All posts tagged PenTile

RGB Stripe as a Benchmark

PenTile display technology is relatively new, so it is often benchmarked against RGB stripe, which has been the legacy technology of the industry.  There have been some who have criticized PenTile technology for color resolution to the pixel level.  Implicit to these comments is the assumption that RGB stripe enables full color resolution that matches whatever is being written to these panels to the pixel level.  In this blog I would like to point out that resolving color to the pixel level is not always possible even with RGB stripe display, or at least it is not what your eyes will see on these displays.

Consider first the simple example of a pattern of yellow and black vertical lines.  To render this on an RGB stripe display you would turn on one column of red and the adjacent column of  green subpixels to create yellow and then turns of the adjacent columns of blue, red, green and blue.  This looks like the image below.  At high magnification one sees the red and green subpixels as stripes, but at a sufficient distance these color merge to give the impression of yellow lines and black spaces.

Now, instead, consider writing a pattern of vertical yellow and blue lines.  To render this on an RGB stripe display, moving from left to right, you would turn on one column of red and one of green and then turn off the  adjacent columns of blue, red and green.  Then you would turn on the next blue column for a blue line, followed by the adjacent red and green to create another yellow line, etc. as illustrated below.  From up close you will see the red, green and blue stripes, but at the same distance as you had previously merged red and green before to get yellow you will now merge red, green and blue to produce white.  But, instead of seeing yellow and blue stripes, you will see white and black stripes! I have demonstrated this effect using MS Paint which will show this effect well if you are viewing this on an RGB stripe panel .  You can use an eyeloop to look  this image and see that it really is the subpixel configuration shown here.

 

Yellow and Blue Stripes Zoomed Out a Little

Closely Spaced Yellow and Blue Lines on an RGB Stripe LCD at High Magnification

Yellow and Blue Stripes on an RGB Stripe Display Zoomed Out Further (check this out with an eyeloop)

While there are those who claim that RGB stripe LCDs have full color resolution which is superior to PenTile, both RGB stripe and PenTile are both dependent upon the proximity of subpixels to produce color and will behave in much the same way.  Resolution of color does not happen independently of the human vision system (HVS).  The HVS must be taken into account when designing displays which are intended to be viewed by people.  PenTile algorithms explicitly take into account the color blending that occurs in the HVS.

 

Can PenTile RGBW be used for TV?

You bet!

Samsung SEC has built prototypes of a 32-inch FHD TV using PenTile RGBW with a white LED backlight.  This was demonstrated at the 2010 Flat Panel Display International (FPDI) conference in Makuhari, Japan.  This PenTile RGBW LCD  TV runs at about 55% the power of an equivalent brightness legacy RGB stripe panel showing the full range of multimedia applications.  The PenTile RGBW TV is capable of even higher savings for TV or video alone.

Due to the white subpixels, the white areas and reflections from things like the metal in a chrome bumper have a real zing that is not seen in conventional RGB stripe TVs.

Just like the mobile displays these are built with one-third fewer subpixels, but from only 2 meters away the pixels are seen as 48 cy/deg* of human vision where the pattern is not easily seen by the best of eyes.

* 48 cy/deg means that in one degree of human vision one can see 24 white lines and 24 black lines

Didn’t Samsung Dump PenTile in Favor of RGB Stripe?

Not hardly!

At the end of February bloggers speculated about this in blogs such as:

http://www.displayblog.com/2011/02/22/samsung-super-amoled-plus-dumps-pentile-matrix-goes-real-stripe-rgb/

At CES 2011 Samsung introduced the new Galaxy S2  phone with a 4.3-inch wVGA RGB stripe OLED display that they refer to as Super AMOLED Plus. The horizontal subpixel dot pitch in this phone is about the same as that of the 3.1″ PenTile OLED panel introduced in products more than a year ago. There have been more than 70 mobile products that have been introduced with PenTile displays, most of then PenTile AMOLED. Many of these continue in production with PenTile OLED displays. PenTile technology remains very useful for the highest resolution AMOLED displays that are still being made.

