SUMMARY
The Grade node mix luminance slider is not a linear mix. Here is an explanation of the mathematics behind the mix luminance slider in the Grade node in Nuke.
MORE INFORMATION
The mix_luminance calculation in Nuke's Grade node is achieved by first converting the RGB values to digitized YPbPr, using the conversion formulae detailed in ITU-R BT.709, which includes a non-linear gamma encoding. The mixed luminance is obtained with mix_luminance as the linear ratio between color-corrected and non-color-corrected luminance values, and then we finally convert back to RGB.
In the this example with RGB values of (0.5, 0.5, 0.5), the non-color-corrected luminance works out to be 0.694, and the color-corrected luminance works out to be 0.524. The blended luminance is then:
0.5 * 0.694 + (1 - 0.5) * 0.524 = 0.609.
Converting this back to RGB values then gives approximately (0.362, 0.362, 0.362).
The the example script compares the Grade node with a straight dissolve of two Constants, where the blending is being done on the RGB values, rather than against the luminance. This is why the results are different, since the luminance conversion is non-linear.
If you paste this example script into Nuke, it will help to illustrate the difference in values.
set cut_paste_input [stack 0]
Constant {
inputs 0
channels rgb
color 0.25
name Constant3
selected true
xpos -37
ypos -54
}
Constant {
inputs 0
channels rgb
color 0.5
name Constant2
selected true
xpos -128
ypos -52
}
Dissolve {
inputs 2
channels rgb
which 0.5
name Dissolve1
selected true
xpos -87
ypos 55
}
Constant {
inputs 0
channels rgb
color {0.5 0.5 0.5 0}
name Constant1
selected true
xpos -291
ypos -53
}
Grade {
white 0.5
mix_luminance 0.5
name Grade1
selected true
xpos -291
ypos 26
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