Calculate screen resolution. What is ppi or how important is pixel density
A long time ago, I worked in the LCD monitor and TV manufacturing industry. And one day I participated in a conversation with engineers from leading companies developing display control circuits. They accused all of us who designed and created screens of “nose on glass engineering” (N.O.G.E.).
In their opinion, we focused on improvements that can only be noticed by burying your nose in the screen. We increased the indicators that everyday use don't play a role. And they were absolutely right.
Today the mobile industry is doing the same thing. Pay attention to what are called the main characteristics of the screen in a tablet and smartphone. By and large, this is only the number of pixels and, perhaps, a certain display technology (IPS, OLED or other). But are these really the only details you need to pay attention to? And in general, are they the most important?
Let's go back seven years to the moment when the iPhone 4 with -display was introduced. Apple chose this name because this screen had a density of 326 pixels per inch, which corresponded to the resolution of the human eye (retina).
Chances are you don't need more high density because you won't be able to tell the difference.
Some experts, including Dr. Ray Soneira of DisplayMate Technologies, disputed this claim. But even critics agreed that this figure came very close to the limit that makes sense for practical application. 300 dpi - the density of photographs in glossy magazines. And no one has ever complained about their quality.
And now about the present. The maximum screen density in a smartphone available in the market is 806 pixels per inch. This is about Sony Xperia Z5 Premium, whose 5.5-inch display holds a full 4K image (2,160 by 3,840 pixels). There are several phones with a resolution of about 1,440 by 2,960 pixels and screen sizes ranging from 5.5 to 6 inches, the density of which exceeds 550 ppi.
Even Apple, which was the first to assure us that 326 pixels per inch would be plenty, increased this figure to 458 units per inch. Super display Retina for iPhone X.
The technical term for all this is insanity.
Without a doubt, you can notice tiny differences down to the 500 ppi density level. Provided that you have perfect vision and keep the phone no further than 30 cm from your eyes. But all the same, if today it is possible to create such products, this does not mean that they need to be created. It also doesn't mean that these displays perform better than others overall.
It takes more to keep all those pixels running computing power and energy. The more dots on the screen, the less room there is for the "open area" - the part that emits light - in each one. Thus, the brightness and energy efficiency of the backlight - or both - suffer.
What parameters should you pay attention to?
Today, displays no longer suffer from problems such as distortion and loss of linearity. We haven't seen one since manufacturers stopped using CRT screens over a decade ago. So aren't our modern displays perfect? The answer is, of course not. I can list at least three display properties that need improvement far more than the number of pixels.
Image quality in bright light conditions
The first is the image quality when sunlight. It can be improved by increasing brightness and user-visible contrast. In order for us to be comfortable looking at an emissive display (emitting light), it must display White color as bright as his surroundings.
In addition to brightness (which wastes energy), the screen must provide enough contrast to work in well-lit conditions. The characteristics of OLED displays usually indicate a contrast ratio of 100,000: 1 or even 1,000,000: 1. But this is also nonsense. You get such numbers only in a completely dark room between black and white display colors.
In real-world operating conditions, contrast is reduced by ambient light. And that's a problem for modern displays. It's rare that a screen can deliver a ratio greater than 50:1 in a typical room, and in brighter lighting conditions the ratio is even lower. We'd like to see full-color reflective display technology, but there's nothing on the market yet.
Color accuracy
The next property that should interest us is color accuracy. But don't confuse it with color palette indicators. The value of the latter determines the range of colors that the display is capable of displaying. OLED and now QLED displays impose a wide color palette, but they do not provide high color accuracy.
A wide color palette would be ideal if there was a source material whose potential it could reveal. But a typical display with a wide color palette It just makes the image too bright and cartoonish.
Instead, we need screens that accurately reproduce the colors in the content creator's palette (sRGB or Rec. 709). Transmission accuracy is expressed by the ΔE* metric, which shows the difference between two colors. If its value reaches 1, the error becomes noticeable. Show me a display option that guarantees low difference in the calculation of ΔE* based on the results of several tests, and then we will have at least something.
