Heatmap with background image on gnuplot

Question

I'm trying to plot a heatmap with the RSSI of a wireless network using gnuplot. Up to now, I partially achieved that. I got the following picture, using the following code:

Partial heatmap

set terminal pngcairo transparent enhanced font "arial,10" fontscale 1.0 size 1440, 900
set output "teste.png"
set size ratio -1
set xrange [ -0.5 : 3510.5 ]
set yrange [ -0.5 : 2480.5 ]
set noxtic
set noytic
r(x) = x >= 0.92 ? 0.0 / 7.0 : \
       x >= 0.86 ? 1.0 / 7.0 : \
       x >= 0.80 ? 2.0 / 7.0 : \
       x >= 0.74 ? 3.0 / 7.0 : \
       x >= 0.68 ? 4.0 / 7.0 : \
       x >= 0.62 ? 5.0 / 7.0 : \
       x >= 0.56 ? 6.0 / 7.0 : 7.0 / 7.0
g(x) = x >= 0.50 ? 1.0 : \
       x >= 0.44 ? 6.0 / 7.0 : \
       x >= 0.38 ? 5.0 / 7.0 : \
       x >= 0.32 ? 4.0 / 7.0 : \
       x >= 0.26 ? 3.0 / 7.0 : \
       x >= 0.20 ? 2.0 / 7.0 : \
       x >= 0.14 ? 1.0 / 7.0 : \
       x == 0.00 ? 1.0 : 0.0
b(x) = x == 0.0 ? 1.0 : 0.0
set cbrange [ -99 : -49 ]
set cblabel "Intensidade (dBm)"
set cbtics -95, 3, -50
set palette function r(gray), g(gray), b(gray)
set colorbox horizontal user origin 0.7, 0.94 size 0.2, 0.01
plot "dados.dat" matrix title "Wireless A" with image

The graph is right, but I wanted to insert the place's plant as a background image. At first, I thought of using the multiplot feature, drawing the background image first and, on top of it, the heatmap. The problem with this is that I couldn't find any documentation on how to make the colors of the heatmap transparent and, as a result, the heatmap covers the background image completely.

Am I on the right track? If yes, how can I make the colors on the heatmap transparent, so that I can see both the background image and the heatmap? Or is there a better approach to solve this problem?

Thanks a lot.

Thanks for the reply @Schorsch, that's similar to what I want to do, but the map is on a different datasource: on the example, the world is a sequence of dots which will be drawn as a line, while the source of my map is an image. I can't just overlay the image as the example because the image would cover the heatmap. I tried setting the white color as transparent before overlaying it, but this creates some artifacts on the edges of the lines (probably a result of some anti aliasing applied previously to the image I'm using as the map). — Luís Fernando Quitaiski
The only way to use transparency for heat maps is to use the plotting style with rgbalpha, but then you can't use a defined palette for the colors. But since you have defined your own functions for red, green and blue anyway, this is no problem. But that also means some fiddling with the color ranges and others to get the correct colorbox. To plot the background image first, you may use something like plot 'bg.png' binary filetype=png w rgbimage, "dados.dat" using 1:2:(rsc($3)):(gsc($3)):(bsc($4)):(127) with rgbalpha. rsc etc. is your function r(x) with an adequate scaling. — Christoph

Greg Minshall Greg Minshall · Accepted Answer · 2016-08-28T06:17:15

this might be of interest: http://gnuplot-surprising.blogspot.de/2011/09/gnuplot-background-image.html

it shows how to have gnuplot display a PNG. assuming one can put that image "behind" (in gnuplot's terminology), and get the coordinates to align (gnuplot's binary keywords "dx", "dy", "dz", and "origin" might be relevant here), it might solve your problem.

Heatmap with background image on gnuplot

1 Answers