dark blue
> background, there will be a light halo surrounding the graphic. For
> this reason, one should be critical of what graphics one makes
> transparent.

Yes, I agree here. Not all graphics are suitable.

> Fortunately, the upcoming PNG format will have gradual transparancy on
> any colour, and even a full alpha channel if needed.

Is this still being developed now the GIF scare has dies down? I hear
that CompuServe are developing a GIF24 which will be based on (but may
not be the same as) PNG.

Of course, TIFF has had a full alpha chanell for a while, but few browsers
implement inline baseline TIFF, let alone the extensions.

> [Phillip Hallam-Baker]
> | What would be nice would be to have a pixmap type affair such that
> | the drawing was stated as being in the foreground and backround
> | colours of the screen. Thus the text colour would match the
> | picture text colour.

You mean, XBM?

The original NCSA docs on IMG specify three image types as usable inline
- GIF87, XBM and HDF. PC Mosaic used to barf on XBM for a while but I
gather it works now.

One problem with XBM (apart from it's sprawling inefficiency) is that
graphics can look rather odd if the foreground colour is light and the
background is dark. Images which use shading tend to look most odd when
reversed in this way. Again, not all images are suitable for XBM.

Chris Lilley
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