Playing a tone
Color accuracy and overall image quality depend largely on tone reproduction, a property better known as proper gamma. Most color errors on LCD and OLED displays are due to incorrect tone reproduction within the three primary colors.
Conclusion
Stop counting pixels. Instead, let's demand improvements to those features that can actually improve picture quality. There are many more ways to do it good screen, in addition to simply measuring the number of points.
The calculators in this article are devoted to the topic of printing digital photographs.
The first calculator helps you select a photo format for printing an image of known dimensions. Let's formulate the problem.
Given: We have a digital image of sizes known to us, for example, 3264 by 2448 pixels, and a set of standard formats offered by photo printing services. The format determines the linear dimensions of the photograph, for example, a 10x15 photograph has dimensions of 102 by 152 millimeters.
Required: Select from a set of formats the largest possible on which you can still print the image without losing quality.
To set photo formats, I created a separate reference book Photo formats, which can be expanded if necessary.
The only specialized knowledge you need to have to find the answer is knowing that quality printing digital image requires a resolution of at least 300 dots (pixels) per inch (300 dpi), and more or less acceptable printing is possible with a resolution of at least 150 dots per inch (150 dpi). Everything else is simple mathematical operations.
Graphically, the task is shown in the figure below.
The logic of finding the answer is simple - the linear dimensions of each format are converted into inches and then into pixels, based on the fact that there are 300 (150) pixels in one inch. Next, the resulting number is compared with the size of the image (there are certain nuances related to the ratio of height and width, but more on that in the second part). If the format size in pixels is larger than the size of our image (in the picture - the format to the right of the photo), then it will no longer work, because the photo will have to be stretched, and we will get a resolution worse than 300 (150) dpi. If the format size is smaller than the size of our image (in the picture - the format to the right of the photo), then it will fit - the photo will have to be compressed, and we will get a resolution better than 300 (150) dpi.
From all suitable formats, the calculator selects the format maximum size(there are no problems with printing smaller images - as far as I understand, you can print with a resolution of 1200 dpi).
Format size in pixels for 300 dpi resolution
Format size in pixels for 150 dpi resolution
The second calculator based on the dimensions of an already printed image and dimensions original image helps determine the resulting image resolution and the part cropped when scaling. Let's formulate the problem.
Given: An image of known dimensions is printed on a photograph of known dimensions. Since the value of the ratio of the height and width of the image and the value of the ratio of the height and width of the digital image, as a rule, do not coincide, when printing the image is scaled, obviously while maintaining the proportions. This is shown graphically in the figure below.
When scaling, as you can see, two options are possible:
the first is scaling with loss of part of the image,
the second is scaling with preservation of the entire image, but with the appearance empty space in the picture.
As an esthete, I chose the first option for calculations.
Thus, the first thing is required: to find the resulting resolution of the image and the part of the image that was not included in the image. Second, accordingly, this will be the difference between the used width (height) and the original width (height) of the image.
Width of the printed image, cm
Height of the printed image, cm
Image width in pixels
Contents:
Theoretical page and calculations
The concept in question stands for pixels per inch, that is, the number of pixels per inch. Also pronounced pee-pee-ay.
It literally means how many pixels fit in one inch of the image that we see on a tablet or other technology.
This concept is also called the unit of measurement of resolution. This value is calculated using two simple formulas:
Where:
- dp– diagonal resolution;
- di– diagonal size, inches;
- Wp- width;
- HP- height.
The second formula is designed to calculate diagonal resolution and is based on the use of the famous Pythagorean theorem.
Rice. 1. Width, height and diagonal size on the monitor
To show how all these formulas are used, let's take for example a 20-inch diagonal monitor with a resolution of 1280x720 (HD).
Thus, Wp will be equal to 1280, Hp – 720, and Di – 20. Thanks to the presence of these data, we can calculate pi-pi-ai. First we use formula (2).
Now let’s apply these data to formula (2).
Note: In fact, we got 73.4 pixels, but there cannot be a non-integer number of pixels, only integer values are used.
