Semi-transparent marks accumulate in visual density where shapes overlap most

When shapes with low alpha (near-transparent fill) are drawn repeatedly at the same location or overlapping locations, their transparency stacks up. A fill(255, 25) drawn 20 times in the same spot appears nearly opaque; drawn only twice it is barely visible. This accumulation effect is a key generative art tool: by not clearing the frame between iterations, you create a visual record of motion or density. Areas visited most appear more solid. The effect is independent of the alpha of individual strokes — it emerges from quantity. It can be used deliberately to reveal structure (where do agents spend most time?) or accidentally as an unwanted artifact.

Examples

Concentric circles with fill(255, 25): the center (where all circles overlap) appears brightest; the outer edge (touched only by the largest circle) appears nearly invisible. Particle trails: particles leaving semi-transparent dots reveal their paths as ghostly streaks.

Assessment

Predict the visual difference between a sketch that draws 100 overlapping ellipses with fill(0, 255) versus fill(0, 10), and explain the mechanism behind the difference.