[argyllcms] Re: Determining proper error value for -r
- From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 23 May 2006 09:37:16 -0700
On 2006 May 23, at 8:11 AM, Graeme Gill wrote:
> Ben Goren wrote:
>
>> Anyway, it occurred to me that the best way to figure out the
>> proper error value to use for the -r option of profile would be
>> to print out a chart with a bunch of patches of the same
>> colors, measure it, and then figure out what the standard
>> deviation is (and convert that into a percentage). So, that's
>> exactly what I did: I created a 39-patch chart, did a copy /
>> paste in a text editor (with some editing) to make eight copies
>> of the patch set, printed it, measured it, imported the results
>> into an OpenOffice spreadsheet, and fiddled around with it
>> until I came up with about a third of one percent as the
>> error--which is in line with my eyeball guesses. (I'll be doing
>> some more testing later today to further confirm and explore
>> all this.)
>
> This is a pretty reasonable way to go about things. It's tedious
> though, and tells you about the level of error at only one point
> in the colorspace.
I think one of us is misunderstanding the other. I got eight
samples each from 39 different colors, the same colors that Argyll
generates from a 39-patch chart.
>> I'm also pretty sure that that value would be much too low for
>> my parents' cheap color laser printer, and probably a bit too
>> high for ``real'' paper (I did this on plain paper just to
>> figure out if it even made sense to do in the first place). I'm
>> also curious to learn just how much of an impact patch size has
>> on accuracy.
>
> The most sensitive "real world" test I stumbled across, was to
> simply make up the profile, and eyeball the gamut surface. I
> found quite noticeable changes in the smoothness of the gamut
> surface, as I varied the -r factor in profile. Too small, and
> the surface was noticeably bumpy. As the number increased, the
> surface got visibly smoother, and looked more like one would
> expect for a well behaved device. The self fit errors rise as
> the -r factor goes up too, so I stopped at a suitable "knee"
> point.
>
> Now, whether this is a good way of doing it, or the only way of
> doing it, I'm not sure.
It sure sounds like an easier way than what I just did.
Might I ask, what do you use to look at the gamut surface? Do you
think OS X's ColorSync utility would be sufficient, or do I
need something with a higher resolution? I've never noticed any
irregularities when looking at the 3-D graph of the profile in
ColorSync, which is why I ask.
It could also be that I don't know what I'm looking for. I just
enlarged the graph in ColorSync, and my profiles do look bumpier
than, for example, Canon's...but I suspect that it's not the best
tool to examine profiles for this sort of thing.
>> There'd be a new flag to targen, similar to the existing -e
>> flag, that tells how many times to repeat /all/ patches, not
>> just the white ones. When profile is fed a chart with repeated
>> patches, it calculates the appropriate value for -r itself,
>> uses that, then creates a profile and reports the recommended
>> value for -r for future use. You can then use the generated
>> profile with the -c option of targen or just toss it entirely.
>
> I' not sure such a thing is worth doing, unless there is
> an indication that such a calculated number is useful and
> meaningful. You've already expressed doubt that the number you
> came up with for your printer seems realistic.
Again, I think there's a misunderstanding here. For the paper and
printer I used, it seems like it's probably the right value. For
other papers and printers, it probably wouldn't be right.
> How does it compare with the number you arrive at in getting a
> smooth looking gamut surface ? Do the numbers agree ?
I'll get back to you as I figure out how to do that....
> What sort of color was your test patch ? It would be interesting
> to do the same test with a quite light, and quite dark color, to
> see if the errors have a different magnitude.
Like I said, I used 39 test patches from an Argyll 39-patch chart,
with eight copies of each patch.
If the number really is at least reasonably close, I'll repeat the
experiment with more patch samples and on different papers to see
how reliable an indicator this is.
Cheers,
b&
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