What is Device Pixel Ratio?
Device pixel ratio is the number of physical display pixels per CSS pixel along each axis, so a ratio of 2 maps one CSS pixel to a 2-by-2 block of device pixels. It influences how much image detail a screen can display within a given layout size.
How Device Pixel Ratio works
A browser lays out pages in CSS pixels and maps that coordinate system onto the display’s device pixels using a scale factor. The factor reflects the viewing environment and can change with display configuration or page zoom, so it is not a permanent hardware specification. Image candidate selection and canvas backing-store sizing can use it to preserve detail at the rendered CSS size. It belongs to client-side presentation decisions, not to the intrinsic resolution metadata of the asset.
Key facts
- The browser exposes the current ratio through window.devicePixelRatio, but moving a window between displays or changing page zoom can alter the value during a session.
- The srcset density descriptor lets the browser compare image candidates against effective display density while also considering its own selection rules and network conditions.
- A canvas whose bitmap dimensions equal only its CSS dimensions can appear soft on a high-density screen; increasing the backing store also raises memory use and drawing work.
When Device Pixel Ratio matters
Use device pixel ratio when selecting responsive image dimensions for sharper rendering. Supplying excessive resolution increases transfer and memory costs without improving detail beyond the display’s capability.