What is the meaning of Han Ganma at the ferry crossing with fragrant grass and wearing Song Niu by the green poplar embankment?

Han Gan's horse is crossing the barren grass, and Song cattle are wearing it by the green poplar embankment. "

These two poems praise two ancient painters:

In the painting circles of the Tang Dynasty, there is a saying that "Han Ma wears an ox". "Han Ma" means Han Qian is famous for painting horses; "Dai Niu" is Dai Song, who is famous for painting cattle and is famous in the Tang Dynasty.

There is an ancient joke that goes like this:

There was a son-in-law who was stupid and incompetent. His wife taught him: "There are two ancient paintings handed down from our ancestors, which are 'Han Ganma at the Fragrant Grass Ferry' and 'Dai Songniu by the Green Yangdi'. If you see it, Use these two sentences to praise this two-axis ancient painting. "

The son-in-law later arrived at his father-in-law's house, and his father-in-law took out the two scrolls of ancient paintings and showed them to his son-in-law. The son-in-law said the two sentences his wife had taught him, and his father-in-law was very happy to hear them.

Later, my father-in-law bought another scroll of paintings by the Eighteenth-century Scholars and asked his son-in-law to come and see if the painting was good. The son-in-law showed the painting and said, "It's a good ancient painting, but it's 'Fragrant Grass Crossing the Head'." Han Ganma', 'Dai Songniu by the Green Poplar Embankment'. "The people watching together burst into laughter. The father-in-law scolded his son-in-law: "You only know cows and horses, when will you know people!" ”

——Anonymous Ming Dynasty’s “Hua Feast, Talking, Laughing and Drinking”