visual-perception on S Anand

Colour spaces

Mon, 27 Aug 2012 14:04:23 +0000

In reality, a colour is a combination of light waves with frequencies between 400-700THz, just like sound is a combination of sound waves with frequencies from 20-20000Hz. Just like mixing various pure notes produces a new sound, mixing various pure colours (like from a rainbow) produces new colours (like white, which isn’t on the rainbow.)

Our eyes aren’t like our ears, though. They have 3 sensors that are triggered differently by different frequencies. The sensors roughly peak around red, green and blue. Roughly.

It turns out that it’s possible to recreate most (not all) colours using a combination of just red, green and blue by mimicking these three sensors to the right level. That’s why TVs and monitors have red, blue and green cells, and we represent colours using hex triplets for RRGGBB – like #00ff00 (green).

There are a number of problems with this from a computational perspective. Conceptually, we think of (R, G, B) as a 3-dimensional cube. That’d mean that 100% red is about as bright as 100% green or blue. Unfortunately, green is a lot brighter than red, which is a lot brighter than blue. Our 3 sensors are not equally sensitive.

You’d also think that a colour that’s numerically mid-way between 2 colours should appear to be mid-way. Far from it.

This means that if you’re picking colours using the RGB model, you’re using something very far from the intuitive human way of perceiving colours.

Which is all very nice, but I’m usually in a rush. So what do I do?

I go to the Microsoft Office colour themes and use a colour picker to pick one. (I extracted them to make life easier.) These are generally good on the eye.
Failing that, I pick something from http://kuler.adobe.com/
Or I go to http://colorbrewer2.org/ and pick a set of colours
If I absolutely have to do things programmatically, I use the HCL colour scheme. The good part is it’s perceptually uniform. The bad part is: not every interpolation is a valid colour.

Comments

isomorphismes 13 Mar 2013 5:20 pm: What colours can’t be created by convex combinations of {R, G, B}? I mean within the visible spectrum.

Change blindness

Tue, 02 Jan 2007 12:00:00 +0000

A cool psychology experiment. A student asks someone for directions. People carrying a door pass between them. Students switch. They check if the person giving directions has noticed that it’s a different person. Many people don’t notice the switch.

Now your tongue can see

Sat, 06 May 2006 12:00:00 +0000

Now your tongue can see. Clive Thomson suggests that this could be a good interface for video games too.

Comments

rajkumarc 6 May 2006 12:00 pm: 10/10

How to tie a bandage

Sun, 23 Apr 2006 12:00:00 +0000

How to tie a bandage without it falling off. It’s in Japanese, but the video is pretty clear.

Neuroscience for kids

Fri, 27 Sep 2002 12:00:00 +0000

I found this site on neuroscience (for kids). Nice. Has lots of brain games (no – not smart ones – more like testing your reaction time, etc.) Actually I was reading up on neuroscience because of this article on how neuroscience helped figure out the meaning behind a Zen garden. via missing matter

Do not look at the needle

Wed, 11 Sep 2002 12:00:00 +0000

I always knew it. If you’re taking an injection, don’t look at the needle.

Pretty faces help you think better

Mon, 12 Nov 2001 12:00:00 +0000

Pretty faces help you think better.

Super vision

Sun, 25 Feb 2001 12:00:00 +0000

It’s possible to have better than 20/20 vision: SuperVision. Apparantly, even 20/2.5 is possible.