Grayscale Heatmap to Color Gradient Heatmap

user_19226934 0
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I am trying to take a set of 256x256px 8-bit grayscale .pngs (with transparency) and convert the grayscale .png to a color .png of the same size, still retaining transparency. The palette I want to use is Zissou1 from the R wesanderson package, which I have gotten into a Python dictionary where each key corresponds to a greyscale value and each value a HEX color.

import os
from PIL import Image, ImageColor

### dic = the dictionary containing the palette in format {grayscale pixel value: "HEX COLOR"},
###       created earlier in the script

with Image.open("3.png") as im:
    fin = Image.new("RGBA", im.size)
    px = im.load()
    px1 = fin.load()
    for x in range(0,256):
        for y in range(0,256):
            px1.putpixel(x,y,ImageColor.getcolor(dic[px.getpixel(x,y)[1]],"RGBA"))
    fin.show()

I am getting the error:

px1.putpixel(x,y,ImageColor.getcolor(dic[px.getpixel(x,y)[1]],"RGBA"))
AttributeError: 'PixelAccess' object has no attribute 'putpixel'
python-imaging-library python
2 Answers
user_580010

To extend on Jason's answer:

The lookup as given by PIL

With Image.point(lookup_table, mode = 'L') you can lookup and transpose the colors of your image.

lookup_table = ...
with Image.open("3.png") as orig:
    image = orig.point(lookup_table, mode = 'L')
    image.show()

To see an example for using the Image.point method with the lookup_table:

Your own implementation (fixed with improved naming)

or implement the lookup against your_dic yourself:

your_dic = ...
with Image.open("3.png") as orig:
    image = colored_from_map(orig, your_dic)
    image.show()

with this alternative function (you almost did):

def colored_from_map(orig, map_to_color):
    image_in = orig.load()
    image = Image.new("RGBA", im.size)
    image_out = image.load()

    for x in range(0,256):
        for y in range(0,256):
            coords = (x,y)
            greyscale = image_in.getpixel(x,y)[1]
            color_name = map_to_color[greyscale]
            image_out.putpixel(coords, ImageColor.getcolor(color_name,"RGBA"))

    return image

Preserving the alpha-channel (transparency)

See the source-code of ImageColor.getColor() at the begin and end of its method body:

    color, alpha = getrgb(color), 255  # default to no-transparency
    if len(color) == 4:  # if your mapped color has 4th part as alpha-channel
        color, alpha = color[0:3], color[3]  # then use it

    # omitted lines
    else:
        if mode[-1] == "A":  # if the last char of `RGBA` is `A`
            return color + (alpha,)  # then return with added alpha-channel
    return color

(comments mine)

So you could simply set the fourth element of the returned color-tuple to the previous value of the original gray-scale image:

            greyscale = image_in.getpixel(x,y)[1]  # containing original alpha-channel
            color_name = map_to_color[greyscale]  # name or hex
            mapped_color = ImageColor.getcolor(color_name,"RGB")  # removed the A
            transposed_color = mapped_color[:2] + (greyscale[3],)  # first 3 for RGB + original 4th for alpha-channel (transparency)
            image_out.putpixel(coords, transposed_color)

Note: because the A(lpha-channel) is provided from original image, I removed the A from the getColor invocation's last argument. Technically, you can also remove the slicing from mapped_color[:2] to result in mapped_color + (greyscale[3],).

user_57302790
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