Get screen scale factor (screen.scale_factor)

Declaration

local factor = screen.scale_factor()

Return Value

• factor
  Number indicating the ratio between device pixels and logical points (pt)

Description

Use the return value to convert between the physical pixel resolution and the logical coordinate system.
Common values: 2.0 (most Retina iPhones and iPads), 3.0 (Plus/Max models and recent full-screen iPhones). Modern iPhones rarely use non-Retina 1.0 scale.
Combine with screen.size() to derive logical dimensions: width_pt = width_px / factor. Image search and color sampling still work with pixel sizes; only when working with point-based UI coordinates do you need to convert.

Example

-- Convert pixel resolution to logical points
local w_px, h_px = screen.size()
local scale = screen.scale_factor()
local w_pt, h_pt = w_px / scale, h_px / scale

sys.log(string.format("Pixel resolution: %dx%d, scale: %.1fx, logical points: %.0fx%.0f", w_px, h_px, scale, w_pt, h_pt))

-- Adjust UI coordinates based on the scale factor
local x_point, y_point = 100, 200           -- Coordinates in points from Apple HIG or third-party UI specs
local x_px, y_px = x_point * scale, y_point * scale

touch.tap(x_px, y_px)

Note: The sample uses functions outside this chapter: sys.log, touch.tap