Transparent GIF's (Was: Re: Image Alignment.)

[Joe Pasqua]

> Is implemetation of transparent gifs
> client dependent? I observed Netscape with its gray backgroud displayed
> images (possibly transparent gifs) with much greater finesse than OW with a  
> gray background (or the default white for that matter.)

This is just a guess as to what is going on. OmniWeb uses an external image  
filter to get GIF's (and other image types) into a form that it can display.  
The transparant GIF's aren't transparent in the sense that that they have an  
alpha channel. They include an indication of a particular pixel value, call  
it T,  that should be considered transparent. That is, anywhere in the image  
data that you see a pixel with value T, you don't display anything - you let  
the background show through.

Since OmniWeb is simply receiving a predigested image back from the image  
filter it has no idea that the original was a GIF with a transparent pixel  
specified. To make this work, the image filter would have to return an image  
object containing an alpha channel with 100% opacity everywhere except  
locations that correspond to the transparant pixel (where the opacity would  
be 0).

Browsers like Mosaic handle GIF internally and make a pass over the image  
data substituting the window's background color for the transparent pixel  
before displaying the image.

As I said, this is just my guess. I'd be interested in knowing what is  
really going on.

Joe Pasqua