To understand how much this is in centimeters, a more common value for our area, you need to divide the resulting number by 2.54 (there are exactly so many centimeters in one inch).
So in our example it is 73/2.54=28 pixels. in centimeter.
In our example it is 73, and 25.4/73 = 0.3. That is, the size of each pixel is 0.3x0.3 mm.
Is it good or bad?
Let's figure it out together.
Is this quantity important?
Pee-pee-ay, based on the above, affects the clarity of the image that the user receives on his screen.
The higher the value of the indicator, the clearer the image the user will receive.
In fact, the larger this value, the fewer “squares” a person will see.
That is, each pixel will be small, not large, and this will make it possible not to pay attention to it at all. The value of the characteristic can be clearly seen in Figure 2
Rice. 2. The difference between indicators is less and more
Of course, no one wants to have a picture like the one shown on the left on theirs.
Therefore, when choosing such equipment, it is very important to pay attention to this characteristic.
This is especially true when you buy on the Internet and do not have the opportunity to evaluate the picture with your own eyes and understand how clear it is.
Finding an indicator in the characteristics of the same smartphone is usually easy. It is usually contained in the "Display" section. An example can be seen in Figure 3.
Rice. 3. Indicator in the characteristics of the smartphone
Important! You can often find information on the Internet that ppi is more important than, for example, resolution or diagonal, and some of these characteristics should play a more important role in the choice. This is not true at all. As we can see above, all these three concepts are inextricably linked.
Advantages and disadvantages
Number of pixels per inch has a positive effect on the clarity of the picture, and, accordingly, on its quality.
It will be much more pleasant for the user to look at an image with a higher indicator.
In Figure 2, the photo on the left has 30 ppi, and the photo on the right has 300. Below is another similar example.
But there is this concept and cons. In particular, we're talking about about the autonomy of the device.
Everything is quite simple - if the picture is clear, a smartphone, tablet or other device with a screen will not be able to work for a long time without recharging.
You can even make a simple rule: the more pee-pee-ay, the less time battery life.
Of course, for a PC this is not a problem, since there the monitor is always plugged into the socket, but for some phones this can become big problem.
Therefore, when choosing a device, be sure to pay attention not only to the number of pixels. per inch, and also per inch!
Thus, we smoothly moved on to the topic of choice.
About choosing displays
There are several rules that will help you choose the display correctly, taking into account the pixels, they sound like this:
1 Be sure to pay attention to the display type. The priority should be AMOLED, even better SuperAMOLED or OLED. Such devices will always be better than.
Let's say we come to the store and see, for example, two excellent devices - and. Their price is almost the same, the second device, by the way, is more powerful.
The specifications indicate that Xiaomi has 400 ppi (for some reason, some write 400.53, but, as we said above, there cannot be a non-integer number of pixels).
Samsung has 267 PPI and the resolution is correspondingly lower (1280x720 versus 1920x1080). The diagonal is the same - 5.5 inches.
But for some reason the picture is clearer on . And all due to the use of proprietary SuperAMOLED+ technology. You can see this for yourself if you pay attention to Figure 5.
2 Try to find an opportunity to look at all the samples you have chosen in person. You can first look at their options on the Internet, and then go to an electronics store and see how they actually display pictures. Personal view in in this case simply irreplaceable.
3 Pay attention to the battery. If we talk about smartphones, then to ensure long work device with a clear image (high ppi and/or good technology), then the battery capacity should be about 3000 mAh.
For tablets it should be even higher, since their diagonal is larger,
4 Remember: the smaller the diagonal and the higher the pixel density (the number of pixels per inch), the sharper the image. Don't deceive yourself - you won't be able to achieve a very clear picture with a huge display and a small pi-ay value. It is important to maintain a golden mean here.
5 Coverage is also important to consider. So matte screens will produce a less clear and saturated image, but will be more gentle on your eyes.
But glossy displays will negatively affect your eyesight, but the image on them will be much more beautiful. In this case, their ppi value may be the same.
This is mostly relevant. If you work on a computer full time or even more, it is better to go for the matte option.
All this will allow you to choose the most suitable display for yourself.