Before
Translucent text must be measured after compositing.
alpha 0.503.98:1AA failSolve alpha for contrast
Find a translucent foreground that reaches the target ratio, then verify the representation you will store or send.
Serialized result
Translucent text must be measured after compositing.
#0000008a4.61:1AA passTarget above the threshold, then verify the bytes
Solving for 4.6:1 returns alpha 0.541176470588. Eight-bit alpha becomes #0000008a, whose composited ratio is 4.607518093747374:1.
Solve, serialize, parse, and measure again
The solver searches continuous alpha. The hexadecimal handoff quantizes that value, so the final check belongs after serialization.
<?php
use PhpColor\Color\Color;
use PhpColor\Color\Contrast\ContrastSolver;
$foreground = Color::parse('#000000');
$background = Color::parse('#ffffff');
$alpha = ContrastSolver::requiredAlpha(
$foreground,
$background,
4.6,
);
$hex = $foreground->withAlpha($alpha)->toHex(true);
$stored = Color::parse($hex);
$ratio = ContrastSolver::compositedRatio($stored, $background);
// #0000008a
// 4.607518093747374Opaque failure means alpha cannot solve the pair
When the fully opaque foreground remains below the target, requiredAlpha() returns 1.0. The return value does not distinguish that unsolved case, so verify the resulting ratio.
InputForeground, background, and target ratio.
Search24 binary-search iterations.
OutputAn alpha value from 0 through 1.
RequiredVerification after the final representation.