If you were to ask most people how television screens and computer displays work, at some stage they would probably mention that each pixel (picture element) is formed from red, green, and blue (RGB) sub-pixels.
For example, the red, green, and blue sub-pixels forming a pixel on a liquid crystal display (LCD)
are rectangular. Combined, these three sub-pixels form a square (or close enough to a square for our purposes here).
But is this the only way to do things? Of course not – someone will always come up with a cunning trick that makes you say "Good grief, that's clever!" For example, consider the folks at Clairvoyante (
www.clairvoyante.com), who have used their expertise in human vision to come up with an incredibly cunning new way of doing things. First, let's consider a 4 x 4 array of standard RGB (red, green, and blue) pixels compared to a 2 x 4 array of Clairvoyante’s RGBW (red, green, blue, and white) PenTile pixels.
To be fair, we should note that the underlying RGBW technology has been around for quite some time for specialist applications such as displays in aircraft. Until now, however, these displays have been unsuccessful when it comes to displaying natural images. In order to address this, the folks at Clairvoyant have come up with a treasure chest of cunning tricks and techniques – such as special sub-pixel-based rendering algorithms – that result in bright, crisp images with natural color.
Observe that each new row of RGBW pixels is shifted by two sub-pixels from the previous row. That is, the first, third, fifth, etc. rows have their sub-pixels ordered R, G, B, W, R, G, B, W, etc. By comparison, the second, fourth, sixth, etc. rows have their sub-pixels ordered B, W, R, G, B, W, R, G, and so on. We'll see how this comes into play shortly.
One key point to note here is that four RGB pixels in a row (in the horizontal direction) are formed from 4 x 3 = 12 sub-pixels. By comparison, two RGBW pixels in a row are formed from 2 x 4 = 8 sub-pixels. Thus, the PenTile Matrix employs only 2/3 the number of sub-pixels required by the traditional scheme. In turn, this means that a PenTile-based screen requires a third fewer transistors, which improves reliability. Moreover, the remaining transistors can be fabricated a tad larger, which makes them more robust.
So why is the smaller number of larger-sized RGBW sub-pixels important? Well, apart from anything else, both arrays occupy the same physical area. Now, if you look at any of the sub-pixels in the illustrations above and below, you'll see them shown as being surrounded by a black line. This represents the real world, where each sub-pixel has an opaque periphery that blocks extraneous internal light coming out (and unwanted external light getting in).
The term "aperture ratio" refers to the transparent area of a sub-pixel compared to the total area occupied by that sub-pixel (including its opaque periphery). The fact that there are only two PenTile RGBW sub-pixels in the same area as three of the standard RGB sub-pixels means that the aperture ratio of the PenTile sub-pixels is larger; in turn, this means that they pass more light per unit area and therefore are more efficient.
However, the real key to the excitement surrounding this new technology is its power efficiency. Although LCDs are considered to be energy-efficient, when it comes to handheld, battery-powered devices such as cell phones, such a display can consume a large proportion of the device's total power budget. And things will only get worse as we start to use new features such as graphics-intensive games and streaming video on these devices.
In the case of today's cell phones using qVGA resolution (240 x 320), for example, the backlighting requires two high-efficiency white light-emitting diodes (LEDs), each of which consumes 50 mW and costs 25 cents (when purchased in extremely large quantities). By comparison, forthcoming cell phones boasting VGA resolution (480 x 640) will require 8 or 10 LEDs for the backlighting, with a combined cost of $2.00 to $2.50 and a combined power consumption of 400 to 500 mW. (Good Grief, Charlie Brown!)
Thus, another really important point about the PenTile arrangement is the fact that the white (W) sub pixels are basically transparent, which means they propagate the backlight with minimal losses (as opposed to the RGB sub-pixels whose colored filters impose significant l
osses). The way all of this works is best described visually (”a picture is worth a thousand words,” as they say). Let’s start by considering a small 3 x 4 array of traditional RGB pixels and the corresponding array of PenTile pixels; initially we’ll assume that none of the sub-pixels are lit up.
