What does the poem mean by saying "he feeds upon her face"?

When the speaker expresses the belief that the artist “feeds upon the face" of the woman he portrays, she calls upon the myth of Pygmalion, in which an artist becomes enamored with a sculpture he has created, until eventually that sculpture comes to life. The implication here, however, is that, although he has fallen in love with his work, the woman he depicts never comes to life. Instead, the artist creates something that approximates an image. Each time he paints a portrait, his desire for presenting her becomes more intense, while the real woman becomes further objectified. Turning towards the final line of the poem, one can see clearly that the artist’s affection is for a dream, and not reality. The artist feeds off of the images he creates, sustaining his artistic endeavors at the price of missing out on the woman herself who ages, and becomes weaker, more wan.