Spiritual Wisdom painting had been granted sentience by the painter, allowing them to absorb spiritual energy from the surroundings to emerge from a painting.
As their existences hinged on spiritual energy itself, their spiritual energy requirement was very high, and they were exceptionally sensitive to fluctuations of spiritual energy in the surroundings. Under normal conditions, it would be impossible to sustain their materialization for too long. In fact, even under the Spirit Gathering Diction of a 5-star master teacher, their materialization will only be extended by a slightly longer period of time.
As such, the test wasn't as easy as it seemed.
In Su shi's view, even though Zhang Xuan was talented in painting as well, he was only a 3-star painter (as well as from Tianwu Kingdom), and not to mention, the content he spoke of was the bare fundamentals. It would be difficult for him to gather sufficient spiritual energy to sustain the Spiritual Wisdom for too long.