What is the differrence between colormap and historam in the identify command output?

Question

I use the identify command in the form below:

identify -verbose image.png

Part of the output is:

  Colors: 8
  Histogram:
     49602: ( 49, 51, 39) #313327 srgb(49,51,39)
     36492: ( 98,121,135) #627987 srgb(98,121,135)
     21728: ( 98,182,240) #62B6F0 srgb(98,182,240)
     39526: (121,131, 75) #79834B srgb(121,131,75)
     34298: (165,171,147) #A5AB93 srgb(165,171,147)
     29957: (185,200,226) #B9C8E2 srgb(185,200,226)
     18767: (210,185, 67) #D2B943 srgb(210,185,67)
     31774: (246, 69, 44) #F6452C srgb(246,69,44)
  Colormap entries: 9
  Colormap:
         0: (121,131, 75) #79834B srgb(121,131,75)
         1: ( 49, 51, 39) #313327 srgb(49,51,39)
         2: (210,185, 67) #D2B943 srgb(210,185,67)
         3: (165,171,147) #A5AB93 srgb(165,171,147)
         4: (185,200,226) #B9C8E2 srgb(185,200,226)
         5: ( 98,121,135) #627987 srgb(98,121,135)
         6: ( 98,182,240) #62B6F0 srgb(98,182,240)
         7: (246, 69, 44) #F6452C srgb(246,69,44)
         8: (255,255,255) #FFFFFF white

I see that the same colors as in Histogram plus white, but in a different order appear also in the colormap.

What is the difference between the two?

Answer 1

The first line under Histogram:

49602: ( 49, 51, 39) #313327 srgb(49,51,39)

tells you that there are 49,602 pixels in the image with the colour sRGB(49,51,39). So it is telling you the frequency of occurrence, or how often, each colour occurs.

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The 9 lines under Colormap: are the palette of the image.

Let's look at the first line:

0: (121,131, 75) #79834B srgb(121,131,75)

That means that wherever the color srgb(121,131,75) occurs in the image, we only store the palette index 0 at that location, rather than the colour 121,131,75. That means we only use 1 byte to store a 0 instead of storing 3 bytes of RGB, which means we save 2/3 of the space. It is a "LookUp Table" or palette.

Palettes trade space for colour accuracy. In general, they are 1/3 of the size of the original image, but can normally only store 256 unique colours rather than the 16,777,216 colours of a conventional RGB image.

Just for fun, let's create this smooth greyscale gradient and some random noise as a conventional RGB888 image (which comes out at 75kB):

magick -size 40x600 gradient: \( xc: +noise random \) +append -rotate 90 PNG24:a.png

And now do the same thing, but oblige ImageMagick to create a palette image (which comes out at 25kB):

magick -size 40x600 gradient: \( xc: +noise random \) +append -rotate 90 PNG8:a.png

There is a longer explanation with example here.