At SID’s Display Week 2011 there was more than one new Samsung PenTile  LCD display technology being shown. New PenTile products continue to be developed by Samsung.

 

Why Bother With a PenTile RGBW LCD if an RGB Stripe is Available?

People have written about the PenTile RGBW in the Motorola Atrix and have wondered why Motorola would choose PenTile if they could have used the conventional RGB stripe. In following some blogs it was interesting to see that some people understood the reason, but let me explain it again here.

The key value proposition for PenTile RGBW LCDs is to save power. And not just a small amount of power but a sizable amount of power. Using one industry standard for a usage model, JEITA, it is easy to show that PenTile RGBW can save about 40% of the power compared to the equivalent legacy RGB stripe LCD. That is nothing to sneeze about. Baseband processor developers have spent countless hours just trying to save 10 microwatts of power, but this saves  tens of milliwatts.

For a phone, a very significant consumer of power is used by the display and in particular the backlight in the display. The PenTile RGBW saves power in two ways. First, it has larger subpixels which means that a smaller percentage of light is blocked by the subpixel transistor and buss-bar and more is available to light up the panel. This is sort of like comparing a fine mesh window screen to a coarser mesh screen. More light gets through the coarser mesh screen.

Secondly, one-quarter of the subpixels are  white subpixels. By white, it means that these are clear, letting light from the white LED backlight  pass though almost unobstructed. R, G, and B subpixels each absorb all colors except for one. Light that is absorbed in color filters is turned into unwanted heat and is wasted. So much of almost all content is white or near white that this is worth writing home about.

This translates into either more brightness, or longer time between charging, or thinner and lighter phones. In a compute intensive phone like the dual-core Atrix saving power is important.

So, not only is a bunch of power saved, but the white subpixels allow you to have really brilliant whites. For images where light reflects off of water or metal it now displays a brilliance that is much more like real life than has been experienced with legacy RGB stripe displays.

Save some power with PenTile RGBW.

RE: Expiance’s post on RGBW, PenTile, Subpixels and Graininess of mobile displays

I wanted to take a moment to respond to Alex Taylor’s blog post on Expiance.com last Friday. For starters, I am very impressed at how much thought and work you put into your post, Alex. Well done!

Still, I feel I need to add some clarification and correction to a couple of things you said in your blog.
Many people like yourself who have, for so long, thought of pixels as having a fixed number of dots, typically three per pixel, so it is not surprising you look at this layout and say there are two per pixel. Certainly, on the average that is true, but it is important to think of pixels in a subpixel rendered display as logical pixels.  This is not unlike it used to be for CRTs. How many subpixels are in a CRT spot? A CRT spot is comprised of a Gaussian distribution of light about a logical pixel center. Such logical pixels can overlap, but when the modulation ratio drops below 50% one loses resolution, per the VESA specification.
PenTile works much the same way.  As many as 10 subpixels can be involved in a given logical pixel, so it is misleading to say one pixel lacks blue and the next lacks red or green. Every pixel is addressed at 8 bits/color and each luminance center is lit by the proper combination of the layout and the algorithms that analyze the image and render the display. It is nothing like compression or zipping.

You seem to agree that pictures look similar for RGB stripe and PenTile. Imaging scientists call these images bandwidth limited images. I would say that these look equivalent between RGB stripe and PenTile displays and can show this show this with MTF characterization plots.

One correction to what you said is that those of us at Nouvoyance never say that PenTile looks identical to RGB stripe. There are differences, but the differences that people point out are sometimes not correct.  For example, we render black and white text perfectly.
I know how tough it is to take a good photograph of a display, but even with the ones you show of very small, single and double stroke  black and white text prove that black and white text is not fuzzy, blurry, or otherwise defective. Look at the single pixel at the top of the “n”, where the curve joins the upright stem. You can see that black pixel every bit as well on the PenTile displays as on the RGB stripe. There is some softness at the edges of each of these that is attributable to the original anti-aliased font, which is also fully and faithfully rendered on the PenTile panels; so there should be no doubt that for black and white text it is rendering perfectly.

