Sensory deprivation, and really paying attention to objects of sight that we usually do not pay attention to (the patterns on the carpet, for example) can have such an effect. These things usually are connected to the mechanics of the visual sense, and often beyond our control. It is just an optical illusion.
A dry as toast, but good book on the topic is Dr. Austin's Zen and the Brain ... he has a discussion of all manner of hallucinations here (from about page 373).
Seeing patterns on the carpet or wall you are looking at, and floor undulation, is kind of like this effect produced by a bad carpet:
Another common effect is to see "spots in the eyes". Most are there all along (floating impurities, early cataracts and such of the eyeball itself), but we just do not notice them until we sit still. Many are just the "cones and rods" of the eye that were there all along. The cones and rods of color, for example, are always present in our eyes, but we do not give them notice so often in day to day life. In Zazen, what is always there just stands out sometimes, and the brain plays some tricks by seeing "connect the dot" patterns.
The eyes contain cones and rods for color that we usually do not notice (but, if you look at any object closely, you will see little dots of color, much like the picture tube of a color tv):
The sensory deprivation effect at staring at the white surface just brings the little dots to our attention, and they play pattern tricks in the brain.
Like a new pair of glasses, the brain will adjust and soon not notice the dots as much. Maybe we are subconsciously looking for the patterns, and thus noticing the patterns. If we just forget about them, they usually go away.
The brain tends to try to recognize objects in formless patterns such as we see the "man in the moon" or bunny rabbits in clouds or Jesus on a piece of toast. There is even a scientific name for it: Pareidolia
However, visual hallucinations are common in Zazen. Not a worry, nor of any particular importance other than as an amusement, possibly with a small lesson about how we create the world through the senses:
Hallucinations and Illusions
Kornfield (1979, 1983) noted that there was a strong correlation between student reports of higher levels of concentration during insight meditation, when the mind was focused and steady, and reports of altered states and perceptions. He reported that unusual experiences, such as visual or auditory aberrations and hallucinations, and unusual somatic experiences, are the norm among practiced meditation students. Walsh (1978) reported that he experienced hypnagogic hallucinations, and Goleman (1978-79) reported visionary experiences during deep meditation. Shimano and Douglas (1975) reported hallucinations similar to toxic delirium during zazen.
... Earlier, Deikman (1966a) reported that during meditation on a blue vase, his subjects' perception of color became more intense or luminous, and that for some of them the vase changed shape, appeared to dissolve, or lost its boundaries. Maupin (1965) reported that meditators sometimes experience "hallucinoid feelings, muscle tension, sexual excitement, and intense sadness."
The contemplative literature contains numerous descriptions of the perceptual distortion produced by meditation. It is called makyo in Zen Buddhist sources, and is characterized in some schools as "going to the movies," a sign of spiritual intensity but a phenomenon that is regarded to be distinctly inferior to the clear insight of settled practice. In some Hindu schools it is regarded as a product of the sukshma sharira, or "experience body," in its unstable state, and in that respect is seen to be another form of maya, which is the illusory nature of the world as apprehended by ordinary consciousness.
In a similar manner, St. John of the Cross described the false enchantments that may lure the aspirant in prayer, warning that "devils may come in the guise of angels."  In his allegory of the spiritual journey, The Pilgrim's Progress, John Bunyan described Christian's losing his way by following a man who says he is going to the Celestial City but instead leads him into a net. In all the great contemplative manuals, one is taught that detachment, equanimity, and discrimination are required for spiritual balance once the mind has been opened and made more flexible by prayer and meditation. Illusions and hallucinations, whether they are troubling or beatific, are distractions—or signposts at best—on the way to enlightenment or union with God.
Move along folks ... nothing to look at here! :-)
Actually, it is all a fine lesson in how the body-mind-self-world
are all interconnected.