All posts tagged LCD

Why PenTile RGBW Improves LCD Efficiency

This blog is being written in response to some misinformation that I saw at AndroidForums today.

http://androidforums.com/motorola-photon-4g/374557-photon-lcd-screen-pentile-not-2.html

Novox77 wrote:

…They (PenTile displays) use less energy because if you look at the surface area that the subpixels cover, it’s less than a standard square matrix. In other words, the black space between subpixels is larger. That is where your power saving comes in, and that’s also why we perceive the screendoor effect.

Which means a PenTile screen is less bright than a square matrix screen. You could compensate by increasing the max brightness of the sub-pixels, but that would nullify the power savings.

This is classic energy-balance. You can’t have brighter AND more power saving, unless you’ve achieved a fundamental efficiency somewhere else, and in the case of PenTile, there’s no difference in efficiency; only the reduction of subpixel size…

Novox77 did pretty well in the past at explaining PenTile OLED screen door effect.  However, in his recent comment  #51,  he had his information confused.  Candice Elliott, Nouvoyance’s CEO, has posted a correction comment at this site as #65, but I would like to restate some of the key points here and add a few points as well.

OLEDs get their power savings by only lighting up what you want to see, and not an entire backlight.  That is true for RGB stripe as well as PenTile.  Power savings is not the primary value proposition for PenTile when applied to OLEDs.  PenTile RGBG has it principal value in extending life by reducing current density, also allowing manufacturers to drive it harder for improved brightness without damaging the panel.

This blog is For LCDs PenTile RGBW improves power efficiency by about 2X in three different ways:

  1. By eliminating one-third of the subpixels each subpixel can be one-third larger.  Opaque portions like the FETs and black matrix border at each subpixel stay the same size for a given backplane technology, but the aperture area increases.  It is like a window screen with a coarser mesh, letting more of the light pass through from the backlight.  Note that display makers can improve the width of the black matrix grid by going to low temperature polysilicon (LTPS), continuous grain silicon (CGS) or high aperture ratio amorphous silicon (aSi) backplanes, which all help to improve light transmissivity, but even these technology have their limitations at high dpi which can be improved though the use of PenTile.  I like to say that nobody is ever too thin or too rich, and likewise display backplanes are never too transmissive.
  2. Clear/white subpixels have no color filters to block light.  With typical image information containing so much white and pastel color, this has a very significant effect of increasing the overall transmissivity of the panel.
  3. Dynamic Backlight Control (DBLC) allows the backlight to be turned down while increasing panel transmissivity for suitable images.  Unlike the conventional global dimming of RGB stripe this is also used to maintain the look of fully saturated colors like yellow to avoid simultaneous contrast error.  It leads to less clipping artifact than typical RGB stripe global dimming since white is never clipped, even in aggressive dimming modes.

The combination of these three factors allows display makers to typically double the light throughput of their LCDs.  This will continue toward even greater savings as resolution presses beyond 300 dpi, primarily due to the effect of #1 above.    You can use the benefit to light transmissivity to either double the brightness, double the battery life or a combination of both benefits.

So to summarize:

  1. The black matrix between the pixels is the same as for the equivalent backplane process used for RGB stripe, but accounts for less percentage space in PenTile RGBW LCDs.
  2. Almost every implementation of PenTile leads to a display that is brighter and has higher contrast http://pentileblog.com/?p=586 than the equivalent RGB stripe LCD.
  3. You can achieve more brightness and more power savings because you have implemented a fundamental change in the display structure.
  4. There is about a 2X improvement in display efficiency for typical usage models with display of 250dpi or higher.

PenTile may look different than RGB stripe panels if you look very closely at fully saturated colors on black, or conversely, but when it comes to saving power it is unbeatable.  It is also faithful for rendering even small, single stroke black and white text.http://pentileblog.com/?p=619

 

Comparing RGB Stripe Text to PenTile RGBW Text

My post showing the comparison of fonts with RGB stripe to PenTile RGBW LCD and PenTile RGBG OLED still left a few people confused, so I am going to illustrate this one more way.  Previously, I compared the results for the same number of columns.  Since PenTile RGBW can represent the amount of information in one-third fewer columns my prior blog showed the PenTile RGBW with one-third more font information.

This time I am comparing the same amount of font information, where PenTile RGBW used one third fewer columns, but have stretched the columns horizontally to match the horizontal space of the RGB stripe, just as it is done in products today.  This allows the comparison of equivalent size single stroke Arial fonts between RGB stripe and PenTile RGBW.

RGB Stripe (Top) - PenTile RGBW (bottom) As Rendered (left) - Blurred to Simulate Viewing Distance (right)

 

Perhaps you may wonder why there are some color subpixels turned on in places such as the vertical stroke of the “D”.  The reason is that if viewed at the normal distance, and if these subpixels were turned off, such a rendering would cause a color error to the white that was adjacent to the black line.  Rendering this as has been done prevents such color error at the edge of this font.  And, yes the PenTile RGBW should look brighter than the RGB stripe since this is a representation of the relative light transmission of PenTile RGBW vs RGB Stripe

The only problem is that this magnification is not at all how the human vision system sees this.  At the viewing distance for which this is design, that which you see will be blurred to hide the subpixels.  Move away from your screen until the subpixels on the lft hand images  are not apparent and you will see this as finished product would appear.

Moving away from the display has also be emulated by taking the images on the left and blurring it in Photoshop HSL space to create the images on the right that more accurately represent how the vision system sees these font renderings.

Perhaps you may wonder why there are some color subpixels turned on in places such as the vertical stroke of the “D”.  The reason is that if viewed at the normal distance, and if these subpixels were turned off, such a rendering would cause a color error to the white that was adjacent to the black line.  Rendering this as has been done prevents such color error at the edge of this font.

The only problem is that this magnification is not at all how the human vision system sees this.  At the viewing distance for which this is design, that which you see will be blurred to hide the subpixels.  Move away from your screen until the subpixels are not apparent and you will see this as finished product would appear.

Moving away from the display has also be emulated by taking the image on the left and blurring it in Photoshop HSL space to create the images on the right that more accurately represent how the vision system sees these font renderings.

PenTile RGBW – Why it has Higher Contrast

Recently, when showing the new WQXGA PenTile RGBW panel to editors I was asked if this was an OLED panel.  When I asked why she thought it was OLED she told me it was because the contrast was so high.  I decided then to generate a blog to explain why PenTile RGBW has such high contrast.

First let me say that OLED panels have the best indoor contrast of any displays.  This is simply because they only emit light where they are driven.  This leads to extraordinary blacks.  LCDs have a lower level of contrast than OLEDs unless one views them outside, where the reflective component of the LCD optics starts to have an impact.

PenTile RGBW, however, has contrast that is higher than other LCDs both inside and outside.  This is entirely due to the nature of light leakage in LCDs.  As it turns out, LCDs primarily have most of their light leakage near the perimeter of each subpixel.  This is a result of the way that an LCD is fabricated and driven.

If one compares RGB stripe to PenTile RGBW you can see that there is far less perimeter for the subpixels in a PenTile RGBW.  Keep in mind that in a PenTile RGBW that we have one-third fewer subpixels that an RGB stripe.  This changes the shape of each subpixel from a 3:1 aspect ratio to a 2:1 aspect ratio as is illustrated below.

RGB Stripe LCD Perimeter

 

 

 

 

Remember that PenTile only uses 2 subpixels per pixel on the average, compared to RGB stripe that used 3 subpixels per pixel.  When one calculates the relative amount of subpixel perimeter in the PenTile RGBW, and compares this to the RGB stripe, one finds that it has 28% less perimeter to leak light.  This directly leads to a comparable improvement in contrast ratio.

Illustrated below is roughly how this leakage would appear if viewed at high magnification.

RGB Stripe LCD Dark State Leakage

RGB Stripe LCD Dark State Leakage

So it is should come as no surprise that a PenTile RGBW LCD appear higher contrast than the comparable RGB stripe LCD.

Motorola Battery Indicator- Really a Color Error?

While I do not have the time to respond to everything that is written about PenTile technology, occasionally I will see something that is not quite right which is then picked up by other blogs and gets a life of its own. This can happen from a single photograph. Perhaps you have seen one or more recent blogs that claim that PenTile cannot render color correctly and then go on to illustrate this using the same badly focused photo.

Here is one that appeared today from androidphonetech.com.

As indicated by the logo, this photo seems to have originated with XDA.CN.

Several bloggers have shown this photo of a Motorola smartphone display stating that this proves that PenTile RGBW displays in Motorola’s phones exhibit this type of color error. I would be the first to agree that photographing high resolution displays is very challenging, especially at such high magnification. It is easy to overexpose the image, as was done here. Overexposure can cause color shifts, because the elements in the camera’s CCD of the proper color will become saturated and can no longer respond. Only at the right hand edge, where there is less light due to blur, did the camera capture the correct color.

Battery indicator on Motorola Atrix

With the proper exposure, from the equivalent display in a Motorola Atrix, you can see above that the highly magnified battery icon is rendered with the correct color over the entire area. You will see that there are more red subpixels on the top of this, since it was the intention to shade the green toward yellow at the top. And, here is the same photo, but zoomed in to show just the battery indicator:This photo matches exactly with what I see with my own eyes when looking through an eye loop.

Some of you may say it is different in a Droid X2 or another Motorola phone, but it is the same technology and the same PenTile firmware.

Of course, it was never intended that viewers examine these displays at this magnification, so you will see pattern visibility in these photos that is not apparent to people with normal vision for normal use. Fully saturated green on PenTile RGBW will have a bit of a checkerboard look that can be seen at this magnification. This does not occur with less saturated colors.

Finally, I tried to reproduce the color error by overexposing my photo of this same display and was able to reproduce the effect below. Of course, this will be slightly different for different cameras with different camera CCDs.

Battery Indicator Overexposed - note the yellow center with the green edge.

As you can see for yourself, PenTile RGBW is rendering the colors in this battery icon correctly in Motorola phone displays.

Fonts – comparing RGB Stripe to PenTile

As promised, this blog will illustrate small fonts compared between RGB stripe LCD, PenTile RGBW LCD and PenTile RGBG .

In the comment from Khalid H. he said that Arabic fonts cannot be read on his HTC phone equipped with PenTile, implying that PenTile cannot render Arabic fonts.  Unfortunately, Khalid did not mention what font size or style of Arabic font, and there are so many. I borrowed a clipped section of an Arabic font from Middle East International Services (http://www.dalilusa.com/arabic_course/text01.asp). I displayed these fonts on my PenTile RGBW LCD and tried to photograph these incredibly small fonts.  Even with the best lens on a Canon SLR, I went beyond the resolution limits of this lens to get a sufficient focus.  So instead, display simulator tools were used to render these characters in RGB stripe, PenTile RGBW and PenTile RGBG as is used for OLED.

I have to agree that I could not read these on PenTile, but then I also could not read these on RGB Stripe, since I do not read Arabic.  Just the same, I am posting the comparison between RGB Stripe, PenTile RGBW and PenTile RGBG .  I compared  these using an eye loop to the actual fonts displayed on our PenTile and RGB stripe screens and the rendering is matched exactly.  As you can see below, the comparison speaks for itself.  PenTile RGBW and PenTile RGBG can render these tiny single stroke fonts equivalently to RGB stripe.  Even the white dot in in the small square is rendered perfectly, or at least it seems that way to a non Arabic reader, but in any case it too compares well to RGB stripe.

The relative brightness is a true representation of how PenTile RGBW will appear on a screen at comparable power.  In each case there are 600 pixels of width, but with PenTile we eliminate one-third of the subpixels, so 600 pixels includes more information than for RGB stripe.

RGB Stripe LCD

PenTile RGBW LCD

PenTile RGBG OLED

For good measure, I did the same exercise with an Arial, English letter font.  Ariel was chosen because there were good options for small single stroke fonts.  I cannot see any problem with how this is being rendered in PenTile.  While some may say that fewer subpixels should lead to fuzzy fonts, it simply is not true.

RGB Stripe LCD

PenTile RGBW LCD

PenTile RGBG OLED

These fonts are zoomed in to a level that one should never look at fonts.  Both PenTile and RGB stripe are looking rather jaggy.  That is true for both RGB stripe and PenTile displays.  Of course, large fonts that are not single stroke, especially when displayed on larger diagonal displays can look better than these small fonts, except that they too will also look more jaggy at the edges.  PenTile is never recommended for low dpi, not because it renders incorrectly, but because the pattern will become visible at lower dpi.

Note that I did not try to test these on phones or with specific types of software, because I don’t have access to the wide variety of phones that some bloggers seem to have.  There are variations in SW and other factors unrelated to PenTile that can make these images look different than the rendering engine is capable of producing.

 

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Update in response to the comment from Khalid H. last weekend.  He suggested that such disconnected fonts were not as difficult and suggested that we look at Arabic Droid fonts which are more connected.  With his help we downloaded this font and generated some text in similar size small fonts to what we used last time.  Then we  rendered these as RGB Stripe, PenTile RGBW and PenTile RGBG to enable a comparison between RGB stripe and PenTile LCD and PenTile OLED versions.  You can see this result below.  Khalid, I hope this is more convincing.

RGB Stripe LCD - Arabic Droid Font

PenTile RGBW LCD - Arabic Droid Font

PenTile RGBG OLED- Arabic Droid Font

Response to Tested Blog – A Review by W. Fenlon

On June 9th Wesley Fenlon posted an article at Tested “How PenTile Displays Are Brighter But Not Always Better.” This article was in some ways even more difficult to follow than the one we recently struggled to translate from Russian.  The biggest part of the confusion was between PenTile RGBG OLED and PenTile RGBW LCDs as well as the unrelated diversion into metal oxide backplane technology.

