Pixel is a unit of length and area

Original Article ↗ Hacker News Discussion ↗

What is a “pixel”? Sample, element, or unit?

  • Several comments define a pixel as a sample or tuple of color values (e.g., RGB) at a location, i.e., a data item, not something with inherent physical size.
  • Others insist a pixel is a picture element on a device (sensor/display), inherently two‑dimensional with area.
  • A third camp calls a pixel essentially a “countable thing” (like apples, eggs, or tiles), not a measurement unit in the strict sense.

Units, counts, and dimensional analysis

  • Many argue pixels are dimensionless counts, like “number of atoms” or “moles,” so “pixel²” is meaningless; you just have a count of pixels in an area.
  • The widespread usage “1920×1080 pixels” or “12 megapixels” is seen as informal shorthand, analogous to “5 blocks away” or “10 tiles high.”
  • Some say the article’s attempt to treat pixels like meters (a true length unit) is fundamentally wrong; meters × meters ≠ meters, but pixels × pixels still yields “pixels” as a count.

Physical realization: sensors, displays, and non-square pixels

  • Discussion notes non-square pixels in DVDs, older VGA modes, some DSLRs, medical imaging, and various displays.
  • Camera side: talk of photosites/sensels, Bayer patterns, sensor size vs megapixel count, and how demosaicing complicates a 1:1 photosite→pixel mapping.
  • Display side: subpixels (RGB emitters), irregular geometries, and differing physical sizes mean no universal pixel length or area.

Point-sample vs little-square model

  • One influential view: pixels should be treated as point samples for signal/image processing; the “little square” mental model is mathematically harmful for resampling and reconstruction.
  • Counterargument: real sensors integrate over finite areas and real displays emit over finite areas, so modeling pixels as small rectangles is often more accurate in practice, especially for graphics, GUIs, and pixel art.

Subpixels, font smoothing, and modern screens

  • Subpixel rendering shows that treating pixels as simple uniform squares is insufficient; exploiting RGB subpixels can increase effective resolution on low‑DPI displays.
  • Others note subpixel techniques are fading on high‑DPI screens and in modern toolkits, where grayscale antialiasing and sheer resolution dominate.

Language, context, and overall reaction

  • Several comments frame this as a linguistic/semantic issue (synecdoche: “pixels” standing for “pixel widths/heights” or pixel counts).
  • Consensus leans toward: “pixel” has multiple, context‑dependent meanings (count, area element, sample, sometimes even time in video), and forcing it into a single “unit of length/area” is more confusing than helpful.