[poppler] [RFC] Add option to keep alpha channel to Qt frontends

[Adam Reichold]

Hello,

Am 07.07.2015 um 23:10 schrieb Albert Astals Cid:
> KeepAlphaChannel = 0x00000080
> ///< Do not compose with the paper color \since 0.34
> 
> Does this really have anything to do with not compositing the paper color? I'd 
> say the paper color is still very much taken into account, it's just that the 
> alpha is not always opaque if you pass that option, no?

As the paper colour that is passed to the Splash output device is
actually NULL, I would say that is not taken into account. Also note
that even though Splash renders colour data and alpha channel
separately, consumers can still access it via the separate pointers. It
is only the Splash::compositeBackground method which currently blends
the colour data and flattens the alpha channel afterwards.

Hence IMHO the functional difference is really not composing with the
paper colour, that we then transfer the alpha channel into the fourth
component of the XBGR8 data is just an extra (and could for example also
be done by the Qt frontends if desired).

> Also the last commit makes me a bit scared to be honest, is very easy to make 
> a mistake in one of those things, I'll put it in my todo but as you know it's 
> long, would appreciate if someone else reviewed/regtested it.

I very much agree and could live without the last commit which only
improves things minimally since the RGB8 and KeepAlphaChannel changes
are useful improvements already. So maybe its best to let the last
commit be for the time being and the changes stew for a few releases?

Best regards, Adam.

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