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The pixel has always been the smallest unit in screen-based design. Because it’s been indivisible, it is the concrete unit of measurement among screen-based designers. The phrase “a pixel is a pixel is a pixel” has been adopted to help print designers not used to fixed-screen density understand the concept. Because of this consistency, web designers have adopted pixels over points and other units to build websites.
Now that hardware is changing and pixel densities are growing, pixels are struggling to find relevance as the stable unit they once were. Browser zooming is one thing and has been covered on QuirksMode. But what is a pixel on high resolution devices today? Why does the 640px × 960px iPhone 4 claim to be 320px × 480px in the browser? The truth is that there are two different definitions of pixels: they can be the smallest unit a screen can support (a hardware pixel) or a pixel can be based on an optically consistent unit called a “reference pixel.”
The hardware pixel
Most of us are familiar with the hardware pixel. It’s the smallest point a screen can physically display and is usually comprised of red, green, and blue sub-pixels. Light from these three sub-pixels is mixed to create the colors we see. Because the hardware pixel relates to a physical element on a screen it cannot be stretched, skewed, or subdivided. These properties make the hardware pixel like the atom: the unit of design on which we build everything.
The reference pixel and splitting atoms
Things are changing for the pixel. The w3c currently defines the reference pixel as the standard for all pixel-based measurements. Now, instead of every pixel-based measurement being based on a hardware pixel it is based on an optical reference unit that might be twice the size of a hardware pixel. This new pixel should look exactly the same in all viewing situations. The beauty of using a reference pixel is that it takes proximity to a screen into account. When using a phone that you held close, a reference pixel will be smaller on the screen than a projection you view from a distance. If the viewer holds their phone up so it is side-by-side with the projection, the pixel sizes should look identical no matter the resolution or pixel density the devices have. When implemented properly, this new standard will provide unprecedented stability across all designs on all platforms no matter the pixel density or viewing distance.