New Approach for Generating Optimal Profile Patch Sets

[Mark D Segal]

Thanks, I've used or created, printed, examined and measured more ramps and patches over the past 19 years than I care to remember and I understand very well what they can and cannot tell us. As I said, I think that target is of indicative value. Measuring makes it more useful. A data file provided more useful still. A lot more patches, more useful again! But we've meandered from the basic point about the linearity or lack thereof of Epson printers, which itself is a tangent from the subject matter of the OP.

[digitaldog]

Actually I see little if any reason to measure the target. More important is to visually see what the profile experience is up against in it's 'raw' unprofiled behavior and the target does just that. But more importantly, it dismisses the generalization that: Most RGB printers are pretty well behaved in terms of linearity.
Make a print, you'll see, at least on every Epson I've ever tried, this isn't at all the case.

[GWGill]

Have you by any chance looked at deltaE2000 distribution of the points centered around the surrounding cubes against the average of the 8 surrounds? Does that make a good way to reduce patches that are providing little benefit?

Sorry, it's a long time ago that I played with BCC sampling, and I moved on to other sampling approaches. Certainly I've used similar ideas to estimate curvature of the device response, and the default targen "Optimised Farthest Point Sampling" uses the difference between linear device value and measured value interpolation of each Voronoi region as a curvature measure.

A lot of the original inspiration came from 3 papers written by Don Bone, and in particular the paper "Adaptive color-printer modeling using regularized linear splines" which was a CSIRO technical report TR-HJ-92-19 and also published by the SPIE summarizes a lot of it, but also includes some interesting work on adaptive sampling. At the time (1993) strip and table reading instruments were rare, and Don was using a point by point measurement, so he came up with an interactive adaptive sampling approach. After measuring a smaller uniform grid, and used a couple of techniques to decide on which points to print and measure next. (Since he was working with a copier, it was relatively easy to print each sample as needed.) He was effectively used a curvature criteria, in that areas which were poorly predicted by linear interpolation were sampled in more detail.

One of the reasons I moved away from regular device grids is that they tended to interact with spline like models badly, since they "resonated" easily. Some sort of damping or regularization is always needed with spline or polynomial models, but it's less critical if the sampling distribution is more stochastic than regular.

[aaron125]

Not to change the subject but wow, Mr Bone was at the CSIRO in Melbourne, Australia. That’s just a few km from where I live. Cool! That’s all. Back to the discussion...


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[Mark D Segal]

Actually I see little if any reason to measure the target. More important is to visually see what the profile experience is up against in it's 'raw' unprofiled behavior and the target does just that. But more importantly, it dismisses the generalization that: Most RGB printers are pretty well behaved in terms of linearity.
Make a print, you'll see, at least on every Epson I've ever tried, this isn't at all the case.

OK - I see where you are coming from: use the target to look at how the printer behaves in its native condition uninfluenced by profiles, then, if I want, see the difference it makes when the same target is printed with the printer profiled. That's interesting and I'll do it. I'lll revert to this discussion after I make the two prints and have a look at them. Meanwhile, if you could send me the reference values for the target it would save me some time, because I still believe that visual impressions should be complemented with data.

[digitaldog]

OK - I see where you are coming from: use the target to look at how the printer behaves in its native condition uninfluenced by profiles, then, if I want, see the difference it makes when the same target is printed with the printer profiled.

Yes. But the profile isn't going to do anything to make the output more linear than it really is.

[Mark D Segal]

I intend to make two prints: one using ACPU and the other using Photoshop with an ICC profile and Absolute Rendering Intent.

When I open the target in Photoshop it tells me there is no embedded colour space and provides the usual options for embedding one: Don't Color Manage, Use Working Space, use another working space. Which option do you recommend for this experiment?

I would also observe this is a very small target at 72 PPI. Would it not be better to do this with a larger version?

[digitaldog]

You want to treat it like a target because that's what it is.
You can resize it but you don't need anything large for output and it should be large enough to measure, if that's the idea, with an i1Pro.

[Mark D Segal]

You want to treat it like a target because that's what it is.
You can resize it but you don't need anything large for output and it should be large enough to measure, if that's the idea, with an i1Pro.

Targets can be treated in various ways depending on context, that's why I asked. So you haven't answered my question in regard to printing it from Photoshop as a hard-proofing exercise.

However, I did print it from ACPU as if a profiling target and in that context and application of course no such options occur, so it's not an issue for that purpose. Now I have this print of your target sitting here in front of me. What am I supposed to infer about linearity from looking at this? It makes no obvious sense to me. Is it simply telling me that if it diverges from how it looks on the monitor my printer is non-linear?

[digitaldog]

Targets can be treated in various ways depending on context, that's why I asked. So you haven't answered my question in regard to printing it from Photoshop as a hard-proofing exercise.

However, I did print it from ACPU as if a profiling target and in that context and application of course no such options occur, so it's not an issue for that purpose. Now I have this print of your target sitting here in front of me. What am I supposed to infer about linearity from looking at this? It makes no obvious sense to me. Is it simply telling me that if it diverges from how it looks on the monitor my printer is non-linear?

Does each step look linear from another, do you see the same separation of each color equally? No.
The monitor has absolutely nothing to do with anything in this respect.

