50% Gray in RGB, Lab ang Gray-gamma 2.2: why are different?

Started Jan 15, 2013 | Discussions thread
NeroMetalliko
Regular MemberPosts: 223
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Re: Here's Why
In reply to technoid, Jan 20, 2013

technoid wrote:

NeroMetalliko wrote:

technoid wrote:

NeroMetalliko wrote:

So, to better understand, if my goal is to create, for example, a simple B&W correction curve to apply to a given printer settings-paper setup (leaving the icc profiles out of the task), the optimal linearization should NOT be calculated basing over an equally spaced K% step wedge, but creating an L equally spaced step wedge and then expecting the Colormunki readings of the printed/measured wedge to be a perfect decreasing straight line.

Is this correct or I still miss something?

Many thanks in advance for the attention.

Ciao

That is roughly right. Printers will exhibit a bit of variation along the line since transitions over the different inks are not completely smooth. A good profile will offer the same capabilities and is more standard.

Hello, many thanks for the answer.

I know that the most proper way should be building a dedicated icc profile.

But keeping for the moment the printer-ink-paper non-linearities and physical limitations apart, and focusing more to the right theoretical approach, given our previous explanations/arguments and the different gamma between L*a*b* and Gray gamma 2.2, what I wonder now is why, at least by a first look by reading online, a frequent approach is to create a B&W step wedge based on equally spaced K% Gray (21 step wedge for example with 5% gray steps in gamma 2.2), print it, read it with Colormunki, export the measured values (which, using Colorpicker, are L*, a*, b* values and reflectances at different wavelengths) and linearize the L line basing the calculation on these data. Maybe I'm wrong but I don't remember to have read someone clearly pointing out that the measured L line, at least ideally, should NOT be a perfect straight line if the step wedge is K% equally spaced (and NOT L equally spaced).

QTR, for example, has a little utility that take the measured L data and build a Gray (or RGB) linearized .icc profile. I don't know yet, but if the created/printed/measured step wedge is K% equally spaced and NOT L equally spaced the final "correction" profile will be slightly wrong with errors ranging in the +4,-4 % amount depending on the Gray zone, so not so perfect at all unless the utility apply a proper compensation for it (that I ignore but easily could be).

In any case I will check this for myself comparing the results. What I need to know at the moment for sure is that, if I decide to build a custom correction curve, the right way to do it is to perform this task creating an L equally spaced (and NOT a K% equally saced) step wedge and then linearize the printed/measured L values with a dedicated L*a*b* curve expecting as a final target an ideally perfect L straight line (paper-ink limitations apart).

Please, feel free to correct me if something is wrong, not clear or missing.

Have a nice day, many thanks in advance.

Ciao

Ah, I see your concern. Not to worry. It doesn't matter whether the wedges that are printed are closer to equally spaced L* or K or something much different than either.

Hello, thanks for the answer.

As you can easily understand my goal at the moment is to evaluate/improve the linearity for BW prints on my Epson R3000 printer using the ABW mode (usually starting from neutral dark settings). In ABW mode the common rgb .icc paper profiles profiles are useless. So, the correction in the usual Color Management style is not allowed.

As per what I have understood up today, there are still two way to compensate for ink/paper nonlinearity in ABW and both are of the type to pre-compensate the image prior to send it to the ABW driver:

- it can be done using a QTR linearized profile, by converting the image in the QTR profile and then assigning a gamma 2.2 profile

- or it can be done by directly apply a properly calculated compensation curve to the image

I'm currently exploring the second method and it could be very useful if you can kindly explain to me why it does not matter wheter the wedges are equally spaced or not for this purpose. I think that the L equally spaced wedge between L:0 and L:100 is mandatory for this method.

The purpose of profiling software is to correct the native printer reponse curves to produce an accurate output. The software will create a profile that will map an image to the proper L*a*b values.

An excellent way to measure the accuracy of a B&W profile is to create an L*a*b series of patches from L=0 to L=100. These should be printed two ways. One with Relative Colorimetric, the other with Absoulte Colorimetric. The patches should then be read with a Spectro.

For Relative colorimetric printing the patches should range from a low value (3 to 10) to a high value (94 to 98) depending on the printer/paper black point/ white point. The L*a*b* will not match the initial patch values as they are compressed between the black/white points.

For Absolute Colorimetric printing the measured L*a*b* values should be quite close* for all patches that are within the white/black point limits and should clip at the white/black points.

*The average difference between the measured and image L values should be around 1 or less with a good profile. An average of 2 or less is still pretty good. More important than average L difference is the smoothness and shifts in a* and b* (Color tints) but that's another topic.

Nice advice for icc prolfile evaluation on BW, many thanks

I will compare for sure the results of the .icc proiles vs the method I'm developing of the direct curve once I have figured out the things and succesfully implemented it.

I have just created and printed an equally spaced L wedge (21 steps at L:5 intervals) and tonight I will measure it. Let me see how it works...

Many thanks in advance.

Ciao

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