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Wand turns transparent background to black – Photoshop

I am trying to grayscale with python using Wand, but when I do

from wand.image import Image
with Image(filename='image.png') as img:
    img.type = 'grayscale'
    img.save(filename='image_gray.png')

it turns the transparent background into black. If I use one with white background it works. What do I do wrong. And also as grayscaling is

Y = 0.2126 * RED + 0.7152 * GREEN + 0.0722 * BLUE

Where can I do that manually in Wand, say if I want to change the values a bit. I looked in the documentation and in various forums but I couldn’t find any answer, only stuff for photoshop.

Thanks!

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2

Answers


  1. 0

    this doesnt answer your question about wand … but you can do it easy enough with just pil …

    from PIL import Image
from math import ceil
import q
def CalcLuminosity(RED,GREEN,BLUE):
    return int(ceil(0.2126 * RED + 0.7152 * GREEN + 0.0722 * BLUE))

im = Image.open('bird.jpg')
# im.convert("L")  will apply the standard luminosity mapping

data = [CalcLuminosity(*im.getpixel((c,r))) for r in range(im.height) for c in range(im.width) ]

#now make our new image using our luminosity values
x = Image.new("L",(im.width,im.height))
image_px = x.load()
for c in range(im.width):
    for r in range(im.height):
        image_px[c,r] = data[r*im.width+c]

x.save("output.jpg")

    or if you wanted to limit extremes based on a threshold

    #now make our new image using our luminosity values
x = Image.new("L",(im.width,im.height))
image_px = x.load()
for c in range(im.width):
    for r in range(im.height):
        image_px[c,r] = 0 if data[r*im.width+c] < 120 else 255

x.save("output.jpg")

    or if you wanted to filter a single color chanel

    def CalcLuminosityBLUE(RED,GREEN,BLUE):
    return BLUE
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  2. 0

    PNG image type set to grayscale removes transparent layer (see PNG docs). One option would be to enable the Alpha channel after setting grayscale.

    img.alpha = True
# or
img.background_color = Color('transparent')

    Depending on which version you have, this might not work.

    Another Option

    Alter the color saturation with Image.modulate.

    img.modulate(saturation=0.0)

    Another Option

    Alter the colorspace.

    img.colorspace = 'gray'
# or
img.colorspace = 'rec709luma'
# or
img.colorspace = 'rec601luma'

    Another Option

    If your version has Image.fx. The following would work

    with img.fx('lightness') as gray_copy:
   ....
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