Marti,
As always, thanks for your quick reply! Lcms is a wonderful library,
and it keeps getting on getting better and better. Thank you.
A couple of quick questions (I've cc'd to the list again as these
may be of general interest):
- I'll try out 1.13. I've been holding off because I see you
made some changes in the cmsTrilinearInterp16 etc area,
and I need to reapply the performance tuning I did earlier
(and from the sounds of things, apply it to the tetrahedral
routine as well).
Do you want the code back? It gives a 25% to 50% performance
boost from memory with no accuracy changes, so might be
worth having in there. This is only for the 3 band 16 bit case
which is what I tuned.
- Once a transform is created with cmsCreateMultiprofileTransform,
the transform no longer uses the original profiles in any does it?
I'm adding transform caching logic to address the issue of initial
calculation delays. If transforms are fully decoupled from profiles
that will make it a bit easier.
- You may recall some time ago I reported a problem with writing
embedded profiles out. I think the problem is related to UNICODE
compilation. I compile with UNICODE. Does lcms handle UNICODE
compilation, given all the strange string work it has to do?
I need to look into this problem in more depth. If you can confirm
that lcms should itself handle Unicode, then I'll keep digging.
Thanks & regards,
Stuart
p.s. I'll get in touch with you off-list sometime to discuss some
spectral based profile research I'm been doing for DSLR profile
generation.
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] Behalf Of Marti Maria
> Sent: Saturday, 17 July, 2004 00:48
> To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> Subject: Re: [Lcms-user] cmsFLAGS_LOWRESPRECALC generates better
> near-black results than the normal CLUT?
>
>
> Hi Stuart,
>
> There was a slightly wrong assumption on 1.12, which was fixed in 1.13
>
> The assumption was to use tetrahedral interpolation on any colorspace.
> This worked well on average, but was giving small errors on some special
> spaces, like Lab.
>
> The reason is, Lab holds luma in the left channel, so, on several cases,
> tetrahedral was using only one gridpoint of L* and remaining 3 on a, b.
> Tetrahedral works well *only* if luma is centered, RGB or CMY for
> example. For other cases, the CMM must use trilinear. In 1.13, the
> interpolation method is dynamically choosen on depending of space.
> Also, the precision of interpolation was improved in 1.13
>
> This would explain half of the weird results you are reporting.
>
> The another half is due to the "smoothing" that precalculation does
> apply to transforms.So, in low res precalc and high res precalc, the
> smoothing was somehow correcting the wrong interpolation.
>
> Let's try with 1.13:
>
> F:\lcms>icctrans -c0 -i*Lab -o "sRGB Color Space Profile.icm"
> little cms ColorSpace conversion calculator - v1.7
>
> Enter values, 'q' to quit
> L*? 0.4
> a*? -0.1
> b*? 0.3
>
> R=1.32 G=1.67 B=0.77
>
> That is right. Using different precal methods (-c0, -c1, -c2 and -c3) gives
> small differences, due to the smoothing induced by the precalculated LUT,
> and the fact Lab not is well centered in ICC PCS, but all inside expected
> tolerances.
>
> So, the conclusion would be: use 1.13 instead :-)
>
> Regards,
> Marti.
>
> ----- Original Message -----
> From: "Stuart Nixon" <[EMAIL PROTECTED]>
> To: "Marti Maria" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
> Sent: Friday, July 16, 2004 5:55 PM
> Subject: [Lcms-user] cmsFLAGS_LOWRESPRECALC generates better near-black results than
> the normal CLUT?
>
>
> 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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