Shiki, the “founder” of haiku as separate from hokku, wrote a verse that has (at least) two possible interpretations:
The first is as a hokku would be written:
A tub of indigo
The waters of spring.
Seen this way, someone involved in dyeing cloth has dumped out a tubful of indigo dye. The dark, greenish liquid runs into and tinges the little rivulets and pools of flowing, springtime water a deeper hue, now that the frozen winter has passed (objects dyed in indigo, by the way, do not turn the deep “indigo” blue until some time after they are removed from the dye liquid).
The second way of understanding this verse is not at all hokku-like, because it makes it a metaphor. Blyth has altered the verse slightly in his translation, making the “tub” a barrel and the “waters of spring” a river:
A barrel of indigo,
Poured out and flowing:
The spring river.
Seen thus, Shiki’s verse is no longer hokku. Instead it is a metaphor used more as simile. The river of spring looks like a barrel of dark, greenish indigo poured out and flowing. This is the same technique used in the popular old poem “The Highwayman” by Alfred Noyes:
The wind was a torrent of darkness among the gusty trees,
The moon was a ghostly galleon tossed upon cloudy seas,
The road was a ribbon of moonlight over the purple moor….
Both ways of reading Shiki’s verse are poetry in some sense, but only the first is the poetry of hokku.
In the first, we deal with the real world, with a poured-out tub of indigo running into and tinting the waters of spring. In the second we are in the world of fantasy, where a river is no longer a river but a giant barrel of indigo poured out and flowing. Those who do not know how indigo dye functions are even likely to visualize the liquid flowing from the barrel as deep blue, when actually it is greenish and only turns blue in items dyed with it that are exposed to air for some time — a chemical process.
Hokku does not use the second method because it takes us away from reality and into fantasy. It mixes two images in our minds, and the mind must jump back and forth between them. Usually the “fantasy” image wins our attention.
That does not mean the second does not create a vivid image and is not poetry in a conventional sense. But it does mean that the “poetry” of the second verse is not the poetry of the first, which deals with the “real world” and does not mix the real world with poetic fantasy.
That is one of the distinctions between hokku and other kinds of verse. Hokku prefers the “thing itself” to metaphor or simile that alters and ultimately detracts from the thing, no matter how conventionally poetic the result in the latter case.