Why is the Unity pixel color read from RenderTexture bigger than 1?

Question

The pixel color component read from the Unity renderTexture is bigger than 1.0 without HDR environment map.

I'm writing a mini program to test the reflectance of Unity PBS. I use six pure white (1,1,1,1) textures to construct a skybox as a GI source. Then I set a plane with Unity standard shader material and camera. In every frame, I change the material arguments (albedo, metallic, smoothness), and get the pixel color at the center of view.

The light source I use is not bigger than 1.0 (pure white skybox, not a HDR texture). So, why is the pixel color (as reflectance) is bigger than 1? I think it supposed to be less than 1.0. The biggest data I get is near to 5. Actually, if I use the color picker to check the color from screen, the color data is never bigger than 1.0.

Here's my code:

// code in OnPostRender()

Texture2D image = new Texture2D(2, 2, TextureFormat.RGBAFloat, false, true);
// only read the center four pixels
image.ReadPixels(new Rect(renderWidth/2, renderHeight/2, 2, 2), 0, 0);

Color a = new Color();
a = image.GetPixel(0, 0);
b = image.GetPixel(1, 0);
c = image.GetPixel(0, 1);
d = image.GetPixel(1, 1);
// average the four pixels to get the exact center color
return (a.r+b.r+c.r+d.r)/4.0f;
```

Answer 1

With GI, every side of the skybox acts as a separate light source. You have 6 lightsources with a strength of 1 each. One of them is behind the object. So measuring a brightness of up to 5.0 is expected.

Your monitor can't display more brightness than 1, so that's what the value you measure with the color picker tool is clamped at.