Re: PNG transparency
On Sun, 3 Aug 2008, Sherman Pendley wrote:
Roedy Green <firstname.lastname@example.org> writes:
Please view the "doctored" and "undoctored" images at the bottom of
http://mindprod.com/applet/masker.html and tell me what you see.
I suspect doctored = mickey mouse and undoctored = alpha channel.
You're correct - I loaded both into Photoshop, which says that the
doctored image is indexed (i.e. mickey mouse) color, and the other is
I believe there is a minor point of confusion here. Both of you are
contrasting some 'mickey mouse' with something proper - in Roedy's case,
alpha channel transparency, in Sherman's case, full colour. But note that
in PNG, these are *not the same*!
You can have a full colour image with alpha (colour type 6), or a full
colour image with no alpha but a special colour used to mean full
transparency (colour type 2 plus a tRNS chunk), or you can have indexed
colour with one transparent palette entry (colour type 3 plus a tRNS chunk
with one entry), or indexed colour with alpha values attached to as many
colours as you like (colour type 3 plus a tRNS chunk with multiple
entries). The single-transparent-colour indexed form is in fact just a
degenerate case of the alpha-channel indexed form.
The reason that indexed colour transparency works in IE6 but full-colour
transparency doesn't has got nothing do with alpha vs binary transparency,
it's just that its full-colour PNG decoder ignores alpha, whereas the
indexed-colour one doesn't (although it does round all nonzero
transparency to full transparency).
unstable orbits in the space of lies