Marti,
Wondering if you have any clues on a problem with near-black
rendering. Using 1.12 I've been seeing a strange problem I
keep meaning to ask you about:
Processing is 16 bits/channel input/output, Perceptual, Black Point
compensation, going from PCS to a common profile such as Adobe RGB or sRGB:
When rendering, I've found that unless cmsFLAGS_LOWRESPRECALC
or cmsFLAGS_HIGHRESPRECALC is present, then near-black values are
rendering to black.
Yet when turning cmsFLAGS_[LOW|HIGH]RESPRECALC on, I get results that are
closer to the expected values. I've done some testing, and
this happens pretty much regardless of output profile, and
also regardless of if Black Point compensation is on or off, and in
all 4 rendering intents (although it may be that not all profiles
have all 4 intents and so are using the same BtoA's).
The values getting reduced to pure black are values that have Lab
with less than about 0.5 L (measured in 0 to 100 range as 16 bit
values). For example:
With cmsFLAGS_LOWRESPRECALC or cmsFLAGS_HIGHRESPRECALC:
Lab(0.4,-0.1,0.3) -> sRGB RGB(0x0202, 0x01a0, 0x00cb)
Without either of these flags set, the same Lab value goes to output RGB(0,0,0)
Any ideas? As you know, I optimised cmsTrilinearInterp16 with
that loop unrolling and fast double to int code I sent you, but that
does not seem to be the cause (I just retested with the original code
and the problem is still there).
It seems a bit strange to me. I thought the default CLUT was 33 point,
and that cmsFLAGS_HIGHRESPRECALC used a 48 point CLUT and LOWRES
uses a smaller one. So why would the problem only occur if you don't
specify either of the two PRECALC flags?
Regards,
Stuart
p.s. The problem is present with or without dwFlags |= cmsFLAGS_NOTPRECALC;
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