如何从黑白背景下视图的RGB值获取单一RGB与透明度值——iOS键盘扩展明暗模式适配

Got it, let’s work through this problem together. You need one semi-transparent gray color that looks exactly like RGB(150,150,150) on a white (light mode) background and RGB(107,107,107) on a black (dark mode) background. Here’s how to calculate it properly:

Step 1: Recall the Alpha Blending Formula #

iOS uses standard alpha blending for colors. For grayscale (all RGB channels identical), the resulting color on a background follows this rule for each channel:

resulting_channel = (foreground_channel * opacity) + (background_channel * (1 - opacity))

Where:

• opacity is the foreground color’s opacity (0 = fully transparent, 1 = fully opaque)
• foreground_channel is the RGB value of your semi-transparent color (we’ll call this X since all channels are the same)
• background_channel is 255 for white (light mode) and 0 for black (dark mode)

Step 2: Set Up Equations for Both Backgrounds #

We know our target results, so let’s write two equations:

1. Light mode (white background): We need the result to be 150

   X * opacity + 255 * (1 - opacity) = 150
   

2. Dark mode (black background): We need the result to be 107

   X * opacity + 0 * (1 - opacity) = 107
   

Step 3: Solve for Opacity and RGB Value #

From the dark mode equation, we get X * opacity = 107 (let’s call this our baseline equation).

Substitute this into the light mode equation:

107 + 255*(1 - opacity) = 150

Rearrange to solve for opacity:

255*(1 - opacity) = 150 - 107 = 43
1 - opacity = 43 / 255 ≈ 0.1686
opacity ≈ 0.8314 (or ~83% opacity)

Now plug this opacity back into the baseline equation to find X:

X = 107 / 0.8314 ≈ 128.7

Round to the nearest integer: X = 129

Step 4: Final Color Values #

Your single semi-transparent color is:

• RGB: (129, 129, 129)
• Opacity: ~0.83 (use the decimal value 0.83 in your code)

Step 5: Double-Check the Results #

• On white background: 129*0.83 + 255*(1-0.83) ≈ 107.07 + 43.35 ≈ 150.42 (nearly identical to your target 150)
• On black background: 129*0.83 ≈ 107.07 (almost exactly your target 107)

This color will perfectly match the iOS keyboard key appearance in both light and dark modes with a single set of values.

内容的提问来源于stack exchange，提问作者ibayramli