Category Archives: Color

The rainbow is dead…long live the rainbow! – The rainbow is dead…long live the rainbow! – Perceptual palettes, part 2: a rainbow puzzle

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ROYGBIV or YOGRVIB?

If you are interested in the topic of color palettes for scientific data, and the rainbow in particular, I would say you ought to read this 2007 IEEE visualization paper by Borland and Taylor: Rainbow Color Map (Still) Considered Harmful. It clearly and elegantly illustrates why the rainbow palette should be avoided when displaying scientific data. I like Figure 1 in the paper in particular. The illustration shows how it is easy to order perceptually a set of 4 paint chips of different gray intensity, but not at all easy to order 4 paint chips colored red, green, yellow, and blue. The author’s argument is that the rainbow colors are certainly ordered, from shorter to longer wavelengths, but they are not perceptually ordered. In this post I wanted to extend the chips example to all 7 colors in the rainbow and try to demonstrate the point in a quantitative way.

Here below is a 256-sample rainbow palette I created interpolating between the RGB values for the seven colors of the rainbow red, orange, yellow, green, blue, indigo, and violet (ROY G BIV):

On this palette I see a number of perceptual artifacts, the most notable ones being a sharp edge at the yellow and a flat zone at the green. The existence of these edges I tried to explain quantitatively in the first post of this series.

Now, to go back to the experiment, from the original RGB values for the non interpolated colors I created the 7 color chips below . Question: can you order them based on their perceived intensity?

I think if you have full color vision (more on the topic of rainbow and impaired color vision in the next section of this post) eventually you will be able to order them as I did.If not, try now below. In this new image I converted the color chips to gray chips using the values obtained in Matlab with this formula:

INT = (0.2989 * RGB(:,1) + 0.5870* RGB(:,2) + 0.1140 * RGB(:,3))';

Give it a try, then hover with your mouse over the image to read the intensity values.

Not surprisingly, the values are not in any particular order. This reinforces the notion that although the rainbow colors are ordered by increasing wavelength (or decreasing in this case) , they are not perceptually ordered. (See this comment to my previous post). Below I rearranged the gray chips by increasing intensity.

And now I reconverted from gray to RGB colors and adjusted the distance between each pair of chips so that it is proportional to the intensity difference between the chips in the pair (I actually had to artificially change the value for green and orange so they would not overlap). That was an epiphany for me. And the name is funny too, BIV R GOY, or YOG R VIB…

I said that it was an epiphany because I realize the implications of trying to create a palette by interpolating through these colors with those distances. So I did it, and I am showing it below in the top color palette. We jumped out of the frying pan, into the fire! We went from perceptual artifacts that are inherent to the rainbow (reproduced in reverse order from blue to red to facilitate comparison as the bottom palette) to interpolation artifacts in the intensity ordered rainbow. Hopeless!

ROYGBIV puzzle

As if what I have shown in the previous section wasn’t scary enough, I took 7 squares and colored them using the same RGB values for Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Then I used the Dichromacy plug-in in ImageJ to simulate how these colors would be seen by a viewer with Deuteranopia (the more common form of color vision deficiency). I then shuffled the squares in random order on a square canvas, and numbered them 1-7 in clockwise order.

Puzzle: can you pair the squares numbered 1 through 7 with the colors R though V? I will give away the obvious one, which is the yellow:

1=Y
2=?
3=?
4=?
5=?
6=?
7=?

Cannot do it? For the solution just hover over the image with your mouse. If you like the animation and would like to use it on your blog, twitter, Facebook, get the GIF file version here. Please be kind enough to link it back to this post.

Conclusion

When I tried myself I could not solve the puzzle, and that finally convinced me that trying to fix the rainbow was a hopeless cause. Even if we could, it would still confuse a good number of people (about 8% of male have one form or the other of color vision deficiency). From the next post on I will show what I got when I tried to create a better, more perceptual rainbow from scratch.

Related posts (MyCarta)

The rainbow is dead…long live the rainbow! – the full series

What is a colour space? reblogged from Colour Chat

Color Use Guidelines for Mapping and Visualization

A rainbow for everyone

Is Indigo really a colour of the rainbow?

Why is the hue circle circular at all?

A good divergent color palette for Matlab

Related topics (external)

Color in scientific visualization

The dangers of default disdain

Color tools

How to avoid equidistant HSV colors

Non-uniform gradient creator

Colormap tool

Color Oracle – color vision deficiency simulation – stand alone (Window, Mac and Linux)

Dichromacy –  color vision deficiency simulation – open source plugin for ImageJ

Vischeck – color vision deficiency simulation – plugin for ImageJ and Photoshop (Windows and Linux)

For teachers

NASA’s teaching resources for grades 6-9: What’s the Frequency, Roy G. Biv?

What is a colour space?

