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

Started Jan 15, 2013 | Discussions thread
NeroMetalliko
Regular MemberPosts: 220
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Re: Here's Why
In reply to technoid, Jan 20, 2013

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

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