image.go

func isIllustration(img []byte) (bool, error) {
	mw := imagick.NewMagickWand()

	err := mw.ReadImageBlob(img)
	if err != nil {
		return false, err
	}

	colorsCnt, colors = mw.GetImageHistogram()

	// Sorting colors by number of occurrences. 
	colorsCounts := make([]int, colorsCnt)
	for i, c := range colors {
		colorsCounts[i] = int(c.GetColorCount())
	}

	sort.Sort(sort.Reverse(sort.IntSlice(colorsCounts)))

	var (
		colorIdx         int
		count            int
		imageWidth       = mw.GetImageWidth()
		imageHeight      = mw.GetImageHeight()
		pixelsCount      = 0
		totalPixelsCount = float32(imageHeight * imageWidth)
		fiftyPercent     = int(totalPixelsCount * 0.5)
	)

	// Going through colors until reach 50% of all pixels
	for colorIdx, count = range colorsCounts {
		if pixelsCount > fiftyPercent {
			break
		}

		pixelsCount += count
	}

	colorsCntIn50Pct := colorIdx + 1

	// Calculate ratio between number of colors used for 50% of the image and 
	// make a decision based on that.
	return (float32(colorsCntIn50Pct)/float32(colorsCnt)) <= 0.02, nil
}