Skip to main content

Notes on Color #1: Misconceptions

In my childhood I learned about the three primary colors: red, yellow and blue. They are "primary" because any other color may be obtained by mixing these colors, on the other hand the primary colors cannot be obtained in this way.

Years later I learned about RGB and HSL for UI and digital painting. At that point I thought I've got a solid understanding of the colors, until now.

Having been learning color therories for a few weeks, I realized that how much I do not know about colors.

This is the first study notes on color theory, which lists a few misconceptions that I have had for years.


Color is a physical property of lights or objects. Right?

Right and wrong. When we say "the apple is red", we actually mean "under the standard daylight illuminant, the spectrum reflected by the apple is perceived similarly as other objects that are commonly known as red".

The light source and the spectral reflectance are apparently important, yet the observer is also a decisive factor. Most of us humans are trichromats, we have 3 types of color receptors. The color we "see" reflects the stimuli of the 3 color receptors. Other species may have fewer or more. Mantis shrimps have 12-16. Therefore a human and a mantis shrimp may not agree on the "color" of an apple. 

Further, some humans are dichromats or tetrachromats. Even trichromatic humans may preceive the colors differently. 

Therefore color is more psycophysical. A standard observer is commonly used to represent an average human's chromatic response.


The 3 types of human cone cells can detect red, green and blue lights independently. Right?

Wrong. The L and M cone cells cover almost the entire visible spectrum. With a weak stimui of a M cone cell, we cannot determine whether it comes from a weak green-ish light or a strong blue-ish light.

Human cone cell response curves.
Source: Wikipedia CC BY-SA 3.0

In fact, the L cone cell has maximal sensitivity around 560-580nm, which is not even "red".

CIE xy color diagram.
Source: Wikipedia CC BY-SA 3.0

Comparing with a pure white object, a 50%-grey object is 50% less effecient in reflecting lights. Right?


Wrong. Our color vision is not linear. The (reflected) luminance of a 50%-grey object is about 18% of pure/reference white.

Munsell Renotation and other curves.
Source: Wikipedia CC BY-SA 2.5




Labels:

Comments

Post a Comment

Popular posts from this blog

Determine Perspective Lines With Off-page Vanishing Point

In perspective drawing, a vanishing point represents a group of parallel lines, in other words, a direction. For any point on the paper, if we want a line towards the same direction (in the 3d space), we simply draw a line through it and the vanishing point. But sometimes the vanishing point is too far away, such that it is outside the paper/canvas. In this example, we have a point P and two perspective lines L1 and L2. The vanishing point VP is naturally the intersection of L1 and L2. The task is to draw a line through P and VP, without having VP on the paper. I am aware of a few traditional solutions: 1. Use extra pieces of paper such that we can extend L1 and L2 until we see VP. 2. Draw everything in a smaller scale, such that we can see both P and VP on the paper. Draw the line and scale everything back. 3. Draw a perspective grid using the Brewer Method. #1 and #2 might be quite practical. #3 may not guarantee a solution, unless we can measure distances/p...
3 comments
Read More »

Chasing an IO Phantom

My home server has been weird since months ago, it just becomes unresponsive occassionally. It is annoying but it happens only rarely, so normally I'd just wait or reboot it. But weeks ago I decided to get to the bottom of it. What's Wrong My system set up is: Root: SSD, LUKS + LVM + Ext4 Data: HDD, LUKS + ZFS 16GB RAM + 1GB swap Rootless dockerd The system may become unresponsive, when the IO on HDD  is persistantly high for a while. Also: Often kswapd0 has high CPU High IO on root fs (SSD) From dockerd and some containers RAM usage is high, swap usage is low It is very strange that IO on HDD can affect SSD. Note that when this happens, even stopping the IO on HDD does not always help. Usually restarting dockerd does not help, but rebooting helps. Investigation: Swap An obvious potential root cause is the swap. High CPU on kswapd0 usually means the free memory is low and the kernel is busy exchanging data between disk and swap. However, I tried the following steps, none of the...
Post a Comment
Read More »

Fix Google Security Code

Google Security Code (http://g.co/sc) is one type of 2-step verification. This is particularly useful when security keys and passkeys are not available. I have been using it in my LXC containers, until today I found out that it stopped working. It just kept saying "The code is invalid". It is easy to rule out some factors: The code works on other browsers on my laptop. The code works on other devices that are directly connected to the router. So it appears that Google also checks IP addresses besides the security code. Recently I have IPv6 enabled, so most devices that are directly connected to the router have both IPv4 and IPv6 addresses. But  I only enabled IPv4 for my LXC containers. So I guess when a code is generated by device A and used by device B, Google should be able to check that device A and device B are closely located. But in my case, IPv6 address appears on device A but not on device B, which may look suspicious. To fix the problem, I just needed to disable IPv...
Post a Comment
Read More »