[RFC] handling of alpha mode (pre-multiplied/straight) in ARGB modes

[Daniel Vetter]

On Wed, Jan 13, 2016 at 03:13:35PM +0100, Marek Szyprowski wrote:
> Hello,
> 
> On 2016-01-11 16:22, Ville Syrjälä wrote:
> >On Mon, Jan 11, 2016 at 04:07:50PM +0100, Daniel Vetter wrote:
> >>On Mon, Jan 11, 2016 at 03:18:44PM +0100, Marek Szyprowski wrote:
> >>>Dear All,
> >>>
> >>>I would like to add support for configuring alpha modes: straight and
> >>>pre-multiplied for blending to Exynos DRM driver. For those who see those
> >>>terms for the first time:
> >>>1. straight alpha mode means means that all A and RGB values are from full
> >>>0.255 range,
> >>>2. pre-multiplied alpha mode means that A is from 0..255 range and RGB
> >>>values are scaled to 0..ALPHA range (where ALPHA is the A value for the
> >>>given pixel).
> >>>The pre-multiplied mode is quite common in texture processing.
> >>>
> >>>I did a little research and found that there is no common approach for
> >>>defining straight or pre-multiplied alpha modes for ARGB framebuffers.
> >>>
> >>>>From reading the sources and the register names I found that following
> >>>drivers use pre-multiplied modes for ARGB framebuffers:
> >>>radeon (at least for ARGB cursors),
> >>>amdgpu (cursor),
> >>>intel (all ARGB planes),
> >>>rock chip.
> >>>
> >>>On the other hand, there are drm drivers which support ARGB modes and use
> >>>straight alpha modes:
> >>>omap,
> >>>msm.
> >>>
> >>>For the rest of drivers, which might deal with per-pixel alpha, I was not
> >>>able to determine from the code if the pixel RGB values are interpreted as
> >>>per-multiplied or straight:
> >>>atmel_hlcdc,
> >>>sti,
> >>>mdp5,
> >>>shmobile,
> >>>rcar,
> >>>vc4,
> >>>imx.
> >>Imo in case of doubt/mixed definitions the semantics of the big desktop
> >>drivers should win. True generic userspace is most likely developed on
> >>those, everything else should just adjust. And in most cases we can get
> >>away with that on arm drivers since they tend to ship userspace and kernel
> >>in lockstep. At least where it really matters.
> >>
> >>>Exynos DRM driver now mixes pre-multiplied and straight modes, depending on
> >>>the CRTC sub-driver.
> >>>
> >>>While checking the code I found a following comment in
> >>>drm/i915/intel_display.c in skl_plane_ctl_format() function:
> >>>/*
> >>>  * XXX: For ARBG/ABGR formats we default to expecting scanout buffers
> >>>  * to be already pre-multiplied. We need to add a knob (or a different
> >>>  * DRM_FORMAT) for user-space to configure that.
> >>>  */
> >>>
> >>>The question is how to cleanup this ambiguities and let generic userspace to
> >>>properly use ARGB/ARGB modes with proper blending mode. I came up with the
> >>>following 3 solutions:
> >>>
> >>>1. Define new fourcc values for pre-multiplied (or/and straight) alpha
> >>>modes,
> >>>2. Introduce a new framebuffer flag for pre-multiplied or straight alpha (or
> >>>both),
> >>>3. Introduce a new plane property for controlling alpha blending mode.
> >>>
> >>>The first solution has serious compatibility problem, because we can either
> >>>map existing fourcc to pre-multiplied (to follow radeon/amdgpu, intel and
> >>>rockchip behavior) or straight mode. Both ways it will break some existing
> >>>applications. To avoid compatibility problem we would need to add 2 more
> >>>sets of fourcc and leave existing ARGB modes as 'driver dependent'.
> >>>
> >>>The second solution is probably a bit less intrusive, but it suffers for the
> >>>similar compatibility problem. To make this feature compatible with existing
> >>>code, probably 2 flags will be needed: DRM_MODE_FB_ALPHA_FORCE_PREMULTIPLIED
> >>>and DRM_MODE_FB_ALPHA_FORCE_STRAIGHT. This way when userspace provides no
> >>>flag, the driver can use its default mode.
> >>>
> >>>Third solution (additional plane property) has been already proposed:
> >>>https://lkml.org/lkml/2015/6/19/330, however I found no follow-up nor
> >>>comments. Separate property lets at least drivers to notify userspace about
> >>>driver's default alpha mode and lets generic application to properly set the
> >>>requested mode. The only problem is that the alpha mode is not directly
> >>>configured on the framebuffer object, where in my opinion it should be
> >>>stored.
> >>>
> >>>Please let me know which approach You like best and which should I take for
> >>>introducing generic way of configuring alpha mode.
> >>One idea that was tossed around a few times is to go full generic and
> >>implement something like glBlendFunc for planes, except that we'd also
> >>pre-multiplied/straight versions where it makes sense. For drivers that
> >>can't support pre-multiplied alpha they could simply leave out these
> >>values from the blend-func property (like we already allow with e.g.
> >>rotation, when a driver can't do 90/270°).
> >>
> >>Wrt backwards compat a property would work well too: If it's not there
> >>then userspace should assume old behaviour (which should be pre-multiplied
> >>really, but might be somewhat broken on some drivers/kernel).
> >>
> >>Damien had some RFC about all this way back, but can't find it right now.
> >I remembe he said he wanted to work on it, but I don't remember seeing
> >anything come out of that. I guess he got swallowed by patchwork, or I
> >just missed the patch.
> >
> >This mail might include some of my more recent thoughts on this matter
> >http://lists.freedesktop.org/archives/intel-gfx/2014-April/042956.html
> 
> I also found this patch: http://patchwork.freedesktop.org/patch/23000/
> 
> However I still don't get a complete picture how it would work. If I got it
> right, the real blending function is defined by a pair of the values from
> the
> proposed enums, where one applies to the background (the blending result of
> deeper planes) and the second applies to the processed plane. Some
> combinations of the proposed enums doesn't make much sense (and especially
> changing the constant zero and one means simply changing the order of the
> planes), so I wonder if it might be easier to simply create an enum with
> all sane values for blending and document it really well? This way drivers
> will simply initialize only the supported blending modes.
> 
> I assume that this way also the separate enums can be defined for straight
> and pre-multiplied alpha blending modes.