You say ”… edges which appear straight on a RGB stripe display will appear jagged, with odd pixels sticking out, just like on those old camera screens, and vertical lines will zig-zag across the screen.” I think that the photos you exhibited to prove this seem to prove the opposite. The same could be said for horizontal line edges on an RGB Stripe, as the red and blue subpixels appear so much more darker than the green. What makes it acceptable in either case is the fact that the resolution is chosen to be high enough that the subpixels blend together by the human eye, when viewed at the appropriate distance.

Let me add that much of the IP for PenTile is in our algorithms. There are several adaptive filters that look at many aspect of images and provide sharpening to edges, especially things like diagonal lines.
You pointed out that this image above demonstrates that the low res layout causes a grid artifact. There is, in fact, some graininess that is possible for a fully saturated green on a black background. The algorithms, for anything less than fully saturated colors, fills in the black regions with white or other color subpixels. This is the same as what is experience on an RGB stripe display that is fully saturated green on black, but in the PenTile case it appears as a checkerboard whereas on the RGB stripe it appears as green vertical stripes, albeit 30% closer together for stripe. Most people will be hard pressed to see this on a 300 dpi screen even at close range. This is especially the case since the human vision system has less resolution in the diagonals. It is also why photographic dot half tones have the same diagonal grid pattern.

As for your assessment that there is a diagonal organization to the display, I would agree, but I disagree with your conclusion. So, allow me to take it up a notch on the technical aspect of the answer. I would agree that the MTF of the display is less on the diagonal for the PenTile OLED RGBG panels than on the horizontal or vertical, but in all directions PenTile can write to the Nyquist limit. While the MTF of PenTile is slightly less than RGB stripe on the diagonal for fully saturated colors, it is still well in excess of the requirement of the VESA/IMID standard of 50% modulation, so it is not reasonable to downgrade the resolution by a factor of 1.4.  For the PenTile RGBW, the MTF, even in the diagonals, is the same as the RGB Stripe panel for black and white, that is to say, that they both will show a checkerboard pattern of equal modulation when the diagonal resolution limit is reached. The checkerboard is the result of an alias that occurs in the original data, before it reaches either display.

Let me turn to one other aspect of fitness for use, which is another name for good engineering, it is known to vision scientists that the human vision system is less capable of resolving detail on the diagonal than on the horizontal or vertical. So the slight fall-off in MTF on the diagonal nearly perfectly matches the sensitivity to detail on the diagonals. Any advantage of RGB stripe in this direction is often not seen, especially as we get into the resolution range of theis WQXGA panel.

So, I am troubled by calling the PenTile resolution claim “somewhat dishonest”. Samsung is saying that our WQXGA display is a PenTile RGBW LCD. And Nouvoyance is even directing those who are interested to our website, showing how it meets the industry standards for modulation contrast ratio when measuring Michelson contrast through a moving aperture grille. This is the test provided by the industry’s leading experts in display metrology. We have disclosed a great deal about what we do.

Do PenTile displays look the same as RGB stripe displays?

No, they look different in some special cases especially at the lower end of the resolution applications for  those with very good vision and well trained eyes, and, as stated above, with a bit more of a textured look for fully saturated green on black. At the higher end of the dpi range, e.g. products with 3.1” wVGA PenTile OLED, I have yet to see even one person who has blogged about a specific product about it looking grainy or less than sharp.

PenTile RGBW can look better than RGB stripe for things like the glint from metal and the reflections from water. It takes that white subpixel to give it that extra punch.

It is very important to apply PenTile to the resolutions where it makes sense – where pattern visibility is not visible for the bulk of the market. To us at Nouvoyance this seems to be good engineering for making a product that saves 40% of the power over the equivalent RGB stripe and meets the needs of the product. Surely, battery life and brightness are important engineering design parameters that as a package make it fit for use.

