For a painter, transitioning from landscapes to abstract art signifies nothing less than a plunge into the unknown: Observation yields to intuition, and the imagined replaces the concrete. Having worked in both styles, Delray Beach artist Michelle Sakhai, 42, appreciates the sense of nuance and rigor they share. For more than 20 years in her native New York, Sakhai painted en plein air, working outdoors and endeavoring to capture the fleeting luminosity of the nature scenes in front of her.