Re: Possible ColorSync discontinuity bug introduced in OS 10.14.6 with display "r/g/bTRC" tags?
Re: Possible ColorSync discontinuity bug introduced in OS 10.14.6 with display "r/g/bTRC" tags?
- Subject: Re: Possible ColorSync discontinuity bug introduced in OS 10.14.6 with display "r/g/bTRC" tags?
- From: William Hollingworth via colorsync-users <email@hidden>
- Date: Wed, 14 Aug 2019 22:29:45 -0500
On 8/14/2019 11:57 AM, Neil Barstow wrote:
So, am I reading this right please?
An LUT profile would be OK and a "MATRIX" not, in this setting.
Is an LUT what you are calling "encoded as - - an array of points"
I guess a 16 bit LUT may be OK? I am just thinking about basICColor
display.
The rTRC. bTRC and gTRC tags in the display profile are "curv" types
encoding the tone reproduction curve of each color. I believe LUT based
display profiles must also include TRC tags, but maybe someone like
Graeme Gill could step in and confirm.
According to the ICC.1:2001-04 spec:
----------------------------------
6.5.3 curveType- The curveType contains a 4-byte count value and a
one-dimensional table of 2-byte values.
The count value specifies the number of entries in the curve table
except as follows:
- when countis 0, then a linear response (slope equal to 1.0) is assumed,
- when countis 1, then the data entry is interpreted as a simple gamma
value encoded as a u8Fixed8Number. Gamma is interpreted canonically and
notas an inverse.
Otherwise, the 16-bit unsigned integers in the range 0 to 65535 linearly
map to curve values in the interval[0.0, 1.0].
----------------------------------
The problem occurs with the case of using a single value to represent a
simple gamma value in the profile. In this case ColorSync seems to mess
up curve generation. It looks like most software nowadays is encoding as
an array of points - which is probably how this one slipped by Apple.
I would have thought that especially canned profiles would use the
simple gamma rather than an array of points. I'd be interested in
knowing the reasoning for not doing this.
Thanks
Will
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