image.go

// isIllustration returns true if image is cartoon like, including
// icons, logos, illustrations.
//
// It returns false for banners, product images, photos.
//
// We use this function to decide on lossy or lossless conversion for PNG when converting
// to the next generation format.
//
// The initial idea is from here: https://legacy.imagemagick.org/Usage/compare/#type_reallife
func (p *ImageMagick) isIllustration(src *img.Image, info *img.Info) (bool, error) {
	if len(src.Data) < 20*1024 {
		return true, nil
	}

	if len(src.Data) > 1024*1024 {
		return false, nil
	}

	if float32(len(src.Data))/float32(info.Width*info.Height) > 1.0 {
		return false, nil
	}

	var (
		colors    []*imagick.PixelWand
		colorsCnt uint
	)

	mw := imagick.NewMagickWand()

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

	if info.Width*info.Height > 500*500 {
		aspectRatio := float32(info.Width) / float32(info.Height)
		err = mw.ScaleImage(500, uint(500/aspectRatio))
		if err != nil {
			return false, err
		}
	}

	colorsCnt, colors = mw.GetImageHistogram()
	if colorsCnt > 30000 {
		return false, nil
	}

	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)
		tenPercent       = int(totalPixelsCount * 0.1)
		fiftyPercent     = int(totalPixelsCount * 0.5)
		hasBackground    = false
	)

	for colorIdx, count = range colorsCounts {
		if colorIdx == 0 && count >= tenPercent {
			hasBackground = true
			fiftyPercent = int((totalPixelsCount - float32(count)) * 0.5)
			continue
		}

		if pixelsCount > fiftyPercent {
			break
		}

		pixelsCount += count
	}

	colorsCntIn50Pct := colorIdx + 1
	if hasBackground {
		colorsCntIn50Pct--
	}

	return colorsCntIn50Pct < 10 || (float32(colorsCntIn50Pct)/float32(colorsCnt)) <= 0.02, nil
}