Output Frame of Weston

[Pekka Paalanen]

On Mon, 10 Feb 2020 13:24:46 +0530
ravi shankar <ravi02431 at gmail.com> wrote:

> Hi Pekka,
> Thank you for reply!. We would like to understand the behavior of weston in
> following use cases
> 
>       Scenario 1: What is the Alpha value on weston composed output buffer
> in following use case

Hi,

my question is: why do you care at all? These seem like the wrong
questions to ask.

Ok, in the following I am assuming that you are giving the whole
scenegraph, and not only application surfaces. By that, I mean that
e.g. weston-desktop-shell is not adding more surfaces than what you
list.

>                    Screen has only one surface and which has alpha '0'
> 
> [image: image.png]

The framebuffer RGB contents are undefined, probably garbage. The alpha
channel is either 255 or garbage, depending on what used to be in the
particular framebuffer and whether the renderer is writing the alpha
channel or not.

A renderer may not write an alpha channel, or writes garbage to it, if
the alpha channel does not exist in the renderer's point of view (e.g
EGLConfig).

>        Scenario 2:    Screen has only one surface and which has alpha '255'
> [image: image.png]

Depends on whether the renderer considers the alpha channel to exist. If
yes then 255, otherwise undefined/garbage.

>      Scenario 3: Screen has two surfaces which has alpha given below
> 
> [image: image.png]      ..

Depends on whether the renderer considers the alpha channel to exist. If
not, it is undefined/garbage. If yes, I guess it might be 255. In
either case, if none of your surfaces is opaque for a certain pixel in
the framebuffer, then that pixel's RGB value is undefined/garbage.

These questions make me think that what you are trying to do does not
make sense, but then you have not explained what you want to do, so I
cannot know.

There is no way a surface could force the alpha channel value in the
framebuffer to anything else than 255 (fully opaque).


Thanks,
pq
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