Science Forums 2

[CraigD]

Originally Posted by Popular

As obvious as it might seem, A and B are the same colour …

Originally Posted by cwes99_03

… neither one of us being able to tell which is clinging and which is not (except by the means I put forth of actually picking apart the picture with a photometer or a digital editing program and measuring the hue saturation and other color levels in the picture)…

As hard to believe from the evidence of our eyes, what the picture’s author (Adelson) and Popular claim is true – the actual color value being sent to your screen is the same for pixels in square A and B is the same.

An easy way to verify this (I just did, since I too couldn’t believe the claim over what my own eyes were telling me) on a Windows machine is:

1. View the picture in you browser (upper left hand corner of you screen will save hassle in later steps)
2. Hold down the Ctrl key and press the PrtSc key to copy the current screen bitmap to the clipboard
3. Run mspaint.exe (or select “Paint” in the start menus).
4. Hold down Shift and press Ins (or click Edit, Paste) to copy the clipboard to the new drawing.
5. Click to select the “Select” tool (looks like a dotted-line box)
6. Click and drag over a portion of the A square in the picture
7. Hold down Ctrl and press Ins (or click Edit, Copy) to copy the selected region to the clipboard
8. Hold down Shift and press Ins (or click Edit, Paste) to copy the clipboard to the new drawing.
9. Drag the pasted region to an unused part of the drawing
10. Repeat steps 6-9 for a portion of the B square, placing the 2 copied regions close together.

You’ll be able to clearly see that A and B are the same color.

Unless you believe that mspaint.exe has hidden software to transform the colors copied via the select tool, this exercise is compelling proof of Popular and Adelson’s claim.

Color perception illusion are an essential, necessary feature of color computer displays, print, painting, etc. Nearly all TVs, computer display, for example, actually emit mostly (or, in the case of laser screens, exactly) photons of 3 wavelengths: 0.000000625 to 0.000000760 meter red, 0.000000520 to 0.000000570 m green, and 0.000000440 to 0.000000490 m blue. Our impression that we are actually seeing photons of many different frequencies is a perceptual illusion. Animals with eyes built radically different from our own might not perceive this illusion correctly, and be quite unable to use our color TVs and computer displays.

If you really want to bathe your retinas in photons of a specific color, you’ll need to use something like the refracted light of a prism. Your retina and brain, however, can’t tell most of these photons from mixtures of the primary RGB colors, so there’s really no advantage of “pure” of “mixed” photons for displaying colors.