fixe Marr-Hildreth Hash kernel error

[leslie-wang]

Signed-off-by: Leslie Qi Wang wqyuwss@gmail.com

Based on https://github.com/aetilius/pHash/blob/master/src/pHash.cpp#L610, and https://web.fe.up.pt/~campilho/PDI/NOTES/EdgeDetection.pdf. Seems kernel should be -1 * a/2 or a/-2.

Thanks for the nice work. It is the only one having all image hashing algorithm!

[ajdnik]

Thank you 😄 I didn't know that color.GrayModel uses the same luminance approach, I assumed that it was just averaging them out.

func grayModel(c Color) Color {
    if _, ok := c.(Gray); ok {
        return c
    }
    r, g, b, _ := c.RGBA()

    // These coefficients (the fractions 0.299, 0.587 and 0.114) are the same
    // as those given by the JFIF specification and used by func RGBToYCbCr in
    // ycbcr.go.
    //
    // Note that 19595 + 38470 + 7471 equals 65536.
    //
    // The 24 is 16 + 8. The 16 is the same as used in RGBToYCbCr. The 8 is
    // because the return value is 8 bit color, not 16 bit color.
    y := (19595*r + 38470*g + 7471*b + 1<<15) >> 24

    return Gray{uint8(y)}
}

[ajdnik]

@leslie-wang Thank you for all the improvements, if you can update the unit tests to reflect these new changes, I'll gladly merge this to master. And I'm sorry for how long this process took.