How did shaped canvases challenge the traditional rectangular painting?

Shaped canvases challenged the traditional rectangular painting by rejecting the conventional role of the support as a mere window for illusions of space, volume, or light. Instead, artists like Frank Stella and Kenneth Noland designed irregularly shaped canvases to be viewed as explicitly flat, concrete art objects.1

By moving away from the standard rectangle, these artists emphasized the physical reality of the painting itself rather than using traditional methods to represent external subjects. Stella, in particular, utilized frame-determined designs where the internal composition of the work directly replicated or interacted with the specific shape of the canvas. This approach helped blur the traditional distinctions between painting and sculpture, as the work ceased to be a flat surface containing an image and became a three-dimensional object that asserted its own presence in space.21