There has been a lot of discussion on the ‘net about the infamous PenTile display. Lots of sites are throwing numbers around, comparing the ‘subpixel resolution’ of competing phones such as the Galaxy Nexus and iPhone 4/4S. Some commentary has been insane, comparing apples with oranges and declaring PenTile an abomination regardless of resolution. Others, like this one, are a lot more reasoned, but still seem to ignore the well-proven fact that the eye’s visual acuity depends on the colour in question. Let’s rattle off a couple of undeniably true facts:
- Hypothetical 1000ppi RGB and PenTile displays can both look absolutely sharp and perfect.
- The acuity of the human eye varies significantly between colours. Acuity is particularly poor with blue light.
Take the following example: these four squares consist of fine lines. From left-to-right, they are: #1: white/black, #2: red/cyan, #3: green/magenta, and #4: blue/yellow.
Stand well back from your monitor, until you can’t tell the lines apart. Now slowly move back in. Notice how the blue/yellow (#4) lines look almost the same as white/black (#1), and red/cyan (#2) look very similar to green/magenta (#3)? Very roughly speaking, this is because our eyes smear out blue light slightly, so we can’t tell the difference between white lines (yellow+blue), or alternating yellow and blue lines. Similarly, the only difference between red/cyan and green/magenta is where the blue lies.
I started this not-terribly-scientific experiment with the “hypothesis” that the eye had a higher acuity to green than red and blue. This is apparently the reason that is used to justify PenTile’s use of twice as many green subpixels as red or blue. However, although my pictures above suggest that green is more visible than blue, my eyes are more-or-less equally acute with respect to red and green light. This is backed up by at least one actual proper study, as well. [Interestingly, the original PenTile technology was RRGGB, which makes much more sense!]
So, we can conclude that the resolution of red subpixels might be the limiting factor for an RGGB “Retina” PenTile display, whereas the resolution of green subpixels might be the limiting factor for an RGB Retina iPhone 4 display. Comparing the two of these numerically is very difficult and highly subjective. The sharpness with which the eye can see these colours is different, and even this difference varies according to the brightness of the image, and the individual. To see numbers calculated with no apparent regard to these factors and presented to five significant figures is just laughable.
The closest you can come to a meaningful comparison is stating what sort of displays would be equivalent if green sub-pixel resolution was all that mattered, and what would be equivalent if red/blue sub-pixel resolution was all that mattered. The linear green sub-pixel resolution is the same for both RGB and PenTile displays, because both feature full green sub-pixels. The linear red/blue sub-pixel resolution of PenTile is compromised by a factor of sqrt(2), because red pixels are diagonally separated. So, here we go:
The effective equivalent RGB resolution of a 1280×720 PenTile display probably lies somewhere between 905×509 (if red/blue is the limiting factor by far) and 1280×720 (if green is limiting factor by far.) That is, somewhere between the-highest-resolution-ever-seen-on-a-smartphone (except the iPhone 4S by a hair) and the-highest-resolution-ever-seen-on-a-smartphone (by a mile, although some [dirty LCD] RGB 720p devices have been launched since.)
And we’ve come all this way without mentioning display size. The bigger the display, the bigger the font can be and the further you can comfortably hold the device from your eyes. A bigger display should therefore mean a lower linear resolution limit before the display be reasonably considered a ‘Retina’ display.
The proof of the pudding is in the eating
My conclusion? It is virtually impossible to pre-judge a display that we haven’t even seen before. I would love to hear subjective comments from people who have seen the device in person in Hong Kong; otherwise, I’m not interested.
But how should I pre-judge this display?
Well, OK, if you must know. Having seen a complete dearth of any meaningful analysis on the web, I decided to sit down and try and figure out how good this display is. Any calculations were out of the question because, like everyone else taking part in this debate, I’m simply not qualified to comment on what the eye can and cannot perceive. At least I’m brave enough to admit it.
So the next best thing is an attempt at simulating a 720p PenTile display. I decided a normal, RGB computer monitor is a pretty good starting point. PenTile is characterised by one of R or B being missing from each pixel. So, for each pixel, I either set the blue component to zero, and doubled the red component, or vice-versa. The end result is shown in the last panel below.
I concede that the second and third panels look markedly different, but they do share the same fundamental properties:
- Green subpixels are everywhere — one for each pixel.
- Red subpixels are in a checkerboard/”diagonal” pattern.
