While I was playing with nodes I wanted to check which gray-scale algorithm blender use. And i came up with this node system: enter image description here But I got this strange result. Where did i make mistake?

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    $\begingroup$ Why is this a strange result? What would be the correct output? $\endgroup$ – Cebbi Jan 13 '17 at 15:34
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    $\begingroup$ Are you expecting the left half to be grey as well? If so, add the outputs from the Multiply nodes together (you'll need two Math nodes set to Add), rather than recombining them. With the setup you have now, you're just remapping the input colours to somewhat different out put colours. $\endgroup$ – user27640 Jan 13 '17 at 15:48
  • $\begingroup$ Wow, @DuaneDibbley, you are actually right! Thank you! I wish I could mark your message as answer! $\endgroup$ – Ihor Salnikov Jan 16 '17 at 11:38
  • $\begingroup$ I think you should accept @troy_s's answer, because it tells you the same thing as my comment, and adds a little to it as well. $\endgroup$ – user27640 Jan 16 '17 at 15:28

The luminance weights are intended to be summed. That is, for any given colourspace, multiply each light by its luminance position, and sum the result with the other two. You will end up with equal values across all three channels.

For REC.709 lights, which are the lights that define the sRGB specification, the formula would be:

Y = 0.2126*R + 0.7152*G + 0.0722*B

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