Is a smartphone camera’s “mono” mode just grayscale, and how is it derived from RGB?
Asked 6/26/2013
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2 answers
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My smartphone camera has a “mono” setting described as black and white, but the result looks more like grayscale than strictly black-and-white. Is that the correct way to think about it?
If so, how is the grayscale value typically calculated from the original RGB data? Is there a practical way to test what conversion the phone is using?
Originally by user20509. Source · Licensed CC BY-SA 4.0
user20509
13y ago
2 Answers
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Monochrome and black and white and grayscale are used randomly these days for the same grayscale thing. Even though strictly speaking Black and white is 2-shade images, monochrome can be any single colour (like old monitors in yellowish), and grayscale is the real term people are looking for.
The computation from the bayer pattern is usually using the Y component from the Y'CbCr compressed image. It can be different in your model but I doubt it. It is a weighted sum of the R,G,B values based on human sensitivities. It is how yellow gets brighter than cyan and magenta (pure mean of R,G and B will result in a dark gray for yellow).
Y = 0.299*R+0.587*G+0.114*B
Originally by user11455. Source · Licensed CC BY-SA 4.0
user11455
13y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—on phones, “mono,” “black and white,” and “grayscale” are often used loosely to mean the same thing: a monochrome image with shades of gray rather than only pure black and pure white.
Strictly speaking, true black-and-white would mean only two tones, while grayscale contains many gray levels. “Monochrome” technically means one color only, which could be gray or another tint, but in camera apps it usually means grayscale.
A common way to create it from color is to use a luminance-style weighted sum of RGB values, often the Y component from Y'CbCr:
Y = 0.299R + 0.587G + 0.114B
This weighting reflects human vision being most sensitive to green, then red, then blue. That’s why colors with the same average RGB value may convert to different gray brightnesses.
There isn’t one guaranteed formula for every phone model, but this is a very common approach. To test it, photograph known color patches, compare their grayscale brightness, and see whether the results match a weighted luminance conversion more closely than a simple average of R, G, and B.
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