- And blue subpixels are the same as red subpixels, except in complementary positions.
So, fundamentally, they both enjoy full linear resolution in the green channel, and suffer from a linear resolution compromised by sqrt(2) in the red and blue channels.
[ For the techies out there, I mostly took care of the sRGB gamma curve (well, as close as I could since sRGB doesn’t follow a perfect gamma curve.) For the non-techies out there who are wondering what I’m on about, if you want to make a pixel output exactly half (or double) the amount of light, you cannot just halve (or double) the luminance value — not even close. I wish it were that simple, but look at sRGB to find out the horrifying truth. Anyway, I digress. ]
The sample images! Finally!
The images below must be viewed at 100%; they’re completely meaningless if expanded or reduced.
One catch is that in order to double the brightnesses of the red and blue subpixels, I had to start with a dim image that was below 50% brightness everywhere. This is why the images below are quite dim. The left side of the image is my PenTile simulation, the right side is a completely standard (dimmed) picture. Now, because you’re monitor isn’t 316 ppi, you need to sit well back from the screen to get an accurate idea of how the display looks.
A ruler helps — the Galaxy Nexus display is, by my calculations, 10.3cm wide. So, hold a ruler at arm’s length, and walk back from the screen until the image “looks 10.3cm wide.” Now you know what the display looks like at arm’s length. If you want to hold the “phone” closer, bring the ruler closer, and move towards the screen until the left of the image is at 0cm and the right of the image is at 10.3cm on the ruler.
If you’re so close to the monitor that you can’t see the ruler in focus, then you’re cheating!
Ignore any colour cast you see on the image — this is to do with the gamma of your monitor (and my monitor, for that matter) and doesn’t represent any real-world property of PenTile displays (although colour rendition is cited as an issue with PenTile displays, see below.)
When I stand/sit just a little way back from the monitor, the images above become crisp and beautiful, even while they’re still very big. On my 94 ppi monitor, the images look absolutely perfect and smooth by the time I’m 80 cm from the screen. On the 316 ppi Galaxy Nexus, that corresponds to 23.8cm, which is less than the 12-inch (30.5cm) metric used by Apple to justify the iPhone’s ‘Retina’ name. More to the point, I’d say that’s unnaturally close. And this is all based on my weird simulation, which definitely ought to be more “speckly” than the real thing, considering the empty black sub-pixels everywhere.
Keep in mind that the right-hand side of these images represent a hypothetical 720p full RGB display, which simply doesn’t exist on any AMOLED smartphone, nor any iPhone*. Whether the Nexus display will be like the iPhone display in terms of being pretty close to indistinguishable from perfection at any face-to-phone distance is still hard to tell. So let’s just all wait for the reviews from people who have seen this phone in person, shall we?
* Some true RGB 720p devices have been announced, such as the HTC Rezound. However, they use LCD technology, presumably with the accompanying poor contrast ratio and performance in bright conditions.
Have we talked about colour rendition yet?
Problems such as poor colour rendition are not necessarily endemic to PenTile displays. It takes time for any new technology to be perfected. RGB has been around since 1953, PenTile RGBG since 2009. Every new display is a brand-new product; you can’t put a display in a photocopier at 50% to get a higher resolution, you need to re-design every aspect of the display to handle the new dimensions. Thus, every different resolution, every different screen size, is a whole new part; a new opportunity to fix problems associated with previous generations of the technology.
It stands to reason that a PenTile display’s colour balance might vary due to viewing angle because the sub-pixels vary in size and shape, which could clearly lead to different emission patterns for each colour of light. Is that a tricky problem to fix? Yes. Should we just blindly assume that the problem hasn’t been fixed? Of course not. In that context, check out the follow short-sighted update to a post:
Update: We just added AnandTech’s analysis at the More Coverage link. They point out that if you like the pixel density on the GS II, you should be happy with that on the Galaxy Nexus — although they don’t address color rendition.
I think this little comment sums up the absurdity of this whole debate nicely — linking to a site which offers some relatively meaningless numbers which ignore screen size, expected viewing distance, and varying subpixel resolutions of different colours, then conceding that the subpixel resolution is probably actually very good, only to complain about color rendition — which is a complete unknown at this point. True, it’s not actually a complaint per se, but reading the entire post leaves a very sour taste in your mouth, when the message should be “just hold on a second, there’s one or two things we need to check before we declare this the best display ever.” Which to me, is just a statement of the obvious.