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

Started Jan 15, 2013 | Discussions thread
technoid
Senior MemberPosts: 1,295
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Re: Here's Why
In reply to NeroMetalliko, Jan 20, 2013

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. 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.

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