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As the full-screen trend becomes increasingly apparent in mobile phones, many people believe that the higher the resolution, the clearer the display. In fact, this idea is not comprehensive. The material of the screen and the arrangement of subpixels are also important factors affecting display quality. There are generally two types of subpixel arrangements: standard RGB and RGB PenTile. What do they mean? Which arrangement is better? OLEDindustry will provide detailed answers.

The basic principle of display is that pixels are composed of red, green, and blue (RGB) light bars. Any color can be created by combining different depths of RGB light. Although the three bars appear separate when viewed up close, they blend into a single color at a suitable viewing distance. A display is made up of many pixels, and to display various colors, each pixel must be divided into three lower-level subpixels: red, green, and blue. In other words, three subpixels make up one pixel. When different colors need to be displayed, the three subpixels emit light at different brightness levels, and since the subpixels are very small, they visually blend into the desired color.

Understanding the concept of subpixels, let’s move on to today’s topic—subpixel arrangement. There are two designs: traditional RGB arrangement and RGB PenTile arrangement. What are the differences?

Definitions of the Two Subpixel Arrangements

RGB Arrangement

RGB arrangement is the most standard arrangement, dividing a square pixel into three equal parts, each assigned a different color, thus forming a color pixel. This method is adopted by the vast majority of LCDs (the order of the three pixels can vary, but it is generally “red-green-blue” or “blue-green-red”).

Therefore, as long as we arrange enough such constructed pixels together, we can display the required patterns.

In fact, most LCDs use standard RGB subpixel arrangements. Its advantage is high pixel independence, allowing each pixel to display all colors independently. However, the downside is that to create an m*n display, a total of 3m*n pixels must be manufactured (during production, subpixels are the basic manufacturing unit, and they have no color; color is produced using filters). This is not a problem for LCDs, as they use printing technology, and the cost impact of producing many pixels is not high.

RGB PenTile Arrangement

PenTile was invented by a company called Clairvoyante in Silicon Valley, San Francisco. It breaks the RGB specification by adding a white (W) light to the red, green, and blue lights, making each of the RGBW colors independent pixels with addressable functionality, allowing each color to operate independently. This contrasts with traditional RGB, where three colors are combined into a single pixel and cannot display any color independently.

Today, RGB PenTile arrangement has become the subpixel arrangement for some OLED-based mobile phones. It differs from standard RGB arrangement, where a single pixel is made up of three subpixels (red, green, and blue), while PenTile’s single pixel consists of only two subpixels (red and green or blue and green). The image on the left shows the subpixel arrangement method of RGB PenTile. As you can see, while displaying 3×3 pixels, RGB PenTile only uses 6 subpixels in the horizontal direction, while standard RGB uses 9, reducing the number of subpixels by one-third. When displaying images, a pixel in RGB PenTile will “borrow” a color from adjacent pixels to create the three primary colors. Horizontally, each pixel shares the color subpixel it lacks with adjacent pixels to achieve white display.

Comparison of PenTile and Standard RGB Subpixel Arrangements

Advantages of PenTile

The image below shows the differences in display technology between PenTile and traditional structures. Traditional RGB pixels are arranged uniformly; for example, to display a white line, all RGB in each pixel must be fully lit, and to display a black line, all RGB must be turned off. PenTile, on the other hand, has each color as an independent pixel, allowing for a chessboard arrangement. Since colors are not connected, each pixel can combine with any surrounding pixel, meaning RGBW colors can combine freely. For example, a white line can be displayed using only two colors, while traditional RGB requires three colors (the left side of the image shows traditional red-green-blue display, while the top and bottom show lines; the right side shows PenTile structure, with the top and bottom showing lines).

The biggest advantage of PenTile is its power-saving capability. LCD displays are very inefficient because RGB color light is produced by filtering through color filters, which block most of the backlight, sometimes up to 90%. We only see a small portion of the light that passes through, which makes the display appear dark. The added white light in PenTile comes directly from the backlight, requiring no filtering, resulting in much higher brightness and lower power consumption.

