How to create heatmap illustraing mesh differences controlling the position of center color for divergence color palette?

Question

I have two 3D meshes of human faces and I wish to use heatmap to illustrate differences. I want to use red-blue divergent color scale.

My data can be found here. In my

Answer 1

There are a couple of things to fix to get what you want.

First, the colours. You base the colours on this code:

color <- colorRampPalette(brewer.pal(n = 9, name = "RdBu"))(99)

You can look at the result of that calculation, and you'll see that there is no white in it. The middle color is color[50] which evaluates to "#F7F6F6", i.e. a slightly reddish light gray colour. If you look at the original RdBu palette, the middle colour was "#F7F7F7", so this change was done by colorRampPalette(). To me it looks like a minor bug in that function: it truncates the colour values instead of rounding them, so the values

[50,] 247.00000 247.00000 247.00000

convert to "#F7F6F6", i.e. red 247, green 246, blue 246. You can avoid this by choosing some other number of colours in your palette. I see "F7F7F7" as the middle colour with both 97 and 101 colours. But being off by one probably doesn't matter much, so I wouldn't worry about this.

The second problem is your discretization of the range of the pd values. You want zero in the middle bin. If you want the bins all to be of equal size, then it needs to be symmetric: so instead of running from min(pd) to max(pd), you could use this calculation:

limit <- max(abs(pd))
breaks <- -limit + (0:nlevel)*2*limit/nlevel

This will put zero exactly in the middle of the middle bin, but some of the bins at one end or the other might not be used. If you don't care if the bins are of equal size, you could get just as many negatives as positives by dividing them up separately. I like the above solution better.

Edited to add: For the first problem, a better solution is to use

color <- hcl.colors(99, "RdBu")

with the new function in R 3.6.0. This does give a light gray as the middle color.