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Bug: Color of a certain image is displayed incorrectly, missing "green" completely [answered]

asked 2014-01-08 14:33:29 +0300

the_mgt gravatar image

updated 2014-01-19 20:46:49 +0300

lk gravatar image

I opened a link with an image of a coat and the color is way off. On my laptop it is a dark turquoise, in reality, dark turquoise (called "petrol") is the color of the coat. My girlfriend owns this coat, so I am able to hold the image directly against the fabric to compare colors. On my Jolla it simply is a dark blue, which is wrong.

The odd thing about this certain image on that certain page is, that the base color of all other images on the page is green. These other greens appear just a hint darker on my Jolla (compared to my Macbook Air), whereas only the image of the coat is missing all green. So I suspect it is rather an error in an image library than with the display.

The webpage

The actual image

(Sorry for the "advertising")

This is not about "perceived quality of the display", the general color temperature (which would affect all images equally) or the philosophical problem of Qualia, it is about one special image which is totally off compared to the depicted object.

EDIT: Some clarifications and added info.

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The question has been closed for the following reason "the question is answered, an answer was accepted" by JSEHV
close date 2016-02-01 14:50:08.102489


The coat certainly looks very different on my laptop's old TN panel compared to Jolla's IPS screen. Colours on this test page are a lot stronger and deeper on Jolla. My autumn foliage photos from Lapland look a lot better on Jolla. Question is, how do we define "incorrect"?

hana ( 2014-01-08 15:32:54 +0300 )edit

@hana: By actually walking into a store and looking at the actual coat, holding the phone next to it. There is always a slight difference between different displays and between displays and "reality". But this case above is outstanding, I saw the coat in a shop, it is turquiose. On Jolla it is blue.

the_mgt ( 2014-01-08 15:48:11 +0300 )edit

According to laboratory measurements by MikroPC, Jolla's colour temperature is over 8400K, which is quite high and makes things look blue. Could this be adjusted via firmware or software, like on PC monitors and apparently some Lumias?

hana ( 2014-01-08 17:27:41 +0300 )edit

For comparison, other smartphones seem to fall in the 7000 to 8000K range. Galaxy S4 and Note 3 have a "Movie" mode which is close to the standard 6500K.

hana ( 2014-01-08 18:43:19 +0300 )edit

Here's a colour that I picked from the coat image. Through the site I found another colour #1f585f which on Jolla looks like dark green. Shades and Tints & Tones are weird, on Jolla some tones look green and others blue. The hue should remain unchanged!

hana ( 2014-01-10 18:49:18 +0300 )edit

1 Answer

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answered 2014-01-16 04:45:19 +0300

Jare gravatar image

Well, color management actually isn't a walk in the park and even the professionals sometimes fail to deliver. To put simply, color management is a chain in which each link has to work. Besides that, in your example there are several things to consider (for example): a) the fundamental difference between a display device (emissive) and fabric (reflective), b) how lighting affects the color of the coat, c) how accurately the image of the coat was retouched color-wise, d) is the image tagged with a color profile, e) does the browser take the color profile into account and f) can the display device show all the colors in the image and is it accurately calibrated and profiled.

Fortunately in consumer products we are mainly interested of a single standard color space called sRGB. It is a de facto standard in the internet and in many many consumer applications. What it means is that we don't have to build a full color management system into devices, if the displays conform with the sRGB. This is the case with most/all smartphones, because it's a much simpler system and it doesn't take any extra hardware resources.

So, how well does the Jolla display conform with the sRGB? Finnish MikroPC released some "laboratory measurements" in their article, but not surprisingly they only scraped the surface. Based on my own measurements taken with an X-rite i1 display pro colorimeter, things aren't that good. When looking at the results, you have to remember there is always some variance between copies. Nevertheless the measured luminance (390cd/m²), whitepoint (9200K), contrast (715:1) and even the RGB primaries (picture 1) are somewhat close to what MikroPC released.

The absolute sRGB coverage of Jolla's display

Picture 1: The absolute sRGB coverage of Jolla's display is nearly 100%.

Anyway, as one can see, the white point, gamma and about all the measured color values are way off from the sRGB requirements (generally ΔE of 3 is the smallest color difference detectable by the human eye). Only the green primary is within a good tolerance.

All hope is not lost though, because the RGB primaries are in the right ballpark and display devices can be calibrated. In case of Jolla, we could alter the whitepoint and RGB response curves, which would bring the display closer to sRGB thus making all colors seem "more correct". This is usually done by modifying the look-up table of the video card or the display, but unfortunately neither of those methods seem to be currently possible on Jolla. And making one of them available might require changes into the display drivers.

Firefox to the rescue! Android version of Firefox actually has the same color management support as the desktop version, which means that it's possible to profile the Jolla display and use the created ICC profile in the Firefox web browser. Now we don't have to worry about meeting the exact sRGB white point or primaries, because eyes will adapt to the current white point and colors will be translated to the nearest possible value in the display gamut. The measurement results after profiling are quite good and the difference to the unprofiled display is huge. However the lack of calibration results into losing some of the display's capabilities. For example due to the white point difference with sRGB the display's sRGB coverage is now only 82% (Picture 2).

The sRGB coverage of Jolla's display, when chromatically adapted to D50 white point

Picture 2: The sRGB coverage of Jolla's display, when the white points of the sRGB and display are chromatically adapted to the D50 white point during ICC profile conversion.

Nevertheless the coat is now turquoise and also all other images look significantly better (but only in Firefox ;)).

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Looks to me this could be quite easily addressed in a future update with an updated display driver.

jsiren ( 2014-01-16 10:04:36 +0300 )edit

Yes, perhaps, at least some lcd drivers contain hard-coded gamma tables, which could be made available for modifying. Users could then choose between different presets in the system settings.

Jare ( 2014-01-16 21:01:40 +0300 )edit

Whoa! Thanks for the answer, I only understand a tiny portion of it. But I do hope it helps Jolla to improve on the display quality.

the_mgt ( 2014-01-19 22:33:51 +0300 )edit

Jare: Sorry for "unaccepting" your answer. While it is technically interesting and correct, the problem remains unsolved since over a year now. So the Jolla Browser should get the same treatment as you did with the Firefox browser or the whole system should get a better color profile.

Whatever, unaccepting the answer in order to bump it, so that somebody could take care of it for the Sailfish 2.0 release. Would be pretty embarrassing, if a bug like this makes it to the tablet.

the_mgt ( 2015-02-25 15:48:42 +0300 )edit

@the_mgt: Well technically it's more like a feature request than a bug. Although you have to remember that a full blown support for color management is very rare in these kinds of applications. And as with Sailfish, there's no system-wide color management support in either Android or iOS. However it's possible to build it into individual apps with some amount of performance degradation.

Creating a display profile doesn't fix a bad display, it only maps what the display is capable of, so that a color management system can then translate the source colors within the display's limitations. Gamma calibrating a display will usually provide some level of improvement especially in the gray-scale accuracy, but too big changes might degrade other aspects of color reproduction. And most of all, gamma calibration doesn't affect the chromaticities of the red, green and blue primaries, so the display will be more or less stuck with the color gamut it came out of the factory. Also it's down to individual variance, how well either or both of these techniques could be used in large scale. For example in iPads the colors are reasonably accurate, because the screen has been designed to match sRGB and the manufacturing tolerances are small enough. So basically what you should really hope/ask for is a better screen (color-wise) than the phone has.

Jare ( 2015-02-27 02:29:04 +0300 )edit

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Asked: 2014-01-08 14:33:29 +0300

Seen: 562 times

Last updated: Jan 16 '14