Python-wand: How can I read image properties/statistics

Question

I am trying to extract statistics for an image such as the "mean", "standard-deviation" etc. However, I cannot find anything related in the python-wand documentation about it.

From the command line I can get such statistics like this:

convert MyImage.jpg -format '%[standard-deviation], %[mean], %[max], %[min]' info:

or

convert MyImage.jpg -verbose info:

How to get such info from a python program using wand?

score 2 · Accepted Answer · answered Oct 16 '14 at 17:48

Currently, wand doesn't support any of the statistic methods from ImageMagick's C-API (outside of histogram and EXIF). Luckily the wand.api is offered for extending functionality.

Find the method you need in MagickWand's documentation.
Use ctypes to implement data types/structures (reference header .h files)

from wand.api import library
import ctypes

class ChannelStatistics(ctypes.Structure):
    _fields_ = [('depth', ctypes.c_size_t),
                ('minima', ctypes.c_double),
                ('maxima', ctypes.c_double),
                ('sum', ctypes.c_double),
                ('sum_squared', ctypes.c_double),
                ('sum_cubed', ctypes.c_double),
                ('sum_fourth_power', ctypes.c_double),
                ('mean', ctypes.c_double),
                ('variance', ctypes.c_double),
                ('standard_deviation', ctypes.c_double),
                ('kurtosis', ctypes.c_double),
                ('skewness', ctypes.c_double)]

library.MagickGetImageChannelStatistics.argtypes = [ctypes.c_void_p]
library.MagickGetImageChannelStatistics.restype = ctypes.POINTER(ChannelStatistics)

Extend wand.image.Image, and use the newly supported methods.

from wand.image import Image

class MyStatisticsImage(Image):
    def my_statistics(self):
        """Calculate & return tuple of stddev, mean, max, & min."""
        s = library.MagickGetImageChannelStatistics(self.wand)
        # See enum ChannelType in magick-type.h
        CompositeChannels = 0x002F
        return (s[CompositeChannels].standard_deviation,
                s[CompositeChannels].mean,
                s[CompositeChannels].maxima,
                s[CompositeChannels].minima)

Thanks for the answer! I already implemented something similar :) — Vangelis Tasoulas, Oct 16 '14 at 20:58

score 1 · Answer 2 · answered Mar 01 '17 at 18:05

Just a note to anyone following the excellent suggestion by @emcconville:

The documentation on the imagemagick site is for v7.x
Wand will only work with imagemagick 6.x
In IM6.x, there is actually one more field on the end of _ChannelStatistics, a double called entropy, and if you leave it out of your ChannelStatistics declaration your structure won't align properly with what you get back and it'll contain a bunch of nonsense

Python-wand: How can I read image properties/statistics

2 Answers2