Additionally, each PenTile pixel can pair with adjacent pixels without the limitation of traditional RGB’s simultaneous operation, allowing many colors to be formed with just two colors instead of the mandatory three colors. The line in the image above is displayed using a two-color combination, reducing the display to two colors while achieving the same effect as three colors. This means that the color blocks used in the entire panel (not the number of pixels) are reduced by one-third with PenTile’s RGBW color blocks, allowing them to be enlarged, increasing pixel transmittance and overall panel brightness. Preliminary estimates suggest that the addition of white pixels and the enlargement of pixel area can double the panel’s brightness, or halve the power consumption while maintaining the same brightness.

Besides increasing brightness or saving power, if the area of PenTile’s color blocks (pixels) is not enlarged and matches the number of traditional RGB color blocks, PenTile’s resolution effectively increases by one-third (as shown in the image, PenTile uses two colors to achieve the same effect as traditional three colors), making multimedia presentations clearer. With the growing prevalence of multimedia use on mobile phones, brightness may be more important than power saving, and PenTile can adjust to increase brightness while saving power. Increased brightness, increased resolution, and clearer display details make it easier to read simple font strokes.

In addition to hardware, PenTile has also made many adjustments on the software side. Although PenTile displays are generally clear and bright, some graphics may appear overly saturated and unnatural. In such cases, software can reduce the brightness of white pixels and adjust other colors to make the images appear more natural. Recently, PenTile demonstrated a color analysis of film images at a conference, using the color values of one image to adjust the backlight for the next image, allowing the eyes to perceive no change in the image while significantly reducing power consumption, estimating that only 40-50% of power is used to view films.

The PenTile subpixel arrangement uses adjacent subpixels to “simulate” a pixel, significantly reducing the number of subpixels. For example, a screen with a normal RGB pixel arrangement would have a total of 15 million subpixels, but with PenTile, it would only have 10 million subpixels.

This situation is very favorable for GPUs, as reducing the number of subpixels directly lowers the GPU load. A certain forum user reported that increasing the resolution of the game “Dirt 3” to 5760*1080 required only a 580SLI to run smoothly!

Roughly calculated, the total pixel count for a 720P screen is 1280*720*0.66=608256 (9100 is 800*480=384000), which is about 158.4% of the clarity of the 9100 screen, where clarity refers to the number of subpixels.

By the way, this calculation is based on total pixel count, not pixel density, assuming the same screen size. If it were a 4.65-inch screen, it would probably be around 130%.

Using “total pixel count” divided by “area” better reflects pixel density; PPI is no longer a good comparison for PenTile’s pixel density, especially since density is often expressed in physics as area or volume.

In fact, the real drawback of PenTile is that the simulated pixel count is too low. The pentile screen of the I9000 only simulates about 66.6% of the pixel count of the 9100, leading to blurry text edges and “grid-like black dots” is inevitable. However, increasing the number of subpixels would cover these visually apparent defects.

Many engineers are troubled by the “graininess” of the i9000 screen. But why do the i9000 and most AMOLED screens exhibit this graininess?

First, it’s important to clarify that graininess is not related to AMOLED material itself but is entirely due to the subpixel arrangement of the screen.

To start with the simplest case, a square pixel is divided into three equal parts, each assigned a different color, thus forming a color pixel. This is also the subpixel arrangement method used by the vast majority of LCDs (the order of the three pixels can vary, but it is generally “red-green-blue” or “blue-green-red”).

As long as we arrange enough of these constructed pixels together, we can display the required patterns.

In fact, most LCD displays use this subpixel arrangement. Its advantage is high pixel independence, allowing each pixel to display all colors independently. However, the downside is that to create an m*n display, a total of 3m*n pixels must be manufactured (during production, subpixels are the basic manufacturing unit, and they have no color; color is produced using filters). This is not a problem for LCDs, as they use printing technology, and the cost impact of producing many pixels is not high.

