# [R] joint histogram for image comparison

Paul Hiemstra paul.hiemstra at knmi.nl
Tue Sep 20 09:38:49 CEST 2011

``` On 09/19/2011 12:49 PM, aurelien wrote:
> Dear all,
> I need to compute a joint histogram to compare 2 images (with image()
> function). I wonder if there is any function available in R to do this
> properly?
> Aurrélien
>
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> and provide commented, minimal, self-contained, reproducible code.

Hi Aurrélien,

You need to be more specific as to what you want, a reproducible example
would be great. To get a joint (if you meanh by joint that you want to
show the next to each other, not joint in the join probability kind of
way) histogram, I would use ggplot:

library(ggplot2)
dat = data.frame(matrix(runif(10e3), 1000, 10)))

# Convert dat to a flat table
# looking at the result is most illustrative
dat_melt = melt(dat)

# Create the set of 10 histograms
# also works for two, facet_wrap
# the joint histograms
ggplot(aes(x = value), data = dat_melt) + geom_histogram() +
facet_wrap(~ variable)

# The following is purely cosmetic
# Changing some colors
ggplot(aes(x = value), data = dat_melt) + geom_histogram(fill = "white",
col = "black") + facet_wrap(~ variable)

# Change the theme a bit
theme_set(theme_bw())
ggplot(aes(x = value), data = dat_melt) + geom_histogram(fill =
"lightgrey", col = "black") + facet_wrap(~ variable)

hope this helps,
Paul

--
Paul Hiemstra, Ph.D.
Global Climate Division
Royal Netherlands Meteorological Institute (KNMI)
Wilhelminalaan 10 | 3732 GK | De Bilt | Kamer B 3.39
P.O. Box 201 | 3730 AE | De Bilt
tel: +31 30 2206 494

http://intamap.geo.uu.nl/~paul