A fixed grid of pixels
Most modern flat panels have a fixed number of physical pixels arranged in a grid, and each pixel can show only its own color. A panel with 1920 columns and 1080 rows has 2,073,600 pixels (1920 times 1080), and that number does not change. This grid is the panel’s native resolution.
Older cathode ray tube screens were different. They drew lines with a beam, so they could show several resolutions reasonably well without a fixed grid. Modern panels cannot do that, which is why matching matters.
When the incoming signal has the same number of pixels as the panel, each source pixel lands on exactly one panel pixel. This one-to-one mapping gives the sharpest result, since nothing has to be invented or averaged.
What happens when they do not match
If the signal has a different resolution, the display (or the computer’s graphics hardware) must convert it to fit the grid. This is called scaling. Each panel pixel now has to be given a value computed from source pixels that do not line up with it.
Imagine a very thin vertical black line, one source pixel wide, in a picture. If the line has to be stretched by a non-whole amount, it might land half on one panel pixel and half on the next. The scaler solves this by blending: both pixels become a dark gray. The line is no longer sharp; it is wider and lighter. Do that for every edge in the picture, and the result is a general softness.
Common ways of resampling include:
- Nearest-neighbor: copy the closest source pixel. This keeps edges crisp but can make uneven widths, so some lines look thicker than others.
- Bilinear or bicubic methods: blend neighboring pixels. This gives smoother results but softer edges, and some methods add slight halos around edges.
- More elaborate filters: try to preserve detail while reducing blockiness. They can look better but are not magic, since detail the source never had cannot be recovered.
Which one your display or graphics hardware uses is usually not something you can see or choose, though some offer sharpness or scaling options.
Why whole-number ratios look better
Scaling by a whole number is easier. If a 1920 by 1080 image is shown on a 3840 by 2160 panel, each source pixel can be covered by a block of 2 by 2 panel pixels, since 3840 divided by 1920 is 2 and 2160 divided by 1080 is 2. If the scaler simply repeats pixels, edges remain crisp; the image just looks like a larger version of the original. In practice, some displays still blur it with a smoothing filter, so results depend on the product and settings.
Non-whole ratios are harder. Showing 1280 by 720 on a 1920 by 1080 panel is a ratio of 1.5 in each direction, so one source pixel must cover one and a half panel pixels. Some source pixels are repeated across two panel pixels and others across one, or they are blended, and either way the result is uneven or soft. Showing 2560 by 1440 on a 3840 by 2160 panel has the same 1.5 ratio.
Aspect ratio mismatches
Resolution also has a shape. A 1920 by 1200 panel is taller than 1920 by 1080 by 120 rows. If a 1080-row picture is shown on that panel without stretching, 120 rows go unused, and you see bars at the top and bottom, or at the sides in other cases. If it is stretched to fill the screen, circles turn into ovals. Displays often offer options such as fit to screen, full screen, or one-to-one pixel mapping. The last shows the image without scaling, leaving unused space around it, and is useful when sharpness is the priority.
Why modern systems make this harder to notice
Many computers and phones now use a scaling factor in software to make text and controls a comfortable size on high-resolution panels. This is a different thing from the panel’s scaler stretching a low-resolution signal. The software redraws the text at a larger size using all available pixels, so it can stay sharp even at non-whole factors, whereas stretching a finished low-resolution image cannot add detail. Some older programs ignore the setting and are stretched instead, which is why they look blurry on a high-resolution screen. Higher pixel density makes the softness less visible at normal viewing distances.
Practical guidance
- Set the computer to the display’s native resolution for general use. The display’s menu or spec sheet states it, and the system often shows it as “recommended.”
- If text is too small, use the operating system’s scaling setting instead of lowering the resolution.
- For games that run slowly at native resolution, a lower resolution is a reasonable trade, and a whole-number ratio of the panel looks cleaner. Some games and graphics drivers offer their own scaling.
- Check the display’s scaling or aspect options to avoid stretching and to choose one-to-one mapping when you want it.
- Beware of the signal path. Scaling can happen in the game, the graphics hardware, an adapter or the display, and doing it twice softens the picture more.
- Remember that scaling can add delay, since the display spends time processing, which relates to input lag.
If a picture looks soft and everything is set to native resolution, the cause may be something else, such as focus-related issues in a projector, a lower-quality source, compression or a cable fault.