SID 2011, the world’s leading display conference, features some new, eye-catching 10.1″ tablet displays from Samsung, as announced on Engadget, with a mind-boggling headline resolution of 2560 x 1600 pixels, aka twice the resolution (and four times the number of pixels) of a conventional 1280 x 800 panel.

This is an amazing breakthrough and they should be thoroughly congratulated as should, if rumours are true, LG who are also likely set to announce something similar.

Less well-received was the news that this product would see the return of the sub-pixel layout known as Pentile: a trick where, for the sake of increasing perceived resolution and effectively shrinking the pixels, part of the colour information for each pixel is thrown away.

The OLED displays used by many HTC and Samsung handsets used this technique, resulting in many users complaining of a graininess to the screen and jagged edges on text and UI components, just like you’d see on older digital camera displays.

Digital Cameras didn’t actually use Pentile, but often used another layout called RGB Delta, with the red, green and blue arranged in triangles. But why? Well, RGB stripe, the conventional approach for computers and smartphones, has its failings too, particularly when displaying softer, natural content like photographs. The camera makers chose RGB Delta because it was, in fact, better for displaying the pictures and that was the top priority.

So should you want a new Pentile LCD display in your next tablet computer, even if it has a high resolution? It’s clearly not a simple answer so that’s what we’re here to dig into.

White sub-pixels

Differentiating itself from the OLED displays which Samsung  produced, with RGBG layout (two greens to each red or blue sub-pixel), the new tablet screens have introduced a white sub-pixel in place of one of the greens.

Since white can be formed by adding Red, Green and Blue together, why would they go to the effort of putting an extra white sub-pixel in, increasing the complexity? Well, the reasons are, on an LCD display, efficiency and brightness.

Most colour LCDs (with the exception of field-sequential designs, such as on Sony’s NGX-VG10 HD camcorder) create colour by passing white light from the backlight through a colour filter, blocking a large amount of the light from passing in order to generate the different colours.

Unlike TVs, smartphone and tablet displays spend much time showing content with a white background, such as documents and web pages.

Adding a dedicated white sub-pixel helps with these bright screen scenarios because, with no colour filter over the top, you increase the amount of light which comes through, even with the same backlight behind it – each pixel becomes much brighter! Because of this you can also make the colour filter over the other sub-pixels more aggressive in order to produce more vivid colours.

On a mobile product, there’s a further bonus advantage to this: mobile displays tend to be partly transflective. Try turning your phone’s LCD backlight down and look at your display in bright sunlight, moving it around different angles. It may surprise you to find that there are some angles wich you can hold it in where the display is still very readable – that’s down to reflected light from deep inside the display. The Nokia E6x series had particularly good displays for this.

Reflected light has to pass through the colour filter twice: both going into the display and coming out, cutting down the brightness twice. Removing that helps with transflective operation so the new tablet screens have the potential to be much better in direct sunlight than their predecessors.

So far, so good, then. Better colours, better battery life, brighter displays and better performance outdoors. So what’s the downside?

Pentile returns

I regard the Pentile pixel layout as cheating. It’s a clever cheat and, for reasons to be explained, not necessarily inherently “A Bad Thing”, but a cheat it is.

Why would you use Pentile? Well, by reducing the number of sub-pixels by 1/3rd (and making the shape somewhat easier to manufacture than the long thin rectangles of RGB stripe), you can pretend that you have more pixels on the screen and, to be fair, you do get some of the benefits of higher resolution. But not all of them and there are drawbacks.

The Pentile Blog, run by the company who invented and licence the technology (who Samsung acquired), attempts to tackle the question head on with their post “Does PenTile have Fewer Pixels“. To quote from this article, their answer is simple: “Pfff… No. PenTile has the same number of pixels as a legacy stripe panel.”

Hang on RGB stripe panels are “legacy”? I thought that RGB stripe was a new feature only available in the latest “Super AMOLED Plus” displays on the Galaxy S II; that’s what the “Plus” stands for after all! Apparently, RGB Stripe is a good feature when it’s in a Samsung display and a bad one when it’s not.

Anyway, in this strange world, all pixels are not equal. A pixel (a point of light) has a colour associated with it. Pentile pixels are normally not the correct colour. On an RGBG Pentile display, the colour of pretty much every pixel will be fundamentally incorrect because each is missing either all the Red or all the Blue data. It’s up to the adjacent pixels to try to add that missing information.

Essentially, the Pentile system compresses (lossily) the colour data, saying that it’s not that important because the sharpness remains, as shown by an ISO test which claims that they are visually comparable from a distance.

To me, that is equivalent to selling a “32GB Flash Disk” that’s really only 22GB and the manufacturer tries to justify that by stating that you can store 32GB of Zipped Word Documents. It’s somewhat dishonest.

Many people saw straight through this when they realised that their displays were not showing the level of detail they expected and had odd visual artifacts. That’s why there was such relief when Samsung appeared to have reverted to the earlier and more loved layout with the Galaxy S II and other phones.

So, is RGBW any different to this or does it also throw away colour data in every pixel? No, it’s not really any different. Half the pixels still have no Blue information and the other half have very little Red or Green (other than what’s in the white sub-pixel). RGBW has some advantages, as said above, but Pentile is still Pentile.

Graininess

But it’s not the lack of colour information which most people complain about. The clever maths is solid and it averages out to look close to the original picture. In fact, on a photograph or video, you may well be hard-pressed to tell the difference between the displays.

What is most obvious, though, is a side-effect of having to deal with the lack of colour information, whenever you look at text or user interfaces.

Consider the layout of these three display types: RGB Stripe, RGBG Pentile (as on many AMOLEDs) and RGBW Pentile (as on these new LCDs).

One way of laying out a screen with 1/3rd the colour information missing could be to stick to stripes.

