MATLAB: how to draw a great heatmap?

Drawing heat maps, which represent matrix data using colors, in MATLAB is fairy simple, but there is a bit of confusion about it. It’s mainly because it’s easier to find quiet unintuitive Bioinformatics Toolbox’s HeatMap object than the standard way of doing it which is imagesc(matrix) supported with colormap(scheme).

Rainbow generated using imagesc() function

If you only need code snippet, here it is:

For the list of all colormaps available by default, see MathWorks’ documentation on colormap. In this article below, I’ll try to explain which one I would choose in most cases and why.

Why default colormap is not good?

Some time ago I was working with thermal images of a palm. Data was actually the temperature, so heatmaps seem to be the best way to represent it 😉 Using image and imagesc I made pretty images that appeared good on my screen, but then following issues arose:

  • How would it look printed in black & white and on my Kindle?
  • How would it look to those 7-8% males color vision deficiency?
  • How will it look on this 10-years-old projector where I need to present my results?
  • How will it look to my dog (kidding, but it’s an interesting question too)

Well, first three are quite easy to check. Few standard colormaps converted to grayscale are displayed in figure 1, below. For simulation of how users with various color vision deficiencies, see this interesting diagram by Vlad Atanasiu (I couldn’t confirm it, but I believe it has some solid foundations). Projector didn’t make much problems in my case, but it’s woth to check if it has enough contrast to see all important color differences.

Colormaps: original and converted to grayscale
Colormaps: original and converted to grayscale. From the top: ‘default’, ‘hot’, and ‘gray’

As we can see here, the ‘default’ (or ‘jet’) colormap, in grayscale turns differently than we might expect. The highest values are not the brightest, so the information on the picture will just look wrong!

To scale or not to scale?

It might be a cliché, but this might be the good place to bring up the difference between image and imagesc.

if drawing 2D array (MxN), by default the data will be directly mapped to colormap colors (with values 1…64), or the closest color available in the colormap
if drawing 2D array (MxN), data will be first scaled to the full colormap range and just then drawn

So the codes image(1:256) and imagesc(1:256) will render different results, as shown in figure 2.

MATLAB: image vs imagesc
Vector [1:256] drawn using image (top) and imagesc (bottom) functions in MATLAB

Be aware that when you scale the colormap to image, images will not be easily comparable for people, and sometimes might be misleading.

Which color is warmer?

Going deeper into «which colormap is best» we need to make the decision whether some color A is warmer than color B. And it is actually not so simple. I was hoping that reading about color theory would give me a clue, but now I think such strict definition may not be possible.

Generally, in color theory warm colors are those associated with daylight and sunset or fire, while cool colors have their peak somewhere blue. This is the way the term warm color and cool color are used in paintings etc. Also, not only hue makes color cold or warm, but also its saturation or intensity. And contrast, as people really perceive color differences than colors itself.

We also have a physical measure color temperature. The problem is that the higher temperatures are blue there, while lower go towards red hues, so it’s just not how we intuitively perceive what is warmer.

The colors are not a simple thing and I’m not an expert, so I’ll just end this section with conclusion that the question «Which color is warmer?» is not an easy one.

Lessons learned

Below there are two heatmaps i generated for my thermal hand recognition project. The default colormap (on the right) seems to look good on the screen, but the one on the left actually represents the data better.

Thermal hand images using two colormaps: hot (left image) and jet (default, right image)
Thermal hand images using two colormaps: hot (left image) and jet (default, right image)

So now here is the conclusion. If you draw a heatmap with a plain range of values, think whether the 'gray' colorscale woudn’t be enough. It will work fine no matter if it’s on the screen, paper, or e-ink display, and should be read without problems by people with some form of color impairment. If you want color, go for 'hot' colormap rather than using default ‘jet’. ‘Hot’ isn’t actually worse than grayscale. Be careful when scaling colormaps. And, what is most important, make sure the figure presents the data in a way that is easy to understand and is well described 😉

(You would probably also not want this comic to be about you)