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This is a bug possibly connected to an unfixed 6-years old bug regarding the value for 50% gray in 16-bit mode in some Photoshop dialog boxes, see: https://feedback.photoshop.com/photoshop_family/topics/photoshop_uses_the_wrong_50_gray_as_the_neutr...
The problem:
Using Apply Image (Blending: Add, Opacity: 100%, Scale: 1, Offset: -128) on two 50% gray (RGB values of: 16384) layers in 16-bit mode produces non-50% gray layer.
Steps to reproduce (with correct 50% gray value of 16384):
Steps to reproduce (with incorrect 50% gray value 16448):
Possible cause:
Apply Images uses an incorrect way for converting -128 (8-bit) to a 16-bit value.
Is: convert_8bit_to_16_bit(128) -> 16448,
Should be: convert_8bit_to_16_bit(128) -> 16384.
Workaround (with some losses):
We want to calculate: for layers A and B create the layer C with pixel values of:
"50% gray" + ((A - "50% gray") + (B - "50% gray")) (clipped at 100% black and white)
1) For layers A and B, shift the correct 50% gray point (16384) to the incorrect one (16448) using the Levels adjustment (NOT the Adjustment layer!) with "input white = 254". We are loosing some data.RackMultipart20181028214096742-40be9124-73af-4e73-9519-3fbc8920d7a6-852546474.png
2) Use the Apply Image as above.
3) Convert the result to the correct 50% gray point (16384) using the Levels Adjustment Layer, with "output white = 254".RackMultipart201810281175713xq-0f513546-e53e-4bc2-9ab1-ee968eaa7ce6-1299189073.png
We fall one short of all values near 50% gray (for example we get 16383 instead of 16384 for 50% gray), so we have to add "+1" to all pixel values in 16-bit mode. Tricky. Create a Fill Layer, Mode: Linear Dodge (Add), Opacity: 1%, Fill: 50%, Color #010101.RackMultipart20181028876pdcqja-1c80c5af-7f10-4c1b-be06-bc0f54dbe7ce-5028913.png
Select layer A, Merge (Ctrl+E). Delete layer B.