What's the exact gamma correction Blender uses?

Question

Whenever talking about gamma correction the answer for the gamma value is always 2.2 or 2.3. I did some tests using standard view transform: Emission shader: 0.5 Render result with sRGB view transform: 0.7354 -> Gamma: 2.25530545664

Is my calculation correct and does somebody know why Blender uses such an arbitrary number?

Answer 1

As Nathan points out in the comments, using a Gamma node with the magic "2.2" value is only approximate because sRGB<->Linear conversions involve a power function.

As I needed a more exact conversion for my specific use case (I'm encoding bitmasks as colours into my vertexes as an optimisation for my game), I have taken the blender source that was linked (https://github.com/blender/blender/blob/594f47ecd2d5367ca936cf6fc6ec8168c2b360d0/source/blender/blenlib/intern/math_color.c) and converted it to blender nodes for your convenience.

Converting sRGB to Linear

Blender's implementation in C

float srgb_to_linearrgb(float c)
{
  if (c < 0.04045f) {
    return (c < 0.0f) ? 0.0f : c * (1.0f / 12.92f);
  }

  return powf((c + 0.055f) * (1.0f / 1.055f), 2.4f);
}

Material node equivalent

Converting Linear to sRGB

Blender's implementation in C

float linearrgb_to_srgb(float c)
{
  if (c < 0.0031308f) {
    return (c < 0.0f) ? 0.0f : c * 12.92f;
  }

  return 1.055f * powf(c, 1.0f / 2.4f) - 0.055f;
}

Material node equivalent

I've implemented both of these as node groups in my project. Here's a minimal use case of it in action, which will read my vertex colour's red channel and convert it to the range 0-255, which matches my setup in Unreal Engine:

Note that some of the numbers in the screenshots have been truncated in blender, if implementing again yourself, you may have to manually punch in 1/12.92/or whatever to get closer to my implementation's results.

Also note that I haven't tested these blender nodes extensively -- I am no expert with this software, but figured this would be a helpful starting point for artists to interpret the C equivalent for conversions.