Method: Magick::Image#color_histogram

Defined in:
ext/RMagick/rmimage.cpp

#color_histogram ⇒ Hash<Magick::Pixel, Integer>

Computes the number of times each unique color appears in the image.

Returns:

  • (Hash<Magick::Pixel, Integer>) —

    Each key in the hash is a pixel representing a color that appears in the image. The value associated with the key is the number of times that color appears in the image.



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# File 'ext/RMagick/rmimage.cpp', line 2830

VALUE
Image_color_histogram(VALUE self)
{
    Image *image, *dc_copy = NULL;
    VALUE hash, pixel;
    size_t x, colors;
    ExceptionInfo *exception;
#if defined(IMAGEMAGICK_7)
    PixelInfo *histogram;
#else
    ColorPacket *histogram;
#endif

    image = rm_check_destroyed(self);

    exception = AcquireExceptionInfo();

    // If image not DirectClass make a DirectClass copy.
    if (image->storage_class != DirectClass)
    {
        dc_copy = rm_clone_image(image);
#if defined(IMAGEMAGICK_7)
        GVL_STRUCT_TYPE(SetImageStorageClass) args = { dc_copy, DirectClass, exception };
#else
        GVL_STRUCT_TYPE(SetImageStorageClass) args = { dc_copy, DirectClass };
#endif
        CALL_FUNC_WITHOUT_GVL(GVL_FUNC(SetImageStorageClass), &args);
        image = dc_copy;
    }

    GVL_STRUCT_TYPE(GetImageHistogram) args = { image, &colors, exception };
    void *ret = CALL_FUNC_WITHOUT_GVL(GVL_FUNC(GetImageHistogram), &args);
    histogram = reinterpret_cast<decltype(histogram)>(ret);

    if (histogram == NULL)
    {
        if (dc_copy)
        {
            DestroyImage(dc_copy);
        }
        rb_raise(rb_eNoMemError, "not enough memory to continue");
    }
    if (rm_should_raise_exception(exception, DestroyExceptionRetention))
    {
        RelinquishMagickMemory(histogram);
        if (dc_copy)
        {
            DestroyImage(dc_copy);
        }

        rm_raise_exception(exception);
    }

    hash = rb_hash_new();
    for (x = 0; x < colors; x++)
    {
#if defined(IMAGEMAGICK_7)
        pixel = Pixel_from_PixelColor(&histogram[x]);
#else
        pixel = Pixel_from_PixelColor(&histogram[x].pixel);
#endif
        rb_hash_aset(hash, pixel, ULONG2NUM((unsigned long)histogram[x].count));
    }

    /*
        Christy evidently didn't agree with Bob's memory management.
    */
    RelinquishMagickMemory(histogram);

    if (dc_copy)
    {
        // Do not trace destruction
        DestroyImage(dc_copy);
    }

    RB_GC_GUARD(hash);
    RB_GC_GUARD(pixel);

    return hash;
}