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I am currently replacing a hand-made image processing library by OpenCV. The programmer who worked on this library had implemented the sobel gradient function. This implementation is right except that he first converts his image to grayscale with the formula thereafter (the arithmetic mean):

Y = 0.333 * R + 0.333 * G + 0.333 * B

In OpenCV, I see that they use the formula:

Y = 0.299 * R + 0.587 * G + 0.114 * B

To convert an image to grayscale.

Is the hand-made implementation wrong or can it be motivated by some purpose. Should I always use the OpenCV conversion version?

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  • $\begingroup$ If the objects you are imaging have a restricted colour palette, you can adjust the coefficients to give you better contrast. $\endgroup$
    – geometrikal
    Jun 26, 2014 at 0:54

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The correct way to compute the luminance depends on how the color primaries (red, green blues) were defined when capturing the image.

The coefficients in OpenCV are taken from the CCIR 601 recommendation (and there are other similar standards).

The arithmetic average is more a Q&D approach.

As @Adi said, the nuance is probably qualitatively unimportant for you, except if you need identical reproduction of the results.

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The values used by OpenCV are values used by Luma coding in video systems. Your original code just used an equal weight for each channel.

This may or may not be important for your application (probably not really).

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