Example implementations relate to swath height error compensation. Some examples may determine a density of an image to be printed in an overlap area of a printing material. The overlap area may include target pixels capable of being printed by a first set of drop ejection elements and a second set of drop ejection elements that are redundant to the first set of drop ejection elements. Some implementations may also determine a mask to apply to the first and second set of drop ejection elements based on the determined density, and the mask may designate at least one additional drop to apply to at least one target pixel in the overlap area by at least one of the first and second set of drop ejection elements. Some implementations may also apply the mask to the first set of drop ejection elements and the second set of drop ejection elements.