[PATCH v5 07/13] drm/mediatek: Support alpha blending in display driver

[AngeloGioacchino Del Regno]

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 at mediatek.com>
> Reviewed-by: CK Hu <ck.hu at mediatek.com>

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