Incense

by Bart Arondson

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Not sure if this is the right forum to ask this.

Here is what I'm looking for:

Example:

  • I have a color photo
    • could be text
    • could be an image

What I would like:

  • if someone photographs this image, the image would only display as black and white, not color

Not sure if this matters but my main focus is digital cameras, like on mobile phones

Thanks for any help with this

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1  
This isn't possible; see my answer. Let me turn this around a little bit, though: what are you trying to accomplish here, and why do you think it would be possible? –  mattdm Apr 16 at 16:53
    
There was a protection scheme against Xeroxing, but that used the fact that the active light in a Xerox machine contains UV. Moreover, that was using two colors only, and looked very weird... –  TFuto Apr 16 at 16:56

1 Answer 1

up vote 8 down vote accepted

This not a thing that can happen. Cameras just don't work that way. For that matter, light doesn't work that way.

Specifically, for digital cameras: every "photosite" — each individual pixel-level sensor — is just a counter of photons. It doesn't have any sense of the wavelength of the light received, and correspondingly no perception of color. In order to make a color image from this, the photosites are covered in a pattern of red, green, and blue filters, allowing only certain wavelengths to be measured by certain sensors. This data is then constructed into a color image. (Some sensors are a little bit different, but this is the vast majority, and the same principles apply.)

In order for this image to be black and white only, these sensors would need to record an exactly equal amount in each channel (red, green, blue). But the only way to do that is for there to... actually be that. I'm oversimplifying a bit (white balance, device color spaces, tone curves, and so on make this not completely true), but no matter what, you can't force a certain false color (grayscale or otherwise).

The perception of color in cameras is actually reasonably analogous to the way our eyes perceive color — and not by accident: color itself is a construct of the way our eyes work and not something inherent in nature; consider that there is no wavelength of light that corresponds to "purple"). Because of this, it's basically the case that if it looks a certain way to the eye, a camera can get reasonably close. The match isn't perfect, and as photographers we spend a lot of time struggling to overcome that gap and get the color rendition to match what we think it should be based on our perception — but that's entirely a matter of taking an image which contains data that can interpreted as many different colors — it's impossible to force an image to somehow pick up exactly what you want it to be, and doubly impossible when you want it to be something different than what it actually is.

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Thanks for the great technical explanation, exactly what I was looking for. One follow up question if you don't mind. This image is an optical illusion: i0.wp.com/listverse.com/wp-content/uploads/2007/09/… would the camera see this as the illusion as well, to clarify if I made a picture to look black and white but upon closer examination it was actually color could I trick the camera ( except at close range ) to take a black and white photo? Sorry I'm talking around the actual problem, it's an idea I had to hide data in plain sight –  Phill Pafford Apr 16 at 17:28
    
@PhillPafford Optical illusions work in the brain, not in the eye. The camera wouldn't "sense" any false motion in the example you give, but if you looked at the resulting image (presuming an accurate enough reproduction) the effect would still be there. You just can't hide data in plain site in this way. You can hide information in images — it's called Steganography — but it doesn't work like that. –  mattdm Apr 16 at 22:37
    
@mattdm: Many optical illusions happen in the eye, as some fine local signal processing/contrast detection happens there. E.g. Mach bands. I quote: "The Mach bands effect is due to the spatial high-boost filtering performed by the human visual system on the luminance channel of the image captured by the retina. This filtering is largely performed in the retina itself, by lateral inhibition among its neurons." –  TFuto Apr 17 at 15:27
    
@TFuto I'm oversimplifying. Even in the quote you give, neurons are involved -- it's processing, not optics. –  mattdm Apr 17 at 16:09
    
@mattdm: I just addressed the "in the brain, not in the eye" sentence of yours. It is the Ganglion cell that does lateral inhibition, and those are part of the retina. And by the way, the entire human vision is mostly "processing", and very little physical optics. :-) Anyway... –  TFuto Apr 17 at 17:22

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