[PATCH] [RFC] desktop-shell: change the alpha of black view in fullscreen mode

[Bill Spitzak]

On 12/04/2015 12:14 AM, Pekka Paalanen wrote:
> On Thu, 3 Dec 2015 14:18:54 -0800
> Bryce Harrington <bryce at osg.samsung.com> wrote:
>
>> On Thu, Dec 03, 2015 at 10:33:27PM +0900, Hyungwon Hwang wrote:
>>> This patch changes the alpha value of black view in fullscreen mode,
>>> when the applications opacity changes.
>>>
>>> Signed-off-by: Hyungwon Hwang <hyungwon.hwang7 at gmail.com>
>>> ---
>>> This patch is incomplete, and just a proof of concept. But I want to
>>> make this patch as the starting point of discussion related the opacity
>>> in fullscreen mode [1]. I tested it with weston-fullscreen.
>>
>> Thanks for sending this as an RFC, as a follow up to the earlier
>> discussion with pq.  I notice some of the points he had raised in that
>> discussion (e.g. avoiding alpha for letterbox edges, etc.) aren't
>> being addressed.  In technical terms this patch doesn't look bad but you
>> might include a discussion of how the remaining problems would be
>> handled?
>
> FWIW, I do think that changing the opacity of the black surface like
> this is a totally wrong approach to any problem.
>
> Either the opaque black borders exist or they do not. They are realized
> by either a single black surface behind the window like currently, or
> several black surfaces around the window. When the black borders must
> not be drawn, the black surface(s) must be destroyed (and re-created
> on-demand).

If the intention is to make an otherwise-opaque window uniformly 
transparent, this is not going to work right.

Say you wish to make the window 50% transparent. If the background color 
of a pixel is B and the window color is A, the desired result is 
B/2+A/2. However if you composite 50% opaque black over the background, 
you get B/2, and then if you composite the window over that with 50% you 
get B/4+A/2.

You could instead do an additive composite of the window over the black 
(ie for alpha a, instead of doing B(1-a)+Aa, do B+Aa. However you have 
to special-case that in the composite stack.

My feeling is that making the black be a "surface" is a mistake, it 
should instead be an aspect of the compositing. When the compositor 
composites in a full-screen surface, it also draws up to 4 black 
rectangles to fill the area between the surface and the edge of the 
screen. Instead of a black surface in the stack, there is a flag on the 
surface indicating this is wanted.