Constancy, in relation to perception, is the brain’s tendency to sense objects as unchanging, regardless of changes to sensory input. Size constancy says that the size of objects is perceived to be constant, even though change in distance makes objects appear smaller or larger.
A good example of this is when you’re driving towards a city on a relatively flat interstate approaching a city. As you near the city, the buildings appear to get bigger and bigger like they are growing (top down processing). We know the city is not growing, but perceptually they are growing (bottom up processing).
Brightness constancy means the brightness of objects is perceived to be constant, even though changes in illumination make the objects appear brighter or darker. This same thing happens with brightness.
A painting in a dark room looks different than the same painting in a bright room. In this example, all four center square are the exact same color. The color can be completely manipulated by the environment (in this case, the colors surrounding the inner squares).
The girl’s eyes are the same color in both of the pictures shown here. It’s the change in the background color, the environment, that make them appear different. In the same vein, a recent grad student did a study on direct print onto a kraft brown board and compared eye-tracking and consumer attention to the same graphics printed on a premium coated board. Though there are clear differences in color and brightness, consumers shopped and viewed these packages the exact same way. It is always worth testing changes ... but in this instance, the gestalt psychology principle reigned supreme - consumers did not change how they behaved and shopped this product. Since the price difference in premium vs. commodity materials is rather large, cost should be the driving factor here since there was no effect on shopper behavior when printing on the kraft board.
This is an important concept that should be considered and applied when it comes to packaging design. To learn more, check out The Packaging School’s Packaging Design Workflow Course at: https://www.packagingschool.com/courses/packaging-design-workflow-101