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Message-ID: <96cbca68-052f-42dd-a410-da9061d54c06@collabora.com>
Date: Thu, 15 Feb 2024 11:46:21 +0100
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Subject: Re: [PATCH v5 07/13] drm/mediatek: Support alpha blending in display
 driver
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To: Hsiao Chien Sung <shawn.sung@mediatek.com>,
 Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>, David Airlie <airlied@gmail.com>,
 Daniel Vetter <daniel@ffwll.ch>, Matthias Brugger <matthias.bgg@gmail.com>,
 Bibby Hsieh <bibby.hsieh@mediatek.com>, CK Hu <ck.hu@mediatek.com>,
 Sean Paul <seanpaul@chromium.org>, Fei Shao <fshao@chromium.org>,
 Jason Chen <jason-ch.chen@mediatek.corp-partner.google.com>,
 "Nancy . Lin" <nancy.lin@mediatek.com>, dri-devel@lists.freedesktop.org,
 linux-mediatek@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
 linux-kernel@vger.kernel.org
References: <20240215101119.12629-1-shawn.sung@mediatek.com>
 <20240215101119.12629-8-shawn.sung@mediatek.com>
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
In-Reply-To: <20240215101119.12629-8-shawn.sung@mediatek.com>
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Il 15/02/24 11:11, Hsiao Chien Sung ha scritto:
> Support "Pre-multiplied" and "None" blend mode on MediaTek's chips by
> adding correct blend mode property when the planes init.
> Before this patch, only the "Coverage" mode (default) is supported.
> 
> For more information, there are three pixel blend modes in DRM driver:
> "None", "Pre-multiplied", and "Coverage".
> 
> To understand the difference between these modes, let's take a look at
> the following two approaches to do alpha blending:
> 
> 1. Straight:
> dst.RGB = src.RGB * src.A + dst.RGB * (1 - src.A)
> This is straightforward and easy to understand, when the source layer is
> compositing with the destination layer, it's alpha will affect the
> result. This is also known as "post-multiplied", or "Coverage" mode.
> 
> 2. Pre-multiplied:
> dst.RGB = src.RGB + dst.RGB * (1 - src.A)
> Since the source RGB have already multiplied its alpha, only destination
> RGB need to multiply it. This is the "Pre-multiplied" mode in DRM.
> 
> For the "None" blend mode in DRM, it means the pixel alpha is ignored
> when compositing the layers, only the constant alpha for the composited
> layer will take effects.
> 
> Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
> Reviewed-by: CK Hu <ck.hu@mediatek.com>

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>