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dmstraker

Identifying/masking burned pixels

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A way to identify pixels which are burned out (white), which can be handy for example in creating mask to clean them up:

 

  • Duplicate layer (Ctrl-J)
  • Invert (Ctrl-I)
  • Blend Mode: Colour Burn

Any burned pixels will appear white. Everything else will be black.

 

This works because (across RGB channels):

  • The blend calculation formula is: Result = 1 - (1 - Base)/Blend
  • For white in bottom layer, Base = 1. For black (inverted white) in top layer, Blend = 0.
  • In this case, Result = 1 - (1 - 1)/0 = 1 (white)
  • For all other colours, say 0.4, Result = 1 - (1 - 0.4)/0.6 = 1 - 1 = 0 (black)

To see what is in the image below, get a white brush and hover over any area - it acts like a torch.


Dave Straker

Cameras: Sony A7R2, RX100V

Computers: Win10: Chillblast Photo with i7-3770 + 16Gb RAM + Philips 40in 4K; Surface Pro 4 i5

Favourite word: Aha. For me and for others.

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Having said all this, in another test I got some coloured pixels - red, yellow, magenta, green, blue.

 

So I did a further test - Duplicate layer. Blend Ranges (gear icon) Pull left node on left graph to bottom right corner. Add empty layer below, filled with black. This also selects white. Comparing the two methods, the coloured pixels are black on the second method.

 

Then a lightbulb went on somewhere. The pixels which appear non-white are saturated in at least one RGB channel but not all of them. So partial burn. I guess that's useful to know too, but as masks are not per channel (without fudging) not so helpful in this.


Dave Straker

Cameras: Sony A7R2, RX100V

Computers: Win10: Chillblast Photo with i7-3770 + 16Gb RAM + Philips 40in 4K; Surface Pro 4 i5

Favourite word: Aha. For me and for others.

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