PenTile and Sharp Eyesight

Here is an email I received from someone who was unable to post his comment:

 

Hi Joel,

 

I could not find a way to leave comments on the PenTile blog site, so I am emailing you instead.

I went to a Verizon store recently to look for a good replacement for my old Droid, and the widespread adoption of PenTile displays is significantly reducing my options.  The contrast, brightness, and saturation of Super AMOLED displays is excellent, and to my eyes they look superior to LCDs from a distance of a few feet or more, but held close enough to read, the PenTile checkerboard pattern is very apparent and distracting, especially at ppis in the mid 200s.  It’s less perceptible at 300-330 ppi in the latest applications, but still noticeable enough that I wouldn’t want to own one.

I understand the principle of the PenTile sub-pixel arrangement and sub-pixel rendering (which can also be applied to RGB stripes or other patterns), and find it very interesting.  However, there’s just no getting around what PenTile actually looks like to people with sharp eyesight.  It makes text, lines, and edges look fuzzy, and solid areas (other than green which is on every pixel) look visibly grainy.  For PenTile to look as crisp and smooth to my eyes as an LCD retina display at 326 ppi, it would probably have to be at least 400 ppi, maybe 450.  Once PenTile displays are in that range and I can no longer see the ugly artifacts, I will have no complaints with them.  Until then people with sharp eyesight like mine will continue to hate them no matter how good the contrast, brightness, and saturation.

For people who just don’t get it, and think PenTile displays look fine, try reading small text and looking at colorful and detailed pictures and icons on one through a powerful magnifying glass for a while.  That’s what it looks like to some people all the time!  Then do the same with an LCD, and even if you can see the sub-pixel pattern it doesn’t look nearly as annoying.  In fact, I would take a full RGB display over a PenTile display with the same number of sub-pixels any day, even though the ppi would be almost 20% lower and the brightness would be somewhat lower.  I can’t wait until Super AMOLED Plus (full RGB AMOLED) or 400+ ppi PenTile displays become mainstream.

Thanks,

- Dave

________

And my reply to Dave:

 

Hi Dave,

Thanks for your note.  I am sorry to hear that you had difficulty leaving a comment on the blog site.  I will look into why that might be.

First of all let me say that I am sorry to hear that PenTile displays are so bothersome to you.  I have no doubts that there are people like yourself who can resolve as high as 50-60 cycles/degree.  For people like yourself there is no doubt that the pattern visibility of PenTile is more apparent and can be bothersome. 

Looking at the chart you can see that normal vision is nowhere this good.  Sensitivity to luminance contrast modulation for normal human vision falls off significantly at 25 cycles/degree.  The ability to resolve chroma is far less as you can see in the 2D version of the plot to the right.

3D Plot of Human Vision System
Luminance Contrast vs Spatial Freq

Nouvoyance has never recommended the use of PenTile for 200 PPI smartphone designs.  In my opinion even a 4” WVGA at 233 PPI is stretching the application of PenTile a bit far, but even that is 24 cy/deg if viewed at 30 cm.  As you can see in the chart the cycles/degree is determined by first stating what your normal viewing distance is for a device.  For example, people rarely complain about TVs which have horribly coarse patterns, because they do not use them up close. 

Perhaps your ability to resolve high cycles per degree is not as much the issue as your ability to look at your phone from an incredibly short distance.  The ability to

Cycles/Degree by Application

accommodate at such short distances can be both a blessing an a curse.  If you can focus on a display from 10 cm could also easily explain why you see these patterns so readily.  Do you, for example, also see the pattern easily on the Galaxy SII which is an RGB stripe OLED?  It is no finer pitch for subpixels than PenTile.  If you see it, but find it less bothersome, this means that you have learned to adapt to that pattern and have not been able to do so for the PenTile pattern.  In the transition from CRTs to RGB stripe LCDs there were many people who had considerable difficulty in adapting to the visibility of the “jailbars”, but, over time, most people have benefited from visual adaption and were able to separate out the image information from the pattern.  Similarly, most people are not trouble by looking through window screens despite their intrusion on visibility. While I have never personally met anyone who could see the PenTile pattern in the Galaxy S3 at ~300PPI, I never doubted that such people like yourself existed.

There are also people who have the ability to see flicker better than most of the population.  They have a horrible time with the field sequential nature of TI based projectors.  They, too, have had to look at projectors in conference rooms that drive them nuts in a market where the technology was proliferating.

So let me give you the bad news first.  There is no technical solution for replacing PenTile in OLED smartphone displays on the horizon.  The highest pitch that has been recently demonstrated is ~260PPI which will go into the Galaxy Note II.  Beyond that, the OLED display industry must use PenTile to maintain a reasonble lifetime and brightness for the blue subpixels.  I had the occasion to chat with Professor Tang, the co-inventor of  OLED technology two weekends ago when he spoke at the 50th Anniversary of SID.  I asked him if the lifetime issue for blue OLED material was fundamental, or if there is a good solution on the horizon.  He explained to me that the energetics involved in blue quantum excitation was at just the right level to break the bonds in the OLED materials.  He doesn’t see a solution in the forseeable future.  For this reason, I predict that you will continue to see many PenTile displays in OLED phones.    Furthermore, we are now seeing critical issues with battery life for tablets, so the ability to cut power consumption in half with PenTile RGBW LCDs will soon bring more PenTile LCDs to the market, starting with WQXGA formats of ~300PPI.  Most people will be delighted, but you, no doubt, will not.

Now for the good news.  There are already LCD alternatives available at your local phone provider for smartphones usng LTPS, and perhaps soon IGZO based backplanes that will give decent performance at 300 PPI.  These may not have the color gamut or response speed of the curent AMOLEDs, but they will be less troublesome for your vision.  The other good news is that the trend for increased resolution continues.  We will be seeing displays continue to 400PPI and 450PPI before too long.  I am guessing that even your vision will not be troubled by PenTile technology when we get there and you may even appreciate the improvements to power efficiency and/or brightness that PenTile technology will bring to LCDs.

Thanks again for your feedback.

 

Joel

 

PS:  Perhaps you would be willing to share with me a specific image or test pattern that you find most objectionable on the PenTile panels you have seen.  The people in our R&D team are always looking at ways to continue to improve PenTile display performance.  We have already made a series of improvements, not all of which have made it yet into products, but sometimes knowing what customers really find to be problematic will enable us to put our efforts into more productive solutions for future designs.

 

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