In the WQXGA product at 300 dpi in a product that is typically viewed from a greater distance in normal use than for a smartphone, so I am hard pressed to think that anyone will feel that this panel will look grainy or textured .

I hope to see you at SID to show you how good this panel can look.


Regarding WQXGA Pixel Density

I’ve come across a number of comments made online today in regard to pixel density in the WQXGA display, and I’d like to address that. One post in particular, in response to an article on PCMag.com, said the following:

Saying that this screen has a resolution of 2560×1600 is pure marketing BS, and I’m kind of surprised that you fell for it. As you noted, “Or, put another way, a panel with the same number of pixels as a traditional screen will have higher resolution.”

That’s because there are only two subpixels per pixel: half of the pixels are red-green, the other half are blue-white. This means that no single pixel can ever produce the color assigned to it without help from its neighbors. If the pixel at x123,y234 is asked to produce color #ABCDEF, it can’t do that because it has no blue if it’s a red-green pixel, and it has no red or green if it’s a blue pixel. These displays fudge a 2560×1600 resolution by using the missing colors from neighboring pixels to fool the eye into thinking that the other pixel is actually displaying a color composed of all three primary colors. If the pixels are small enough, you may actually be fooled, but the image will be subtly inferior to a true 2560×1600 display.

I appreciate the passion that DOSGuy and others show, but I must disagree with assertions about resolution. They seem to be confusing pixels with subpixels or dots. And DOSGuy, in particular, sounds like he’s of the camp that believes that resolution is based upon counting dots.

That assertion is, however, in contradiction to how the world’s leading metrologists for the display industry, who have written the accepted standard for measuring display resolution. Display resolution is based upon measurement of modulation contrast ratio—more specifically Michelson contrast ratio. There is currently only one display metrology standard organization, Video Electronics Standards Association (VESA), who’s standard is now being combined with the ISDN standard for a new and comprehensive standard to replace the VESA Ver 2.0  standard by an SID subcommittee. Nothing in this standard talks about counting dots—nothing.

So why not? The reason is partially historical. At one time the display of choice was a CRT. It had a Gaussian shaped spot. Two adjacent spots overlapped and compromised resolvability when the overlap reduced  the modulation contrast ratio. The lower limit was then set  50%.

Later passive matrix LCDs came out that had well defined dots in an RGB stripe pattern and people could count dots. A white pixel had a combination of one red, one green and one blue dot. Still there was crosstalk, so modulation contrast ratio could be compromised.

Display metrologists realized that modulation contrast ratio was where the rubber met the road. It was what the eye really saw and what could easily be measured. What good did it do if there were lots of dots that bled into one another for any reason? For many years this definition using modulation contrast ration has stood the test of time with display experts.

PenTile displays meet and exceed the definition of modulation contrast ratio for the resolution as defined. In this case for 2560 x 1600, meaning that one can write a series of 1280 black and white line pairs in one direction and 800 line pairs in the other direction and can write diagonal line pairs in any other azimuth of the same pitch and always meet or exceed 50% modulation contrast ratio. While the spec on speaks of black and white lines the PenTile display can do the same with all other colors.

So can the PenTile display write any color at any pixel? Yes it can. Remember that we are dealing with logical pixels that are comprised of varying numbers of subpixels. What the instrument measures and what the eye sees is the center of luminance energy at each and every one of the corresponding 2560 x 1600 pixels and it is possible to write every color to each of these that one can write to an RGB stripe display. The instruments can measure this and the human eye can see all of these dots of all colors.

For more detail on Michelson contrast modulation measurements and PenTile, please read this white paper on the topic.

It’s definitely not marketing BS — it’s perfectly within the specifications defined by the display industry’s metrology experts.

