How to apply gradient map on a grey scale image

Question

I've searched for any possible solution for that, but all the answers are not really clear or incomplete.

So, say I have the image uploaded into memory:

image = Image.open('image.jpg')

How do I apply this gradient (#582f91 to #00aeef):

To this image:

So it becomes this:

Thanks ahead.

Answer 1

I've previously solved this problem using a cython script (as need to update at high frame rate) to achieve this. Here the input cmap to the funciton is a flattened array of a matrix where each row corresponds to a colour and coloumns to RGB values. I used a website to generate the gradient though can't remember which one. The image was flattened for speed and scaled between 0 and 255 int values.

To be able to import and use the cython function you will need to run the setup script from command line, once you have install cython using pip ie

pip install cython
pyhton setup.py build_ext --inplace

This should then produce ac file and .so file.

Cython code:

import numpy as np
cimport numpy as np
cimport cython

DTYPE1 = np.float
ctypedef np.float_t DTYPE1_t

DTYPE2 = np.int
ctypedef np.int_t DTYPE2_t

@cython.boundscheck(False)
@cython.wraparound(False)

def mat_to_im(np.ndarray[DTYPE2_t, ndim=1] data, np.ndarray[DTYPE2_t, ndim=1] cmap):

    cdef int wid = data.size
    cdef int x, x1, y

    cdef np.ndarray[DTYPE2_t, ndim=1] im = np.zeros([wid*3], dtype=DTYPE2)

    for x in range(wid):

        x1 = x*3
        y = data[x]*3

        im[x1] = cmap[y]
        im[x1+1] = cmap[y+1]
        im[x1+2] = cmap[y+2]

return im

Setup file:

from distutils.core import setup, Extension
from Cython.Build import cythonize
import numpy

setup(
    name='image convert',
    version='1',
    description='color map images',
    author='scooper',
    install_requires=['numpy'],
    ext_modules=cythonize([
        Extension("image_convert", ["image_convert.pyx"], include_dirs=[numpy.get_include()])])
)

This should help with any issues (I've cut the setup from larger code file and haven't tested it): http://cython.readthedocs.io/en/latest/src/quickstart/build.html

Answer 2

Just use LinearSegmentedColormap :

# make a cmap
mycm=matplotlib.colors.LinearSegmentedColormap.from_list('',['#582f91', '#00aeef'])

# apply on a canal
imgrad=mycm(image[:,:,0])