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

[Adam Reichold]

Hello again,

Am 02.07.2015 um 23:01 schrieb Albert Astals Cid:
> Another thing i wanted to try for a while but didn't get to was killing the 
> Splash XRGB8 mode (which is an extension i created) and using the plain Splash 
> RGB8 mode since now QImage is smart enough to know how to use that, would you 
> mind testing if that yields any benefit?

It does improve rendering that includes the background composition by
more than two percent with structurally simple documents (i.e. some
mathematical texts which do not contain any images themselves).

Not doing the alpha blending (but exporting the alpha channel) is still
faster, i.e. also slightly above two percent in the same benchmarks.
(Meaning XBGR8 with paper colour is 2.2 % slower than RGB8 with paper
colour which itself is 2.2 % slower than XBGR8 without paper colour.)

An extended patch is attached. (I am not totally sure about the
endianess conversion there.)

Also note that while conversion from QImage to QPixmap did show no
difference between Qt's handlign of ARGB32 and RGB888, painting the
resulting QPixmap is 40 % faster for ARGB32, i.e. XBGR8 in Splash
parlance. But since painting those images is usually very fast, this
will very probably not offset the 2.2 % gain for rendering.

Best regards, Adam.
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