Digitize a filled contour plot

Question

I have an RGB bitmap. Actually it is the contour plot of some scalar field plotted with the 'jet' colormap. I need to reverse the bitmap and obtain the source data. Is there a ready-to-use ond open-source tool for that? Python module is OK too.

Answer 1

Well, as nobody did it, this is a lasy algorithm that do the job:

import numpy as np
import scipy.misc
import matplotlib.pyplot as plt

## The digitized field will be scaled to range (0,1)
scale = np.linspace(0.0, 1.0, 300)
## Palette is a curve in RGB space
jet = plt.cm.get_cmap('jet')
palette = 255.0 * np.array([ jet(s)[:3] for s in scale ])
## Read the field as RGB image
field_0 = scipy.misc.imread('field.png')[:,:,:3]
ny, nx, _ = field_0.shape
## Use Euclidian norm to find a closest point in the palette
dist = lambda v : np.array([ np.linalg.norm(p - v) for p in palette ])
field = np.array([ [ scale[np.argmin(dist(field_0[i,j]))]
                     for j in range(nx) ]
                   for i in range(ny)[::-1] ])

## Plot
fig, ax = plt.subplots(1, 2)
ax[0].imshow(field_0)
ax[1].contourf(field, cmap='gray')
plt.show()

Thanks to everyone who cared.