From: eric@anholt•net (Eric Anholt)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v4 1/5] drm/blend: Add a generic alpha property
Date: Wed, 04 Apr 2018 09:03:36 -0700 [thread overview]
Message-ID: <87a7ujc61z.fsf@anholt.net> (raw)
In-Reply-To: <e0c1e8af3c8e9a8283d78d4cdeb2fe92147c5945.camel@bootlin.com>
Paul Kocialkowski <paul.kocialkowski@bootlin•com> writes:
> [ Unknown signature status ]
> Hi,
>
> On Fri, 2018-03-30 at 13:37 -0700, Eric Anholt wrote:
>> Maxime Ripard <maxime.ripard@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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next prev parent reply other threads:[~2018-04-04 16:03 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-13 20:54 [PATCH v4 0/5] drm/blend: Support generic plane-wide alpha Maxime Ripard
2018-03-13 20:54 ` [PATCH v4 1/5] drm/blend: Add a generic alpha property Maxime Ripard
2018-03-30 20:37 ` Eric Anholt
2018-04-04 9:15 ` Paul Kocialkowski
2018-04-04 9:46 ` Laurent Pinchart
2018-04-04 16:03 ` Eric Anholt [this message]
2018-04-04 8:47 ` Paul Kocialkowski
2018-03-13 20:54 ` [PATCH v4 2/5] drm/atmel-hclcdc: Convert to the new " Maxime Ripard
2018-03-13 20:54 ` [PATCH v4 3/5] drm/rcar-du: " Maxime Ripard
2018-03-13 20:54 ` [PATCH v4 4/5] drm/sun4i: Add support for plane alpha Maxime Ripard
2018-04-04 8:49 ` Paul Kocialkowski
2018-03-13 20:54 ` [PATCH v4 5/5] drm/docs: Remove the rcar alpha from the csv file Maxime Ripard
2018-03-26 7:47 ` [PATCH v4 0/5] drm/blend: Support generic plane-wide alpha Maxime Ripard
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