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//******************* Color Spaces - February 11th, 2019 ********************//
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- I might have finished a project...early? What world have I trespassed into?
- Apparently 5 minutes ago, Professor Turk has also decided that project 2A will be due this Thursday (St. Valentine's National Singles Remembrance Day), and part 2B will be due next week on Feb. 23rd
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- We wrapped up last week by talking about the cones in the human eye, and how we perceive colors. A few important takeaways:
- There are significantly less blue cone receptors, which means we don't focus on it as well as green/red; this is why many people recommend avoiding blue text
- There IS some overlap between each of these cones frequency
- We also started talking about the "CIE" chromaticity diagram that shows all the colors visible to the human eye
- This is actually a 3D diagram, where 2D slices of it are shown at a time; the X-axis is, the Y-axis is , and the Z-axis as brightness
- Everything on the curved portion of the diagram are "visible chromaticity values," meaning they correspond to visible wavelengths - i.e., we can find a photon for that color!
- On the flat part, though, are colors we see that don't match up with an exact wavelengths (mostly shades of magenta, i.e. mixes of purple/red) - we only see them as mixes of different colors of light! There are NO photons with this color, we just perceive mixes of light as such!
- These are known as NON-SPECTRAL colors (because they're SPOOOOOOOOKYYYYYYYY...and, well, not part of the visible spectrum)
- Another thing to note for this diagram is the notion of COMPLEMENTARY COLORS, i.e. colors that are opposite from each other on the diagram (blue-yellow, green-magenta, etc.)
- For a given display device, we say the GAMUT of the device is the range of colors that it can display
- But can't my smartphone display ANY color? NO! Most devices can only display a medium-ish-sized triangle of color inside the CIE diagram - there are vivid colors that your monitor simply can't display
- Why a triangle? Because most displays are just RGB, with only 3 colors - we could add more types of colors to add more corners, but there's still (at least for right now) limitations on how extreme the colors can get
- What about making the corners encompass the whole CIE diagram? Well, to get the triangle's corners that far out, you'd need a monitor that could produce X-rays, which are expensive (and mildly dangerous)
- How do we display colors that are outside our device's range, then? There are 2 main schools of thought:
- PROJECTING the color to the nearest point on the triangle
- CLIPPING any of the color's extreme R/G/B values to the maximum possible with the monitor
- So, most monitors have the RGB color space with ADDITIVE colors, where we produce light of a given wavelength for the color we want
- You can think of the colors we make as a 3-circle Venn diagram, with a circle each for red, green, and blue
- Red + Green = Yellow
- Blue + Green = Cyan
- Red + Blue = Magenta
- Magenta photons do NOT exist; you won't find them on the color spectrum by themselves
- Red + Green + Blue = White light
- Similarly, there is no such thing as a pure-white photon
- You might also think of this scheme as an RGB "color cube" with RGB coordinates from (0,0,0) to (1,1,1), with red, blue, and green all in opposite corners, yellow/cyan/magenta at the in-between corners, black at (0,0,0) and white at (1,1,1), and the mixes of colors all in-between
- This'd mean that gray is in the very middle of the cube
- In a computer, of course, we represent these colors as discrete values between 0 and 255, instead of a continuous [0, 1] range, since it's convenient to represent each color channel with a single byte
- This is starting to change, however - most digital cameras nowadays have more than 8 bits of colors, and high-end monitors have higher color ranges as well
- An alternative to this is SUBTRACTIVE color methods, like with printers and inks
- Excepting really weird cases, paints and inks don't produce photons - they just absorb particular wavelengths of light, and reflect the remaining ones
- In this scheme, the primary colors change to CYAN, MAGENTA, and YELLOW
- "But Greg, what about red, blue, and yellow? What happened to them? Well, while that was still a subtractive color scheme, your teachers didn't want to confuse you by teaching about these weird cyan colors!"
- Here, in the "CMY" color space, the colors are:
- Cyan + Magenta = Blue
- Cyan by itself absorbs red light, and magenta by itself absorbs green - so when we mix them, only blue remains unabsorbed!
- Magenta + Yellow = Red
- Cyan + Yellow = Green
- Cyan + Magenta + Yellow = Black
- ...and if all colors are left out, you get white, since none of the colors are absorbed!
- In the "real world," black is often included as its own pigment because it's so common, and because mixing cyan/magenta/yellow perfectly into black is more difficult to get right than just having it by itself. This is known as the "CMYK" color space ("K" instead of "B" so we don't confuse black and blue)
- We can think of this sort of color scheme as subtracting RGB colors from white light, like so:
|1| |R|
|1| - |G| = final color
|1| |B|
- So, we talked about non-spectral colors already, like magenta and white - but for some colors, there are more than one way to get them
- These colors are known as METAMERS: "two spectral distributions that look the same to humans"
- Cyan light, for instance, can either be gotten directly as a photon w/ a wavelength between blue and green, OR we can get it by having blue AND green light of the same intensity together!
- Colors we can get just on the spectrum directly, without having to mix two different colors of light, are known as "spectrally pure"
- Colors like magenta, on the other hand, can only be gotten by mixing two colors together - the fact we can see them at all is down mostly to human biology interpolating the colors
- On Wednesday, we'll start talking about some other color spaces - not for human perception, or for display devices, but for it to be easier for human designers to get the color mix they want. It's hard to tell what makes off-pink color in RGB, for instance - but a clever color scale can help us out!
- So, that's what's on the menu - see you for dinner (in the most platonic of senses) on Wednesday!