Re: PNG transparency

Tom Anderson <>
Sun, 3 Aug 2008 16:54:39 +0100
On Sun, 3 Aug 2008, Sherman Pendley wrote:

Roedy Green <> writes:

Please view the "doctored" and "undoctored" images at the bottom of 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
RGB color.

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).


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