Constant MATERIAL_SPREAD 

pub const MATERIAL_SPREAD: f64 = 1.5; // 1.5f64

How far, as a contrast ratio, a translucent surface may stand off its ground anywhere, whatever is behind it. Every ink drawn on a surface is chosen to clear 7:1 on the opaque ground (SMALL_TEXT_MIN_CONTRAST; a disabled row’s dim ink included), so on the material it still clears 7 / 1.5 ≈ 4.7:1, above body text’s 4.5:1 with room for 8-bit rounding.