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Since I am deconstructing the rainbow/spectrum in RGB, HSV color spaces, I will show it in dichromat “color space”, and finally will then make a new one in CIE L*a*b* color space I thought I should include a review of the basics. So what is a color space? This good post answers that question.

Color Use Guidelines for Mapping and Visualization

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I find the Color Use Guidelines by Cynthia Brewer (of Color Brewer fame) very well done and extremely useful. Below is a screen captures of the main page. To see an explanation of and example for each color scheme visit the interactive guidelines.

The rainbow is dead…long live the rainbow! – Perceptual palettes, part 1

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Introduction

This is the first  post in a series on the rainbow and similar color palettes. My goal is to demonstrate it is not a good idea to use these palettes to display scientific data, and then answer these two questions: (1) is there anything we can do to “fix” the rainbow, and (2) if not, can we design a new one from scratch.

The rainbow is dead…some examples

In a previous post I showed a pseudo-3D rendering of my left hand x-ray using intensity (which is a measure of bone thickness) as the elevation. I mapped the rendering to both grayscale and rainbow color palettes, and here I reproduced the two images side by side:


I used this example to argue (briefly) that the rainbow obscures some details and confuses images by introducing artifacts. Notice that in this case it clearly reduces the effectiveness of the pseudo-3D rendering in general. It also introduces inversions in the perception of elevation. The thick part in the head of the radius bone, indicated by the arrow, looks like a depression, whereas it is clearly (and correctly) a high in the grayscale version.

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An iPhone and Android App to “correct” for Color Blindness

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Just posted a tweet yesterday on this great iPhone and Android App to correct for Color Vision Deficiency:

And here’s a screen capture:

colour maps

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The perfect lead into my series on perceptual color palettes. Great post!

The original article on the Guardian is here. And here is the conversation that lead to improved map, as put together on Storify.

I thought it’d be interesting to run a simulation of what the map would actually look lie to viewers with the 3 types of color deficient vision. Below are my results for the first map. It is obvious from this simulation that while the map is OK for Tritanope viewers, the green and red areas are very confusing for Protanope and Deuteranope viewers.

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A good divergent color palette for Matlab

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INTRODUCTION

Before starting my series on perceptual color palettes I thought it was worth mentioning an excellent function I found some time ago on the Matlab File Exchange. The function is called Light and Bartlein Color Maps. It was a Matlab Pick of the week, and it can be used to create four color palettes discussed in the EOS paper by Light and Bartlein. Each of these palettes is suited for a specific task, and the authors claim they are non confusing for viewers with color vision deficiencies.

In the remainder of this post I will showcase one of the palettes, called orange-white-purple, as it is good divergent scheme [1]. With the code below I am going to load the World Topography Matlab demo data, create  the palette and use it to display the data.

%% load World Topography Matlab demo
load topo;

%% create Light Bartlein orange-white-purple diverging scheme
LB=flipud(lbmap(256,'BrownBlue')); % flip it so blue is for negative(ocean)
                                   % and green for positive (land)

%% plot map
fig2 = figure;
imagesc(flipud(topo));
axis equal
axis tight
axis off
set(fig2,'Position',[720 400 980 580]);
title(' Non-symmetric divergent orange-white-purple palette','Color',...
    'k','FontSize',12,'FontWeight','demi');
colormap(LB);
colorbar;

And here is the result below. I like this color scheme better than many othera for divergent data. One only issue in the figure, although not inherently due to the palette itself [2], is that the centre of the palette is not at the zero. This is a problem since the zero is such an important element in ratio data, in this case representing sea level.

MAKING THE PALETTE SYMMETRIC AROUND THE ZERO

The problem fortunately can be easily fixed by clipping the data limit to a symmetric range. In Matlab this has to be done programmatically, and rather than going about it with trial and error I like to do it automatically with the code below:

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A rainbow for everyone

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Traffic lights for everyone

Stephen Westland of Colour chat recently posted about a clever new LED traffic light developed in Japan. Here’s my tweet with the link to Westland’s original blog post:

I really like the idea of making a traffic light that works for everyone: for people with full color vision and people with color vision deficiencies. In fact, I think we should do the same with our color palettes. Why do I say that?

A rainbow for everyone

Take a look at  Figure 1 below. This is a map of the Bouguer Gravity (terrain corrected Bouguer Gravity to be precise)  in Southern Tuscany, colored using a rainbow palette. I intentianally left out the colorbar. For a moment ignore the sharp gradient changes at the yellow and cyan color (that is one of the topics of my upcoming series “The rainbow is dead…long live the rainbow!”). Can you tell which color is representing high values an which low? If you have used a mnemonic like ROY B GIV and can tell that highs are towards the Southwest and lows towards the Northeast, then you are right and you also have full color vision, just like me. Great, because this post is exacly for us, the “normals”.

Figure 1

Take now a look at Figure 2:

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