All posts tagged pixel density

Regarding WQXGA Pixel Density

I’ve come across a number of comments made online today in regard to pixel density in the WQXGA display, and I’d like to address that. One post in particular, in response to an article on PCMag.com, said the following:

Saying that this screen has a resolution of 2560×1600 is pure marketing BS, and I’m kind of surprised that you fell for it. As you noted, “Or, put another way, a panel with the same number of pixels as a traditional screen will have higher resolution.”

That’s because there are only two subpixels per pixel: half of the pixels are red-green, the other half are blue-white. This means that no single pixel can ever produce the color assigned to it without help from its neighbors. If the pixel at x123,y234 is asked to produce color #ABCDEF, it can’t do that because it has no blue if it’s a red-green pixel, and it has no red or green if it’s a blue pixel. These displays fudge a 2560×1600 resolution by using the missing colors from neighboring pixels to fool the eye into thinking that the other pixel is actually displaying a color composed of all three primary colors. If the pixels are small enough, you may actually be fooled, but the image will be subtly inferior to a true 2560×1600 display.

I appreciate the passion that DOSGuy and others show, but I must disagree with assertions about resolution. They seem to be confusing pixels with subpixels or dots. And DOSGuy, in particular, sounds like he’s of the camp that believes that resolution is based upon counting dots.

That assertion is, however, in contradiction to how the world’s leading metrologists for the display industry, who have written the accepted standard for measuring display resolution. Display resolution is based upon measurement of modulation contrast ratio—more specifically Michelson contrast ratio. There is currently only one display metrology standard organization, Video Electronics Standards Association (VESA), who’s standard is now being combined with the ISDN standard for a new and comprehensive standard to replace the VESA Ver 2.0  standard by an SID subcommittee. Nothing in this standard talks about counting dots—nothing.

So why not? The reason is partially historical. At one time the display of choice was a CRT. It had a Gaussian shaped spot. Two adjacent spots overlapped and compromised resolvability when the overlap reduced  the modulation contrast ratio. The lower limit was then set  50%.

Later passive matrix LCDs came out that had well defined dots in an RGB stripe pattern and people could count dots. A white pixel had a combination of one red, one green and one blue dot. Still there was crosstalk, so modulation contrast ratio could be compromised.

Display metrologists realized that modulation contrast ratio was where the rubber met the road. It was what the eye really saw and what could easily be measured. What good did it do if there were lots of dots that bled into one another for any reason? For many years this definition using modulation contrast ration has stood the test of time with display experts.

PenTile displays meet and exceed the definition of modulation contrast ratio for the resolution as defined. In this case for 2560 x 1600, meaning that one can write a series of 1280 black and white line pairs in one direction and 800 line pairs in the other direction and can write diagonal line pairs in any other azimuth of the same pitch and always meet or exceed 50% modulation contrast ratio. While the spec on speaks of black and white lines the PenTile display can do the same with all other colors.

So can the PenTile display write any color at any pixel? Yes it can. Remember that we are dealing with logical pixels that are comprised of varying numbers of subpixels. What the instrument measures and what the eye sees is the center of luminance energy at each and every one of the corresponding 2560 x 1600 pixels and it is possible to write every color to each of these that one can write to an RGB stripe display. The instruments can measure this and the human eye can see all of these dots of all colors.

For more detail on Michelson contrast modulation measurements and PenTile, please read this white paper on the topic.

It’s definitely not marketing BS — it’s perfectly within the specifications defined by the display industry’s metrology experts.