// Copyright 2017 The goimagehash Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package transforms

import (
	"image"
	"image/color"
)

func toGray(r, g, b uint8) uint8 {
	// 19595 + 38470 + 7471 equals 65536.
	return uint8((19595*uint32(r) + 38470*uint32(g) + 7471*uint32(b) + 1<<15) >> 16)
}

// Rgb2Gray function converts RGB to a gray scale array.
func Rgb2Gray(colorImg image.Image) []uint8 {
	colorImg.Bounds().Dx()
	bounds := colorImg.Bounds()
	w, h := bounds.Dx(), bounds.Dy()
	pixels := make([]uint8, h*w)
	for i := range pixels {
		x, y := i%w, i/w
		var R, G, B uint32
		switch v := colorImg.At(x, y).(type) {
		case color.NRGBA: // pillow discards alpha during conversion and provides no way to pre-multiply it
			R, G, B = uint32(v.R)<<8, uint32(v.G)<<8, uint32(v.B)<<8
		default:
			R, G, B, _ = colorImg.At(x, y).RGBA()
		}

		// Pillow only operates on 8bit data, operating on higher bit data produces rounding differences
		pixels[i] = toGray(uint8(R>>8), uint8(G>>8), uint8(B>>8))
	}

	return pixels
}