Convert RGB Image to Grayscale

QUESTION: How do I convert a color image (say a JPEG image) to grayscale?

ANSWER: This answer from Craig Markwart on the IDL newsgroup:

There is no one "correct" conversion from RGB to grayscale, since it depends on the sensitivity response curve of your detector to light as a function of wavelength. A common one in use is:
    Y = 0.3*R + 0.59*G + 0.11*B

Using Craig's formula, then, here is an example:

   filename = Filepath(Subdir=['examples', 'data'], 'rose.jpg')
   Read_JPEG, filename, rose
   rose_gray = 0.3*Reform(rose[0,*,*]) + 0.59*Reform(rose[1,*,*]) + 0.11*Reform(rose[2,*,*])

Here is the result.