Now, let's assume that we want to fully light up the traditional RGB pixel in row 2 column 2, which will require us to turn each of its sub-pixels on 100%. We can achieve the same effect with the PenTile matrix by activating a collection of sub-pixels as shown below:
As a second example, let's assume that we wish to fully light up the traditional RGB pixel in row 3 column 2; as before, this will require us to turn each of its sub-pixels on 100%. Once again, we can achieve the same effect with the PenTile matrix by activating a collection of sub-pixels as shown below:
As one final example, suppose that we wish to fully light up two of the traditional RGB pixels in row 2 column 2 and row 3 column 2. In this case, we can achieve the same effect with the PenTile matrix by activating a collection of sub-pixels as shown below (actually, this final example is something of a simplification, because
the actual values for each RGBW sub-pixel will be tweaked [adjusted] based on the values of surrounding pixels.):
The fact that the RGBW sub-pixels have a larger aperture ratio than the traditional RGB sub-pixels – coupled with the fact that the W (white) sub-pixels propagate the backlight with minimal losses (as opposed to the red, green, and blue sub-pixels in which there are significant losses) – makes the PenTile matrix much more efficient as a whole.
And what do we mean by more efficient? Well, taking VGA resolution as an example, a PenTile screen will be 100% brighter than a traditional screen using the same number of backlight LEDs. Alternatively, a PenTile screen can achieve the same brightness as a traditional screen using half the LEDs (and therefore consuming half the power). "But wait," I hear you cry, "how can we achieve a 100% increase in brightness or a 50% reduction in power when the previous image showed the center group of RGBW sub-pixels at 62.5%?" Well, the answer to that question is a little too complex to go into here, but it's a good one (if you are interested, you can discover more in the "Alternative Subpixel
Technology" topic of my paper on the
Origin and Evolution of Computer Displays).
Last but not least, in addition to the concept of the PenTile matrix itself, the folks at Clairvoyante have developed corresponding sub-pixel image processing algorithms that take images intended for standard displays and convert them into equivalent images for PenTile matrix displays.
But what about the fact that there are 1/3 fewer RGBW sub-pixels in the PenTile matrix as compared to the RGB sub-pixels in the traditional displays. Or, to put this another way, a single RGBW-based pixel replaces (occupies the same area as) two of the traditional RGB-based pixels in the horizontal direction. Let's remind ourselves as to what this looks like:
So, doesn’t the fact that the PenTile matrix effectively has half the number of pixels (2/3 the number of sub-pixels) as compared to a traditional RGB-based display affect the resolution of the display. Well, it all depends what we mean by resolution, doesn’t it? The conventional way of specifying resolution is to report the number of whole pixels forming the display. Thus, if the arrays in the above image represented the total display, we would say that the RGB version has a resolution of 4 x 4 pixels, while the RGBW has a resolution of only 2 x 4 pixels. Is this bad? Well, let's see. . .
Before we proceed, consider the previous illustration, in which all of the pixels are fully lit and both screens are pure white (although the RGBW screen would be twice as bright using the same amount power as discussed above). Now consider what would happen if we tried to present the maximum possible number of alternating black-and-white lines on our displays in both the vertical and horizontal directions as shown in the following illustration:
As we see, we can display exactly the same number of black-and-white lines on both displays. On this basis, we might say that both displays have the same "visual resolution", even though they have different numbers of pixels/sub-pixels. To put this another way, it's the amount of detail that can be perceived in the displayed image that is important, not the number of pixels/sub-pixels used to produce that image.
OK, black-and-white lines are one thing, but what about real-world images including text (both regular and italic fonts) and graphics? Well, the point here is that the human vision system can detect variations in luminance (the intensity of light per unit area of its source – what we poor folks may consider to be "brightness") at a much higher resolution than it can detect variations in color.
As we have observed during our earlier discussions, illuminating different combinations of RGBW sub-pixels results in the same brightness as their RGB counterparts, but we achieved this by spreading the colored sub-pixels around. Having said this, Clairvoyante's sub-pixel rendering algorithms perform an extra step that sharpens the image by moving the energy back onto the original pixel locations whenever possible. Furthermore, this sub-pixel color "spreading" can actually be advantageous in the case of things like italic fonts, because it can end up minimizing unwanted visual artifacts.
The bottom line is that, by means of incredibly cunning tricks, Clairvoyante's sub-pixel rendering algorithms uses this knowledge of the human visual system to present us with images that have the same perceived resolution as traditional displays. If you are interested, the "Alternative Subpixel Technology" topic of my paper on the
Origin and Evolution of Computer Displays shows an image of a cell phone using a traditional RGB display compared to the same image using an RGBW display. This image provides a mixture of black-and-white and full color, including a photo of a person and textual information (both regular and italic fonts).
I tell you – it doesn’t matter how much you learn, there's always a different way of doing things that makes you say: "Wow, what a great idea!"
