Alpha channel in JPEG's

Andreas Raab andreas.raab at gmx.de
Sun Jan 12 23:16:39 UTC 2003


> Got any tips on the Squeak end on how to read PNG's, putting one PNG 
> over top of another to form a new picture?

Err ... not sure what you are asking for. How's this:

form1 := Form fromFileNamed: 'image1.png'.
form2 := Form fromFileNamed: 'image2.png'.

form1 displayOn: form2 at: 0 at 0 rule: Form blend.
form2 display.

Cheers,
  - Andreas

> -----Original Message-----
> From: squeak-dev-admin at lists.squeakfoundation.org 
> [mailto:squeak-dev-admin at lists.squeakfoundation.org] On 
> Behalf Of Nevin Pratt
> Sent: Sunday, January 12, 2003 10:26 PM
> To: squeak-dev at lists.squeakfoundation.org
> Subject: Re: Alpha channel in JPEG's
> 
> 
> OK.  Sounds like I've got a bit to learn.
> 
> Off hand, though, I don't think the picture format on disk is 
> going to 
> matter, as long as I can read the pictures into a Squeak image and 
> overlay one on top of another, with the top one having a transparant 
> portion so that the bottom one shows through at the 
> transparant areas. 
>  Then, once the overlay happens, it can become "one" picture again 
> inside the Squeak image, ready for the next overlay.
> 
> So, on disk, it sounds like PNG is the file format for this.
> 
> Got any tips on the Squeak end on how to read PNG's, putting one PNG 
> over top of another to form a new picture?
> 
> Or am I still "up in the night", "all wet", or "a greenie", etc., as 
> they say :-)
> 
> Thanks for your input.
> 
> Nevin
> 
> 
> Andreas Raab wrote:
> 
> >Nevin,
> >
> >JPEG does not support transparency at all. You can work 
> around this by
> >using various techniques but "overlaying transparent JPEGs" 
> will simply
> >not work (never has, and probably never will). Here are a few things
> >that can be done:
> >* Use PNG - it supports transparency (doesn't compress as 
> well though)
> >* Use a separate image encoding the transparency as gray 
> scale. In this
> >case you could use either GIF or PNG (for masks with very few alpha
> >levels) or even JPEG itself (for masks with very many alpha levels).
> >In any case don't expect anyone to "add transparency to JPEG". This
> >isn't even part of the spec.
> >
> >Cheers,
> >  - Andreas
> >
> >  
> >
> >>    
> >>
> 
> 
> 




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