[PATCH renderproto] Add floating point transforms

[Alexander E. Patrakov]

19.08.2014 03:02, Keith Packard wrote:
> Siarhei Siamashka <siarhei.siamashka at gmail.com> writes:
>
>> Do you have a reproducible testcase for this problem? Being off by
>> several pixels seems to be very wrong. Especially if this happens
>> often.
>
> Non-affine transforms cause problems. I noticed this with RandR when
> doing keystone correction.
>
> Here's a simple example:
>
> Desired matrix:
>
>   33.13915858	15.17037037	-16384.00000000
>   0.00000000	23.73634938	0.00000000
>   -0.00057170	-0.00182142	25.70348287
>
> Rounded to 16.16 fixed matrix:
>
>   33.13916016	15.17036438	-16384.00000000
>   0.00000000	23.73634338	0.00000000
>   -0.00056458	-0.00181580	25.70347595
>
> Actual bits:
>
>   { 0x00014dba, 0x000098c6, 0xfd7b7d45 },
>   { 0x00000000, 0x0000ef09, 0x00000000 },
>   { 0xffffffff, 0xfffffffb, 0x000102d9 },
>
> Take the point 2560,1600 and transform:
>
> desired:        4348.04, 1780.87
> actual:         4342.50, 1778.60
>
> error:          5.99

Sorry, but I think that we have bigger problems. Please look at 
https://bugs.freedesktop.org/show_bug.cgi?id=39949 (which was a blocker 
for xserver-1.12 and makes mouse input on scaled displays unusable in 
some cases). Since there was no manpower to review that bug for years, I 
think that the best course for now is to prohibit all transforms that do 
not amount to mere translation. You can change my opinion by reviewing 
the patch in the bug.

>
>> Floating point format supports a wider range of values (unnecessary
>> if the rest of XRENDER remains restricted to 16-bit coordinates) but
>> has much worse accuracy in corner cases and is poorly predictable.
>> The 32-bit floating point format does not look like an obvious
>> upgrade over 16.16 fixed point.
>
> Transforms are different than specifying geometry though; they actually
> need the wide dynamic range offered by floating point.
>
>
>
> _______________________________________________
> xorg-devel at lists.x.org: X.Org development
> Archives: http://lists.x.org/archives/xorg-devel
> Info: http://lists.x.org/mailman/listinfo/xorg-devel
>

-- 
Alexander E. Patrakov