Yeah, aliasing is always a big concern for stuff like this, hence why we
have normalized zpos, functions to reduce rotations and stuff like that.

If it's possible to define a set of enums that work, and dont accidentally
interact with zpos in fun ways then I guess we could go ahead with that.
But would need some serious thought.

I think in the end it's probably best if we look at blending/zpos as one
overall feature, where the kerneldoc/spec for the feature explains how
things should work and how it all interacts.

> The other problem mentioned somewhere is where to attach the blending
> property. Some parameters (like straight/pre-multiplied or enabling global
> alpha) can be set on per-plane basis, but I can imagine that some parameters
> are global for the given crtc.

Yeah, we can do both per-plane and global props on the crtc. E.g. we
probably need a backgroun color (for chips that support anything else than
black), and that needs to be on the crtc and would always be the lowest
plane.

Lots of interactions here.

Thinking about this: Should we maybe extract a new drm_prop_blending, so
that we can pull together zpos, background color, blending modes and all
the helpers to register props into one file? Plus then add one overview
section which explains in detail the model we expect this to work with?

> The another item related to blending is color keying. It was not mentioned
> in the above proposal at all, but some hardware can do that. I've noticed
> that some hardware also is able to do color keying only for exact RGB(A)
> values and some have separate registers to set RGB tolerance. This also
> need to be standardized somehow.

tbh I have no idea how color keying should interact with blending. At
least with intel hw it's either alpha or keying, and color keying has
become fairly unpopular.

I think it'd be best if we just consider alpha blending for now, and then
figure out in a 2nd step how to integrate color keying (if anyone really
cares enough about this). Otherwise this will become too big and we'll
never get it merged.

One more: Where/how to you plan to create the open source demonstration
compositor? Just testcases arent' enough, it should be something real like
wayland, hwc, ozone or xf86-video-modesetting.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch