All posts tagged Subpixels

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.

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

Does PenTile Have Fuzzy Text?!

Some people claim this to be true based on what they expect they should see with fewer dots, but for typical black text on a white background, this is not the case.

Take a look at this recent post by Droid Life: http://bit.ly/jCgVLj

Black and white text used on PenTile is every bit as sharp as that used on an RGB stripe display – even for single stroke fonts. Evidence of this is shown in the image below – where the font is magnified to a size well beyond what’s used for normal viewing.

While the subpixels appear different than the RGB stripe, at normal viewing distance the fonts are indistinguishable between the two.

There are those who assume that this isn’t the case, as they do not understand the nature of subpixel rendering or PenTile’s edge sharpening filters. But photographs prove that this is the case!

The image featured below was published on Engadget and presumably showed that PenTile OLED text (on the left) was not as sharp as the SLCD stripe display (on the right) by enlarging it to a level beyond normal use.

Look closely at the single stroke letter “n” which is white on black font in the word “Calendar” and judge for yourself.

Will RGBW help with a checkerboard pattern

The ability to observe pattern visibility is a function of visual acuity in cycles/degree of human vision and the size the pattern.  A person with the ability can see such a checkerboard pattern in a 4-inch WVGA pattern from normal viewing distance can also see stripe patterns on many popular RGB stripe panels being sold today.  For an RGBW panel the checkerboard pattern at the lower dpi range of PenTile OLED applications is only visible for fully saturated green or red on black. With an RGBW panel at any lesser color saturation the blacks in the checkerboard are filled in with the white subpixels to improve the brightness and to reduce this effect.

At 300 dpi on a tablet panel, which is typically viewed from 50% further away, about 18 inches, even the most saturated red and green on black as the worst case condition will not have any apparent checkerboard since the ability to resolve this will require vision of nearly 50 cycles/degree.  Few of us have vision which is that good.

So that brings me to the comment on zooming in to see text that is less clear.  First of all, your eyes do not have a zoom feature.  At least mine don’t.  These panels are designed to be viewed at a certain distance.  It is like saying that a tapestry when zoomed in shows the artifact of the stitches, or that a half tone print when zoomed in shows dots.  Of course they do, but that is not how they are used.  Secondly, if you do zoom in on black and white text, you will see the same detail in even single stroke fonts for PenTile as you do for RGB stripe.  Examples that have been shown in one recent blog say that it makes it look less sharp, but these images show otherwise.  Look at the black spot at the top of the “n”, where the curve meets the straight stem in the RGB stripe and compare that to PenTile. If it were less than sharp that single pixel black dot would not be there.

Single pixel dot at top of PenTile "n" is equal to RGB stripe

 

DisplayMate’s Twitter on PenTile Resolution

We came across a few tweets from Ray Soniera of DisplayMate yesterday where he made several comments about PenTile. And we wanted to address that.

When Ray Soniera says that PenTile has half of the R, G, & B subpixels as an RGB stripe he is correct. What Dr. Soneira is doing is counting only the R, the G, and the B subpixels… ignoring the W… and ignoring subpixel rendering.

First, imagine that we had a purely monochromatic B&W panel. Do we say it has no resolution because it has no color? (Seriously?) In the PenTile case, each white subpixel would be half of the B&W resolution, and the combined RGB triplet would be the other half. So, it looks like a full resolution B&W display. This is good, because the human eye can see tremendous detail in black & white… But not so well in color!

The human vision system can only see a very low resolution in pure color. In pure red to green color our eyes see only one tenth as well as it can in pure black & white. What we see in everyday life is a combination of pure color and pure black & white, so it seems very richly detailed.

So, having high resolution in black & white, but only half the resolution in pure color, is still more than we need. Why waste putting detail into color which we can’t see?

So, Dr. Soneira is right, we do have only half the number of RGB triplets… but that is a good thing.

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.

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.

Does PenTile Have Fewer Pixels?

Pfff… No.

PenTile has the same number of pixels as a legacy stripe panel. It achieves this with one-third fewer subpixels, using subpixel rendering (SPR) technology, and adaptive filtering algorithms… but these panels still meet and exceed the rigorous requirements of the display industry for the definitions of resolution, measuring modulation contrast ratio of black and white lines through a moving aperture grille.

Take a look at the image below. That’s a comparison of RGB stripe and PenTile RGBW LCD. Hopefully this will help you see how four columns write a black and white line pair as compared to the six columns required for RGB stripe.

We love talking about this, so if you have questions, please feel free to comment.

RGBW: Why Add the White Subpixel?

Brightness, really.

Legacy LCDs act as optical light valves working in conjunction with polarized light, so white light from an LED backlight is passed through Red, Green and Blue color filters.

When you do that though, those filters end up absorbing much of the light.

Since so much image content is comprised of white or desaturated pastel colors, we recognized that the use of a clear/white subpixel would add considerably to the efficiency of the overall system by absorbing far less light in the color filters.

Which means a brighter display and less battery used.