[Mark D Segal]

If a picture is worth a number of words, here is a scan of what emerged from the printer. To me, depending on what "linear" looks like, it would seem the top and bottom several rows appear non-linear.

[Stephen Ray]

Just for reference, the attached image is Andrew's InkDensity.tif placed into a 4x6 inch Epson photo glossy paper print imaged with a $50 Epson XP-440 all-in-one printer using "printer manages color" and then scanned along with a Kodak Gray Scale with the same machine.

[Doug Gray]

What's important for making good profiles is smoothness. Another way of describing it is that if you divide up the curves into 36 segments (The I1Profiler 3D LUT spacing) the LUTs can do a good approximation treating the printing as linear over each of the small segments as long as the curve doesn't abruptly change.

Here's an example of how the Canon Pro1000 is smoother, but also deviates more in a* and b* over the device space neutral tone curve (RGB 0 to 255).

Of note is that on the Epson 9800 the a* and b* are "lumpy" and these lumps are rapid enough that a significant part cannot be compensated for by a profile.  Even partially compensating for these lumps requires requires a lot of extra, near neutral patches.  The Canon Pro 1000 shows much larger excursions but these are easily compensated with the exception of the large changes as low L*. Additional, near neutral patches in this area works well because the curve is smooth and without the rapid changes in slope that occurs over most of the 9800 curves.

[Mark D Segal]

Does each step look linear from another, do you see the same separation of each color equally? No.
The monitor has absolutely nothing to do with anything in this respect.

Again, what does "look linear" mean? Maybe I don't know what linear looks like even though it may be staring me in the face.

What the monitor has to do with is that without file reference data, what I see on the monitor is the only other available comparator. Anyhow, I'm moving on from this - no more time for it and less important to me than the overall performance of the PPP system as a whole (printer/paper/profile).

[digitaldog]

If a picture is worth a number of words, here is a scan of what emerged from the printer. To me, depending on what "linear" looks like, it would seem the top and bottom several rows appear non-linear.

Yup!

[Stephen Ray]

Exactly! Profiles do not make linearity when non linearity exists.

If a printer were non-linear, say heavy in the cyan channel, would a profile not correct the colors to be where they should?

[digitaldog]

If a printer were non-linear, say heavy in the cyan channel, would a profile not correct the colors to be where they should?

Printer profiles do as the name implies: it profiles (fingerprints) device behavior. Such profiles know nothing about "correct" colors, certainly in context. They take the data provided (in this case RGB) and convert to another RGB color space through a profile connection space, usually Lab and as such, by the time the profile receives this Lab data, this output profile has no idea about the 'original' RGB color space.

[Stephen Ray]

Printer profiles do as the name implies: it profiles (fingerprints) device behavior. Such profiles know nothing about "correct" colors, certainly in context. They take the data provided (in this case RGB) and convert to another RGB color space through a profile connection space, usually Lab and as such, by the time the profile receives this Lab data, this output profile has no idea about the 'original' RGB color space.

In this context of InkDensity.tif, your answer is true because the file is without an ICC tag to begin with?

[aaron125]

If a printer were non-linear, say heavy in the cyan channel, would a profile not correct the colors to be where they should?

I’ve always thought that the easiest way to remember what a profile is does is to think of it in terms of: calibration/characterisation. If one thinks of a monitor and before it was common for the computer to adjust the internal monitor LUTs, one used to have to rely on altering brightness, RGB gains, etc oneself. So, first one calibrates the monitor, putting it into a particular state such that the brightness is equal to x, whitepoint is equal to y, etc. and then it is profiled, characterised in said state. So the character of the monitor is determined from flashing up a bunch of different colours and this character is akin to a map. If I want colour w to be displayed, what signals do I need to send to the monitor such that it will display w as close as it physically can? In that regard, whether it be a monitor or a printer (rather than flashing a bunch of colours, the printer prints out said colours in terms of being fed a bunch of colours from a patch target; the calibration part for most RGB printers these days is commonly selecting a particular driver setting for the media, so choosing Epson Luster configures the printer to put down a specific amount of ink from a specific head height, etc.) or pretty much and device, I don’t think a profile is able to correct much at all. It’s simply a map of the device capabilities, which is it’s character/peculiarities. To that end, no, I don’t believe a profile is able to correct some colours to put them where they should be, because how is the profile supposed to know where those colours should be placed(?).


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[digitaldog]

In this context of InkDensity.tif, your answer is true because the file is without an ICC tag to begin with?

No, as it has to be assumed to be in some color space for any conversion. It is untagged because it's to be treated as a target. But you can't run it through any output profile without some assumption to get to Lab in the CMM. Depending on what software is being used for the conversion, that assumption is either sRGB or whatever RGB working space is set in Photoshop.
The idea is to print this out without color management. To test differing media settings prior to making a profile. To visually see what media setting, based on the output without color management might be the best choice. The idea is to dismiss the generalization as I noted, that RGB printers are well behaved. Some are, some are not. Much has to do with the driver handling the data. As you can see from Mark's output, or if you print this yourself, certainly on an Epson, the native driver isn't close to producing linear output. So we're kind of going around in circles, the target was created for a specific goal and I brought it up simply for others to use to see how their drivers behave with a fixed media setting and without color management.