Let me begin by differentiating these two implementations of PenTile technology:

PenTile RGBG OLED

  • Value proposition:  enables OLED to achieve higher resolution with equivalent brightness
  • How accomplished:  eliminating one-third of the subpixels reallocates the dead space around each subpixel into useful luminous areas, reducing current density and allowing high resolution panels to appear as bright as lower resolution panels without lifetime degradation.
  • PenTile algorithms:  subpixel rendering SPR, edge sharpening

PenTile RGBW LCD

  • Value proposition:  improves transmissivity of the LCD
  • How accomplished:
  1. Adding a clear subpixel to pass white light from backlight without significant attenuation
  2. Eliminating one-third of the subpixels allows each subpixel to be one-third wider and have better aperture ratio
  3. Using dynamic backlight control (DBLC) allows images to be analyzed to add global dimming, to reduce power to the backlight , while preserving the color of high luminance saturated colors and at the same time increasing the contrast.
  • PenTile algorithms:  RGB to RGBW gamut mapping, subpixel rendering SPR, edge sharpening , image analysis for dynamic backlight control

Let me add another clarification:  RGB stripe can be used in both OLED and LCD.  PenTile can be used in both OLED and LCD.  Wesley confuses things by comparing a PenTile RGBW LCD to a RGB stripe OLED like the Samsung 4.5-inch Super AMOLED Plus

So Wesley had it right when he said that the PenTile RGBW used in the WQXGA panel had a fourth subpixel (clear or white) to let more light through, “making the screen brighter as it consumes less power”.  He also had it right when he said that”… backlight dimming for darker images, the 2560 x 1600 PenTile display clearly has an advantage in power efficiency…”

Wesley says that “PenTile screens have fewer subpixels overall than RGB (stripe) LCD arrays.  The pixels are also bigger, which means screens for these display(s) often have to be slightly larger than (RGB stripe) LCDs, hence that 4.5” Super AMOLED Plus on the Samsung Infuse…”   Let me correct and clarify this.  It is true that for a given diagonal size that the subpixels are larger, than the equivalent RGB stripe panel, however the pixels are equivalent.  PenTile displays use logical pixels that contain varying numbers of subpixels with luminance centers equivalent to an RGB stripe panel.  RGB stripe displays are comprised of pixels that each have 3 subpixels and are always the same size.  This is true for both LCD and OLED.  Having fewer and larger subpixels does not cause image quality issues that were supposedly illustrated with pictures in Wesley’s blog and have previously appeared in other blogs.  Shown below are images that I took myself from an RGB stripe display and from a PenTile RGBW display.  You will see that PenTile images with equivalent number of pixels (not subpixels) as RGB stripe do not suffer from blurriness or lack of crispness (note that some small amount of aliasing/moire on both images is due to the digital nature of such image capture). I did nothing in Photoshop to change the quality of either image other than equivalent scaling to fit the format needed for the blog.  In a future blog I will show such a comparison of B&W text to illustrate again that such text is not degraded by PenTile

 

My photo of a PenTile RGBW LCD

My photo of an RGB Stripe LCD

 

 

Samsung has offered WVGA 4.3-inch and 4.5-inch diagonal AMOLED.  These are RGB stripe LCDs.  Samsung offers WVGA AMOLED that are 4.1-inch and smaller that are PenTile OLED.  The reason that PenTile is not offered in these larger sizes is that it would exhibit pattern visibility at such a low dpi and offers less value to these OLED designs.  If however OLED were needed in high dpi at these larger diagonal sizes thenPenTile OLED would again have utility.

In the final paragraph of Wesley’s blog he states that “…the RGBW layout, (for) the 2560 x 1600 PenTile does seem to be employing that same improvement…”  I am not entirely clear which improvement he is referencing, but since it follows the reference to the Super AMOLED Plus, I think he means the use of RGB stripe.  It is true that the 2560 x 1600 panel does not use RGB stripe, but it also is not AMOLED.  The reason it does not use RGB stripe is that an amorphous silicon TFT backplane at 300 dpi with today’s state of the art technology would have about a 30% aperture ratio, making it impractical for power consumption for a table application.  Perhaps LTPS or metal oxide transitors might improve this aperture ratio, but uniformity of these technologies for 10.1-inch diagonal has yet to be demonstrated in production.  Once that technology is proven for this diagonal, it may allow practical tablet applications, but at that point PenTile technology could again be applied for an additional 40% improvement in power efficiency.  When comes to saving power, saving more power is always better.  I like to say that people are never too thin or too rich and likewise panels never save too much power.

