Paul Kocialkowski <paul.kocialkow...@bootlin.com> writes: > [ Unknown signature status ] > Hi, > > On Fri, 2018-03-30 at 13:37 -0700, Eric Anholt wrote: >> Maxime Ripard <maxime.rip...@bootlin.com> writes: >> >> > Some drivers duplicate the logic to create a property to store a >> > per-plane >> > alpha. >> > >> > This is especially useful if we ever want to support extra protocols >> > for >> > Wayland like: >> > https://lists.freedesktop.org/archives/wayland-devel/2017-August/034 >> > 741.html >> > >> > Let's create a helper in order to move that to the core. >> > diff --git a/drivers/gpu/drm/drm_blend.c >> > b/drivers/gpu/drm/drm_blend.c >> > index 5a81e1b4c076..05eda2d57c77 100644 >> > --- a/drivers/gpu/drm/drm_blend.c >> > +++ b/drivers/gpu/drm/drm_blend.c >> > @@ -88,6 +88,12 @@ >> > * On top of this basic transformation additional properties can be >> > exposed by >> > * the driver: >> > * >> > + * alpha: >> > + * Alpha is setup with drm_plane_create_alpha_property(). >> > It controls the >> >> s/setup/set up/ >> >> > + * plane-wide opacity, from transparent (0) to opaque >> > (0xffff). It can be >> > + * combined with pixel alpha. >> > + * The alpha value is represented as premultiplied alpha. >> >> I don't think this premultiplied comment makes any sense. What are >> you saying it's premultiplied with? Maybe you mean that the output >> pixels will have both their color and alpha channels multiplied by >> this alpha? >> I'd just drop it. > > I disagree here: since there are multiple ways to blend the pixel alpha > value and the plane-wide alpha value, I think it's important to clearly > specify which blending equation DRM expects here. Otherwise, the plane- > wide alpha value just doesn't have a specified meaning and driver > implementations can't figure whether the hardware uses the same equation > or not and whether adaptation to this coefficient is needed. > > The equations for premultiplied alpha blending are at: > https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending > > What do you think?
"Premultiplied alpha" only has a meaning for an RGBA value, where RGB have been already multiplied by their own A. In this case, you're saying that an alpha-only value is "premultiplied", but with what? What would an alternative to it being "premultiplied" do instead? That's what I'm confused about.
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