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Testing monitor gamma with the Lacom test image - a warning

Doug Kerr

Well-known member
This is to summarize a matter that was developed over a series of messages in another thread in this section.

************

The Lacom monitor test suite, developed by Han-Kwang Nienhuys, allows us to ascertain various aspects of the behavior of our our display chain ("monitor") without need for a colorimetric instrument (such as is used in the Spyder test packages). (Thanks to Maggie Terlecki for calling this test suite to our attention.)

That test suite can be accessed here:

http://www.lagom.nl/lcd-test/

One test in the suite allows us to ascertain the "gamma" of our display chain by visual observation of a special test image.

This parameter characterizes the nonlinear transfer curve between RGB coordinate values and the resulting luminance on the display. It applies in a peculiar way to the transfer curve prescribed to directly support the sRGB color space.​
The various test images in the Lacom suite are available for download (in a ZIP archive) from the Lacom site. They can then be observed via a competent viewer and the various conclusions drawn from what we see. But a more convenient way is to view them in your browser, working from the Lacom test procedure pages, which give detailed instructions for interpreting the images.

However, with respect to the gamma test image, there is an important prerequisite for doing so, not clearly stated on the site. The gamma test image must be presented pixel-for-pixel on the screen by the browser. If not, the gamma test just does not work.

In the case of my browser, Mozilla Firefox (currently at version 30.0), attaining that requires both the following:

• The browser zoom level must be set to "normal" (or "reset"), as done with Ctrl+0.

• The system display resolution must be set to 96 DPI (100%).

Thus, if you normally operate with another display resolution setting, you essentially must make this particular test "offline", using the downloaded gamma test file and observing it through a competent viewer.

It is entirely possible that others of the Lacom tests may be subject to this same prerequisite. I have not done any analysis of those.

Best regards,

Doug
 
Doug,

When I first read the title of this thread, my first reaction was, "oh, no!, Doug has found something else terribly wrong with the test".

I am wondering if the word "warning" is the best to be used in the title, as for some they may not read your summary and simply steer away from trying the test while if instead you would use something like "advice", they would be more open to read on.

I'm not a grammarian but because of my initial reaction, I thought I'd mention it.

Again, thank you for verifying his work as it feels good to know that it is pretty accurate when done correctly.

:)
Maggie
 

Doug Kerr

Well-known member
Hi, Maggie,

When I first read the title of this thread, my first reaction was, "oh, no!, Doug has found something else terribly wrong with the test".

I am wondering if the word "warning" is the best to be used in the title, as for some they may not read your summary and simply steer away from trying the test while if instead you would use something like "advice", they would be more open to read on.

I had considered "alert" and some others ("advice" is in fact quite good), and even considered no such "alerting" term in the title, but then nobody would read the piece!

I'm not a grammarian but because of my initial reaction, I thought I'd mention it.

Well, it's a good point.

But I stand by "warning". Those who do not hark to it may well have erroneous results.

Again, thank you for verifying his work as it feels good to know that it is pretty accurate when done correctly.

Thanks. And thanks for your contribution to the whole matter.

Best regards,

Doug
 

Doug Kerr

Well-known member
I note that on the Lacom site, for some tests, there is a warning that the "For this test your monitor must be in its native resolution."

But there is more to it than that.

Then, there is a panel that reports the "monitor resolution" (as reported by the browser), which we are asked to check against the monitor's "native resolution". The intimation is that this should be the operating resolution of the monitor, but a note comments that the browser may not actually know that.

The latter is certainly true, but does not get to the heart of the actual issue here. But gladly, that report is truly the "monitor resultion" upon which the browser actually bases its scaling of graphic images. It embraces three separate. but all pertinent, factors. I will discuss them with examples from the situation here.

1. My display system is set to operate my "monitor" at its "native resolution" (1680 px × 1050 px).

I could set it (actually a monitor driver setting) to virtually operate at other resolutions, but that is not a good idea, as there will always be interpolation needed (done in in the device driver) to mediate between the "interface" resolution and the actual pixel grid resultion of the display device itself.

The actual spatial resolution of the monitor in this state is very nearly 98.48 px/inch.​
2. I have my operating system (Windows 7) set to assume a spatial resolution of the monitor of 120 px/inch. What does that do?

Well, the original intent is that for objects that are actually defined in an "inch size" (notably, text characters, whose size is denominated in "points", a fraction of an inch), they will be scaled to render on the user's screen in the physical size implied by their point size.

But in fact, the user is free to set this display parameter to other than the actual spatial resolution of their display system to arbitrarily control the on-screen size of text in various point sizes (which in fact is how the setting is explained in Windows).

Thus, I tell my O/S that the spatial resolution of my display system is 120 px/inch. so on-screen, text in "12 point size" will be rendered as if in 14.4 point size. This is good for an old geezer.

Yes, I know you are not used to hearing this described this way!​

3. The Firefox browser scales the on-screen presentation of graphic images (from a pixel-to-pixel basis) based on:

3.1 The "zoom level" to which the user has set the browser ("normal" leads to scaling of 100% from that factor).

3.2 What the user has told the O/S is the spatial resolution of the display system (see 2, above), a setting of 96 "DPI" leading to a scaling of 100% from this factor.

It is not clear that this is a good idea.​
So the condition for proper operation of various of the tests is actually:

1. The monitor driver must be set to operate the monitor at its "native" resolution,

2. The O/S must be set to an assumed display resolution of 96 "DPI", and

3. The browser must be set to "normal" zoom.

That means (at a minimum) "the resolution assumed and reported by the browser must be the native resolution of the monitor", although we can imagine a special situation in which that is true but all the conditions above are not met, so that is not a sufficient condition.​
Nicely, the report (from the browser) on the Lacom pages properly includes all those factors. So, excepting a very unlikely special situation, it will give us a good tip as to whether we are all right or not.

Thus for this situation here:

• Monitor native resolution: 1680 px × 1050 px.

• Monitor operating resolution: 1680 px × 1050 px.

• O/S "DPI" setting: 125% (96 "DPI").

• Browser zoom setting: "Normal".

the Lacom page reports the "monitor resultion" as: 1344 px × 840 px.

This is in fact exactly 125% of the monitor operating resolution (1680 px × 1050 px).

And that is not the monitor native resolution

Thus we cannot expect proper operation of those test images for which that "warning" is given.

Nor of the "gamma test" image, for which no such "warning" is given.

I will make recommendations in this regard to Meneer Mienhuys. (I have already written him on this general matter but have not as yet received a reply.)

Best regards,

Doug
 
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