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I am wondering, how can I perform conversion from RGB to YUV 420P image in Go (more precisely *image.RGBA to *image.YcbCr). I havn't found any standard method in image package for that. As I also didn't find any library for that, my only idea was to make it pixel by pixel using color.RGBToYCbCr(), but I realised that *image.YcbCr hasn't Set() method for working directly on pixels, so I'm a bit confused. I would be gratuful for some directions or code for that. Greetings

MikolajMGT
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1 Answers1

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You could use image.NewYCbCr() to create an instance of image.YCbCr whose pixels you can directly manipulate. Then it is just a matter of using color.RGBToYCbCr() for each pixel in the image:

bounds := original.Bounds()
converted := image.NewYCbCr(bounds, image.YCbCrSubsampleRatio420)

for row := 0; row < bounds.Max.Y; row++ {
    for col := 0; col < bounds.Max.X; col++ {
        r, g, b, _ := original.At(col, row).RGBA()
        y, cb, cr := color.RGBToYCbCr(uint8(r), uint8(g), uint8(b))

        converted.Y[converted.YOffset(col, row)] = y
        converted.Cb[converted.COffset(col, row)] = cb
        converted.Cr[converted.COffset(col, row)] = cr
    }
}

In the snippet above original is a regular image.RGBA and converted is image.YCbCr. See that the corresponding position pos in the arrays of the color components of converted is computed from the pixel coordinates in original.

Oscar Luis Vera Perez
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  • Thanks for answer, this is what I'm looking for, but as I specified in question, target format is YUV 420, so I change declaration of `converted` to `image.NewYCbCr(bounds, image.YCbCrSubsampleRatio420)`, but now I am getting `panic: runtime error: index out of range [76800] with length 76800` - I thnik, that `pos` need to be calculated in different way for that format, as there is chroma subsampling active. Do you know how should it be computed? – MikolajMGT Apr 12 '20 at 11:43
  • @MikolajMGT I see the issue, in fact, `image.YCbCr` provides the methods `YOffset` and `COffset` for that. I updated the answer. – Oscar Luis Vera Perez Apr 12 '20 at 13:52