However, this issue changes in the AMOLED era. AMOLED faces two problems: first, the total number of pixels directly determines production costs; second, AMOLED’s luminous efficiency is not high. If the same manufacturing process as LCD is used, higher luminous brightness is required to achieve the same visual effect as LCD, which also increases manufacturing costs. Therefore, Samsung adopted a different subpixel arrangement method when manufacturing AMOLED panels, called RGB PenTile, with many variants.

The left side of the image shows the subpixel arrangement method of PenTile RGB used in the i9000. As you can see, while displaying 3×3 pixels, PenTile only uses 6 subpixels in the horizontal direction, while standard RGB uses 9, reducing the number of subpixels by one-third. In other words, under PenTile technology, a pixel contains only two subpixels, either green + red or green + blue. You might wonder how PenTile can reduce subpixels by one-third while maintaining the total pixel count. Since it lacks one subpixel, how does it still display 3×3 full-color pixels? The key lies in the “sharing subpixels” between adjacent pixels.

Next, let’s focus on how PenTile lights up subpixels during operation.

First, let’s display horizontal white lines.

As you can see, horizontally, each pixel shares the color subpixel it lacks with adjacent pixels to achieve white display.

Next, let’s display vertical white lines.

The sharing situation is the same.

Now, let’s display a black-and-white dot matrix.

Notice that the expected blue pixel is missing! This is because each pixel has lost its neighbor and cannot share subpixels, resulting in PenTile screens being unable to accurately display such patterns. This issue is very troublesome. To ensure the display result remains white, the originally off blue pixels need to be lit, resulting in a failure to display white dots.

Now we know that the essence of PenTile technology is to achieve shared subpixels between adjacent pixels. This requires that any pixel displayed on the screen must have neighboring pixels present. However, this is not always feasible. For example, the following scenarios may arise. What problems will occur in these situations?

First, consider displaying vertical black-and-white boundary lines, which may occur at the edges of text.

As you can see, in the leftmost line, there is a vertical alternating arrangement of red and blue pixels. This visually causes a noticeable “color edge” phenomenon.

Next, consider a 45-degree diagonal black-and-white boundary line, which may occur at the edges of text.

As you can see, the expected white edge turns red.

There are many more situations that could be analyzed, but in these cases, the problem is that there will be non-white edges on the screen, which is far from our expectations. After all, no one wants to see colorful displays of black-and-white photos, right? Therefore, PenTile technology must make certain corrections for these situations, which involves lighting some subpixels that should have been off, artificially creating some neighboring pixels to achieve normal color display. However, this introduces a problem: the originally smooth edges become jagged. This is also the reason for the edge fringing in PenTile displays. I won’t illustrate specific images here.

The discussions above have been based on displaying black and white. When displaying colorful images, PenTile encounters even stranger issues. For example, when we need to display pure yellow, all blue pixels on the screen must be turned off. However, since red pixels are spaced apart rather than tightly arranged, the black spots between them can be easily seen by the naked eye, and their distance is twice that of pixel distance, resulting in a “grid pattern”. When displaying light orange, red and green pixels will emit at 100% brightness, while blue pixels will emit at 50% brightness. The unlit blue pixels will create dark spots, leading to a surface that should be pure color appearing with a “grainy” texture distributed diagonally at twice the pixel distance.

In essence, PenTile is a method that reduces the number of subpixels by allowing adjacent pixels to share subpixels, thus achieving a low-resolution simulation of high resolution. The advantage is higher visual brightness at the same brightness and lower cost, but the disadvantages are also evident—simulated displays cannot compare to the real thing. When fine details need to be displayed, the essence of PenTile becomes apparent, and clarity significantly decreases, making small fonts difficult to read. To compensate for color issues, PenTile displays will produce jagged edges in color segmentation areas, resulting in jagged edges instead of smooth lines.

The final point is that as long as the displayed content is not white, grid-like spots will appear at twice the pixel distance. Therefore, PenTile displays must have sufficiently high resolution to compensate for the visual effects caused by the twice pixel distance texture. Thus, using PenTile technology on a 4-inch AMOLED display like the i9000 can lead to noticeable issues. While not causing significant problems, those who are sensitive to screen graininess should consider checking the actual device before deciding.

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