The problem with that is that the oddness of the missing colour information would be very easy to spot – big stripes running down the screen of alternating purple and green hues so, naturally, the alternative is to break the pattern up and alternate the order between the rows of the display.

So we end up with this:

Looking at these, you can see two things instantly: 1) The RGBW looks instantly brighter – as we predicted – and 2) The staggered rows gives a sort of checkerboard effect that’s much lower resolution than the pixels, so is much easier to spot to the naked eye. Remember, according to the Pentile proponents, the three images above are all directly equivalent, despite the obvious lower level of detail in the Pentile layouts.

It’s this checkerboard pattern which gives the graininess to solid areas of colour which is so noticeable on many Pentile displays. But it’s not just the graininess, it’s also the fact that 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.

Now that’s all well and good, but how does it affect real-world performance? Here is a close-up shot of a RGBW display from AnandTech’s Motorola Atrix 4G preview.

You don’t need to be a display expert so see that the graininess is still there, as are the jagged edges. RGBW didn’t solve this problem.

Simulated performance

On Twitter, Engadget’s Richard Lai pointed out to me that he would be interested to see how the technologies compared at the same resolutions.

Sadly, I wasn’t able to get the different display types today to do a direct comparison so here are some mock-ups I made to demonstrate what’s going on with different types of content. These pictures pretty much speak for themselves. They have been slightly blurred to simulate the impact of your eye (all adjusted by an equal amount),

Text	Lines
White text on black background compared on RGBW, RGBG Pentile and RGB Stripe displays	2 pixel wide vertical stripes in Blue, Grey and Red compared on RGBW, RGBG Pentile and RGB Stripe Displays
Dark Text	Photogaphs
Black text on white background on RGBW, RGBG Pentile and RGB Stripe displays	Photographic content on RGBW, RGBG Pentile and RGB Stripe displays

Interpreting these pictures is somewhat subjective, but there are a few observations from the above pictures which I hope you will agree with:

1. Looking at the text, the higher resolution and more uniform pattern of the RGB stripe makes the text look much cleaner. Stray red sub-pixels on the vertical edges of the letters look messy; the white background makes things worse. The white sub-pixels of the RGBW arrangement stand out quite prominently compared to RGBG, making the checkerboard effect worse.
2. On the lines, representative of UI components, the vertical lines exaggerate this stray pixel problem on both the Pentile layouts. When the line is only two pixels wide, you see the red and blue lines jump their way from one side to the other, zig-zagging its way down the screen as the colour information gets dropped.. But, on the grey colour, the RGBW layout looks quite a bit smoother than RGBG.
3. The photograph is the most interesting one: here, patterns in both layouts are very clearly visible. The RGB stripe does look most artificial, almost robotic and, well, stripey. That said, the checkerboarding is still very visible on the and the 50% higher resolution of the RGB stripe shines through too but it’s not a clear win for either.

Anand Lal Shimpi summarises it very well, again from his Atrix review:

“Personally, I was bothered a bit by text rendering (particularly aliased text on zoomed out web pages) on the qHD screen. For the most part, the qHD display was pleasant to look at and its PenTile upbringings didn’t bother me.”

A subjective difference?

Here we get into the territory of “Retina” displays and the ilk: at what level will all these pixels (and, of course the sub-pixels) blur together, rendering all arguments about jagged edges and visible patterns moot.

For every person, this point will be different, depending on a) the dpi (dots per inch) of the display, b) how far away you hold it from your eyes and c) how good your eyesight is. Many people won’t ever have noticed this even on the worst Pentile screens.

For Pentile, the checkerboard pattern has an interesting effect, because what is important for determining this visual dpi is the distance between the visual artifacts (the dots of the same colour), rather than the distance between adjacent “pixels”. This is much lower. In fact, looking at these dots, you see that the effective grid is rotated 45 degrees from the normal display angle and the gap between the subpixels is 1.41 times that implied by the quoted resolution.

A lot has been written on the subject of Apple’s Retina Display so I won’t re-hash this, but it’s worth doing a few quick sums on Samsung’s new panel just to see how it compares.

X-pixels	2560
Y-pixels	1600
Aspect Ratio	16:10
Diagonal	10.1″
Height	5.35″
Width	8.56″
Pixel DPI	299
Pattern DPI	211

Now, at 300dpi, the pixel resolution is clearly right up there with what Apple claim to be “retina”-class, for a mobile phone screen. But, at 211dpi, will the checkerboard pattern could well be visible to many people.

In his article “Resolving the iPhone resolution“, Phil Plait uses the assumption that people hold a phone around 12″ (30cm) away from their eyes. I did a quick check and arrived at pretty much that same distance for holding an iPad so the assumptions for DPI necessary to be “retina”-class in his analysis hold true for a tablet too.

“Let me make this clear: if you have perfect eyesight, then at one foot away the iPhone 4?s pixels are resolved. The picture will look pixellated. If you have average eyesight, the picture will look just fine.”

Conclusions

Samsung’s new tablet display is retina-class, but many people may well still be able to detect the visual artifacts coming from the Pentile layout.

To fully get rid of the effects on a Pentile layout, you would need to increase the resolution to around 420dpi, which may mean that sticking to a 300dpi RGB-stripe display is the best solution.

Ultimately, if you think it will annoy you, you need to try it out to see if your eyes are sharp enough to spot the problem; if not, then don’t worry about it! But bear in mind, when you’ve seen this artifact, you will never unsee it.

If  you do end up getting a new retina class tablet with this Samsung display, then the addition of the white sub-pixel should have some really nice benefits to battery life, colour reproduction and outdoor performance, whcih are very good things.

I guess we’ll just have to wait and see what the rest of SID shows and what Apple turn up with in the next iPad!