Imagemagick: How to increase the number of colors in binary image?

Question

It is a black-and-white image. Its palette consist from only two colors.

When I try to threshold this image in ImageJ (Fiji), it cann't do this because of the image has only two colors. (I don't know why it cann't do this.)

So, the questions is: how to increase the number of image colors (number of colors in image palette) automatically using ImageMagick?

Update 1

One of the posible solution is to compose the image with a grayscale box which includes 256 colors. But is there another solution?

Update 2

The color palette of the image contains only two colors. The question is how to increase the size of the color palette (adding< for example, random colors)?

Update 3

This is a Threshold dialog of ImageJ.

Answer 1

You can put arbitrary entries into a PNG image palette, but it won't help you for all the reasons you have been given in the comments.

#!/usr/bin/env python3

from PIL import Image
import numpy as np

# Open your image and ensure it is in Palette mode
im = Image.open('P4mFP.png').convert('P')

# Check first 2 RGB palette entries are black and white
print(im.getpalette()[:6])            # prints [0, 0, 0, 255, 255, 255]

# Stuff in new palette with 2 original values plus 254 other, random RGB values
im.putpalette([0, 0, 0, 255, 255, 255] + list(np.random.randint(0,256,254*3)))

# Save the new image
im.save('result.png')

Check the palette with ImageMagick:

identify -verbose result.png                                
Image:
  Filename: result.png
  Permissions: rw-r--r--
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: PseudoClass
  Geometry: 1920x1080+0+0
  Units: Undefined
  Colorspace: sRGB
  Type: Bilevel
  Base type: Palette
  Endianness: Undefined
  Depth: 8-bit
  ...
  ...
  Colors: 2
  Histogram:
       2005891: (0,0,0) #000000 black
         67709: (255,255,255) #FFFFFF white
  Colormap entries: 256
  Colormap:
    0: (0,0,0,1) #000000FF black
    1: (255,255,255,1) #FFFFFFFF white
    2: (102,218,69,1) #66DA45FF srgba(102,218,69,1)
    3: (76,252,109,1) #4CFC6DFF srgba(76,252,109,1)
    4: (141,189,112,1) #8DBD70FF srgba(141,189,112,1)
    5: (98,185,56,1) #62B938FF srgba(98,185,56,1)
    ...
    ...
    252: (44,1,152,1) #2C0198FF srgba(44,1,152,1)
    253: (244,134,106,1) #F4866AFF srgba(244,134,106,1)
    254: (254,129,16,1) #FE8110FF srgba(254,129,16,1)
    255: (41,166,211,1) #29A6D3FF srgba(41,166,211,1)

Answer 2

I try different command-line options of ImageMagick and it looks that there is no simple command that can increase the number of colors used in image palette.

The -colors option and the -depth option doesn't help. The palette size stays the same.

So the only way I've found is to compose image with thin grayscale line that contains 256 levels (colors) of gray. It is the example of such line.

The following ImageMagick command composes this line with the original image and this command can be automatized.

magick bw-image.png gray-line.png -compose plus -composite composition.png

The palette of the composite image contains 256 colors nad the added line does not interfere with further image processing.

Answer 3

Here is a binary test image showing values 0 and 1 (see histogram to the right) that can be thresholded by ImageJ (see below).