PenTile Math

Every launch of the great product contains seed of disappointment. After all the excitement what it could be, harsh reality of the real thing hits us hard. Samsung Galaxy S III was not different: why, oh why they had to put awful PenTile display in? How bad is it?

What is PenTile?

Almost all displays produce colors by mixing three basic ones: red, green, and blue (RGB). Every pixel on the screen is made of three miniature sub-pixels, each in one of the basic colors. If you come very close to the old CRT screens, you can actually see them.

PenTile displays are  a bit different. Each pixel is made of just two sub-pixels, the repeating sequence of green-red and green-blue ones. As human eye is more sensitive to green than other colors, this can give better color reproduction. Similar pattern is used on many camera sensors.

Check this article on Engadget with excellent pictures showing side-by-side microscope shots of different screen technologies.

Why is PenTile bad?

Density of the pixels on the screen is measured in pixels per inch (PPI). The higher the PPI, the sharper the picture. As PPI of typical screen is too low to render small letters nicely, manufacturers use algorithms that render letters not on pixel, but rather on sub-pixel level. Although it is not same as higher resolution screen, results are much better.

Here is how it works in practice. For me, anything above 220 PPI is good enough for reading. Let's compare 220 PPI RGB and PenTile screen. If we convert this to sub-pixels, RGB provides 660 (220x3) by 220 DPI, while PenTile has only 440 (220x2) by 220 DPI. Convert PenTile sub-pixels back to RGB 3-sub-pixel per pixel PPI and you will get equivalent of 179 PPI, which is bad.

Math: square root of (220 * 220 * 2/3).

So why putting PenTile in S III, when S II had RGB screen?

Aha! That's the one million dollar question. S II Super AMOLED+ has 217 PPI while S III has 306 PPI Super AMOLED screen (no "+" here). Convert this to sub-pixels:  651 vs 612. Not very different, right? S III equivalent RGB resolution is 250 DPI, which is actually better. My best guess is that there are technical limitations how many sub-pixels Samsung can squeeze into AMOLED screen. They wanted HD resolution for S III, but couldn't put enough sub-pixels in 4.8 inch display. Making screen 20-30% larger was not an option, so they had to switch to PenTile again.

Having the similar number of sub-pixels, both screens can produce text of similar sharpness.  With advancement of AMOLED manufacturing process, I am sure we will see RGB HD screens in sub-5" sizes.

But people say that screen is bad, and Samsung justifies it as longevity trade-off

Some say that PenTile is bad. It is true for low-resolution PenTile displays, like in the first Galaxy. Anybody who actually saw the screen says that screen is excellent.

Pushed by bad press, Samsung felt obliged to respond claiming that PenTile screens last longer. Blue pixels are less efficient than red and green and they have to be driven harder, which reduces their lifespan, but not so much on PenTile screens. This explanation sounds a bit stretched, but it might be true: red pixels look two-time smaller than the blue ones and screen is not very bright, like they intentionally dimmed the screen to last longer.

Whatever is true, ignore the critics and check it for yourself. That's the only sensible test.