[argyllcms] Re: Argyll-reset and Windows-reset gamma curves
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 08 Aug 2013 16:55:44 +1000
Ivan Kolesov wrote:
> Well, the main question is how come Argyll's and Windows' default gamma
> curves are different? Also, what can be done about it, or should anything
> be done? How does this affect the overall calibration and profiling
> processes?
Hi,
I think this may have come up some time ago - summary is that
some people don't know how to scale integers to a higher precision.
The Windows VideoLUTs structure table entries are 16 bit, irrespective
of what the hardware is. The correct values for a 256 step linear curve
are:
0 = 0
1/255 * 65535 = 257
2/255 * 65535 = 514
.
255/255 * 65535 = 65535
A way of computing this using integer code is:
val16 = val8 << 8 | val8
The wrong way of doing it is to truncate it:
val16 = val8 << 8
This will only be the same as above if the entries are 8 bit.
If the entries are more than 8 bit, you aren't getting the
full range of video levels. My guess is that this may be what you
are noticing.
Doing a "dispwin -s filenameX" on system start and after dispwin -c
and comparing the values would indicate exactly what's going on.
V 0.7 of xcalib fixed this problem I think. From what you say,
MSWin has a similar problem.
Graeme Gill.
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