One other small point is that this WQXGA demonstration panel already has a very wide viewing angle technology that is very similar in performance to IPS.  PenTile can be combined with any WVA technology for wide viewing angle performance equivalent to that seen on an RGB stripe panel.

To conclude

  • PenTile technology does not create blurry images or black and white text.
  • PenTile RGBW LCDs are more power efficient than any other technology in the market.
  • PenTile RGBW LCDs can be combined with IPS or any other wide viewing technology.
  • PenTile RGBW LCDs are fit for use on tablets due to the demands for high resolution and low power consumption.

How Does PenTile RGBW Save 40% of the Power?

The power in an LCD primarily goes into the LEDs in the backlight. The problem is that a great deal of light is lost between those LED and your eyes. Losses occur in channeling the light from the LEDs to a light guide and more is lost in diffusers to make that light look uniform behind the LCD panel. Light is lost due to internal reflections even before hits the LCD. About half of the light is lost in the polarizers on the display cell. More is lost in trying to go through opaque cell structures like the FETs at each subpixel. Stillmore is lost in the color filters associated with each subpixel. In the best of cases 10% of the light makes it from the backlight to your eyes. For high resolution panels it can be less than 5%.

Since backplane FETs can only be made so small, the higher the resolution of the display the poorer the ratio between open area where light is transmitted through a subpixel and the total area of the subpixel that includes the FET and the buss bars. Each subpixel is also surrounded by a black matrix to prevent the backlight from photoactivating the subpixel FET, keeping it from turning on when you don’t want it to.

Color filters work by passing some color and absorbing all of the rest. Those other colors are just turned into heat.

1. In a PenTile RGBW there are one-third fewer subpixels used to create same number of pixels. That allows each subpixel to be one-third larger to pass that much more light to your eyes. In a display with 300 dpi it is a significant factor. So, this helps to fix the problem with aperture ratio.

2. Another  part of the savings  comes from the use of clear subpixels that pass the white light from the backlight nearly unaffected. Since some much content is white or pastel, this becomes very important. This helps improve on the losses through color filters.

3. Additionally, PenTile RGBW displays analyze each image for peak luminance and for highly saturated colors to minimize the backlight with Dynamic Backlight Control (DBLC). This is similar to content adaptive backlight control, but also examines saturated colors to optimize the choices between power savings and maintenance of high luminance saturated color. This takes advance of the natural occurrence of white to reduce color filter loss and enable less clipping artifact than is seen in RGB LCDs with CABC.

When it comes to a tablet 40%* can be a substantial saving. PenTile displays may look a little different than RGB stripe panels in some circumstances, but 40% is a number that is big enough that it is difficult to ignore when making a high resolution display product.

*  based upon JEITA usage model

Can You See PenTile Pixel Pattern Up Close?

Sure you can. But pattern visibility is possible with almost every display – whether it is RGB stripe or a PenTile display. Viewing TVs at a retail store allows you to see all kinds of TVs from a much closer distance than you would normally watch television, but in all cases, you can see the RGB stripe TFT LCD pattern or RGB stripe plasma pattern.

Since PenTile displays have  fewer and larger subpixels, such pattern visibility would be more apparent, but PenTile is only used in very high-resolution displays.

For users who have exceptional close-range vision capabilities, they will see pattern visibility more readily in PenTile than the equivalent size and resolution RGB stripe displays, but that certainly doesn’t meant that it has less resolution.

What to Expect from Us at SID

While PenTile is generally associated with mobile products, where customers have seen all of our current design-ins, we have been applying the technology to larger sizes as well.

At Display Week 2011 in Los Angeles, along with Samsung, we will demonstrate new technology for two applications…

  1. A 10.1” tablet demo using PenTile RGBW LCD, with the highest resolution ever made
    It runs at 55% of the power of an equivalent RGB stripe display and enables a thin and light tablet without sacrificing ultra-high resolution. This tablet is about the same pixel pitch as the highest resolution smart phones, but now in a 10.1-inch diagonal size. You’ll love it.
  2. The future of television
    For future TV, power savings will be critical. New technology from us and Samsung SEC combines a PenTile multiprimary LCD with field sequential color (FSC) and a 2D RGB LED backlight to achieve a very wide color gamut at the same time as the highest level of power savings. If you are display technologist, this demo will surprise you. You will see that it is now possible to remove color breakup, the main artifact associated with FSC.