WQXGA: What People are Saying

“The debate about when a “retina display” for tablets will exist is over: Samsung’s new 10.1-inch, 2560×1600 display is it. With a crazy pixel density of 300dpi, it rivals what Apple considers a retina display for a phone. But it’s for tablets… Our eyeballs can’t wait.”
- Gizmodo

“So what does all this mean to consumers? Basically — we’re going to be getting some seriously high quality displays in some of the upcoming tablets. Luckily for us, those displays will be lighter and 40 percent more power-efficient. Which, is a great thing — that stuff leads to lighter, visually better and longer lasting tablets.”
- Android Central

“From Samsung we’ll be seeing its 10.1-inch 300ppi prototype LCD panel, which rakes up an astonishing resolution of 2,560 x 1,600 under the battery-friendly PenTile RGBW matrix (not to be confused with AMOLED and Super AMOLED’s RGBG arrangement).”
- Engadget

“And if you weren’t already thinking it—yes, this is perfect for tablets.”
- Wired/Ars Technica

“[Samsung] just announced a 10.1-inch LCD display with 300 dpi (a measure of how many dots they’ve crammed into an inch—300′s high for a tablet) and WQXGA resolution, a staggering (and tablet-record-setting) 2560 by 1600 lines.”
- TIME’s TechLand

“Samsung’s new display does prove that high-res, tablet-sized displays are indeed possible without giving up power efficiency…”
- TUAW

“Samsung subsidiary Nouvoyance is set to reveal an impressive 10.1-inch LCD next week that could be used in future tablet computers.”
- CNET

“The new PenTile WQXGA display has double the resolution found in the forthcoming Samsung Galaxy Tab 10.1 and has more than five times as many total pixels as the iPad’s 1024-by-768 display.”
- PC World

“Samsung has fit a version of its upcoming Galaxy Tab 10.1 tablet with a high-resolution screen that will rival high-end screens such as Apple’s retina displays.”
- LA Times

“Thought the iPad 2′s 1024 x 768-pixel screen was the last word in high-resolution tablet displays? How wrong you were.”
- Laptop Mag

“Samsung [is] getting ready to show the world their stunning new 10.1 inch LCD screen which has a staggering resolution of 2560×1600 pixels, normally reserved for 30 inch panels.”
- KitGuru

“While current display technology works just fine in tablets, having a higher resolution display will certainly improve the tablet experience, especially as displays approach the sharpness of print media.”
- MobileBeat

Is Sharpness the Same as Graininess?

I recently came across this FoneHome article comparing the Galaxy S 2 Super AMOLED Plus to the HTC Sensation qHD and noticed that there seems to be some confusion about graininess and sharpness.

It is possible to have a grainy appearance and still have a sharp display.  And if so, how?

When people talk about a grainy appearance they are really speaking of pattern visibility. The structure of the subpixels can be seen, especially if the display is larger diagonal and the format is low. That is why PenTile is not applied to very low dpi applications. Since PenTile has fewer dots, but the same number of pixels, it is easier to see the pattern than in an RGB stripe display of the same format and size. I have met some people who have exceptional vision, or the ability to view displays from very close range, who can see such graininess even in the iPhone 4 display, so this is a very individual thing.

For qHD displays like the Motorola Atrix, it far more difficult to see the pattern. There are, for example,  3.1-inch PenTile OLED displays where nobody has ever commented in any blog on any graininess or that these phones use PenTile technology.

So can a grainy display still be sharp? Yes, sharpness is measured and perceived as modulation ratio. When you write a black and white line pair can you still see the difference between the white and the black and you must be able to measure 50% contrast modulation or more. PenTile displays have no difficulty meeting and exceeding 50% contrast modulation ratio to the full resolution of the panel and in any direction. So a WVGA panel should be capable of 400 black and 400 white lines in one direction and 240 black and 240 white lines in other direction. This is possible because the algorithms for PenTile display analyze each image and apply adaptive filters to enhance edges of text and fine line graphics, thereby assuring sharpness.

In short, the answer is yes; a display that may look a bit grainy can still be sharp.
Hopefully that helps clear things up (no pun intended).