使用CIAdditionCompositing叠加纯黑图像为何结果不符合预期？

Great question—this is a classic gotcha with Core Image tied to color space handling, so let's unpack what's happening here.

The Root Cause: Linear vs. Non-Linear RGB #

Core Image runs almost all its operations, including CIAdditionCompositing, in a linear RGB color space. The catch is that most standard image formats (like sRGB) use a non-linear curve to encode brightness, since human vision doesn't perceive light linearly.

Here's the step-by-step breakdown of what likely went wrong for you:

• When you created your mid-gray image, you probably used a value like 0.5 thinking it's sRGB (which maps to an 8-bit pixel of 128, the "middle" gray we visually recognize).
• But Core Image automatically converts that sRGB value to linear RGB for processing: 0.5 (sRGB) translates to roughly 0.218 in linear RGB.
• CIAdditionCompositing adds the linear values: 0.218 + 0 = 0.218 (still linear RGB).
• The problem hits when you view the result: if your pipeline doesn't convert the linear value back to sRGB, or if you accidentally defined your gray as linear RGB to begin with, the output looks brighter. For example:
  • If you defined your gray as 0.5 in linear RGB (not sRGB), that's equivalent to ~0.729 in sRGB (≈ 186 in 8-bit)—way brighter than the mid-gray you expected. Adding black doesn't change that linear value, so the output stays bright.

How to Fix This #

To get the exact mid-gray you're expecting, make sure your input images are tied to the sRGB color space explicitly:

• When creating colored images in code, specify sRGB instead of relying on defaults:

  let sRGBSpace = CGColorSpace(name: CGColorSpace.sRGB)!
  let grayColor = CIColor(color: CGColor(gray: 0.5, alpha: 1.0, colorSpace: sRGBSpace))
  let grayImage = CIImage(color: grayColor)
  

• After compositing, ensure the output image is converted back to sRGB before displaying or exporting it.

Quick Test to Confirm #

Check the raw pixel values of your composite image:

• If it's the correct mid-gray, you'll see an 8-bit value of 128.
• If it's brighter, you'll see a value around 186—confirming your input gray was treated as linear RGB instead of sRGB.

内容的提问来源于stack exchange，提问作者stkent