Monochrome/greyscale and software dithering in the kernel

[Daniel Vetter]

On Thu, Aug 3, 2017 at 5:24 PM, David Lechner <david at lechnology.com> wrote:
> On 08/03/2017 09:58 AM, Noralf Trønnes wrote:
>>
>> Hi,
>>
>> The tinydrm/repaper driver is monochrome, but I have just used an
>> emulation format XRGB8888, since monochrome support is scarce in
>> userspace. I'm using ITU BT.601 to convert from rgb to greyscale and
>> then use the msb for monochrome.
>>
>> Now I'm asked if we can implement Floyd–Steinberg dithering in the driver
>> to make it more useful, so userspace can work reasonably well with
>> default tools/libs. I did have that in the first stage of the driver
>> development, but decided against it, since I figured this was the
>> responsibility of userspace not the kernel. But personally it doesn't
>> really matter to me either way.
>>
>> So the question is, can I say yes to such an addition?
>>
>
> IMHO, if we add monochorme/grayscale support to DRM, then I think using
> dithering for the "compatibility mode" XRGB8888 emulation would be a good
> idea. But until then, I don't think I would like it because unless you are
> displaying photos it will probably causes some noticeable distortion.

I'd suggest for monochrome displays we forgo dithering entirely. The
XRGB8888 stuff is just there to make sure userspace can expect that
format to work, but that's it. If you want your greyscale to look
good, then pls directly render greyscale in userspace.

There's 15+ years of history on why drm doesn't do any rendering in
the kernel, and there's reasons for that. XRGB8888 really was an
exception since otherwise normal distro's won't even boot up, but
that's going to be the _only_ exception. And we're going to make it
work with the minimal amount of kernel code, i.e. no fancing
dithering, scaling, interpolation or anything like that.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch