A method, system, and computer-readable storage medium are disclosed for simulating bristle brush behavior in an image editing application. User input may represent a stroke made by a paintbrush comprising multiple bristles sweeping across a canvas. The application may calculate a per-bristle opacity value using an exponential function dependent on a target opacity value for the brush stroke and an expected number of overlapping bristles during the brush stroke. The per-bristle opacity value may be used to determine per-bristle effects of the deposition of paint on the color and/or opacity values of the canvas pixels affected by the brush stroke as these per-bristle effects are composited to determine the cumulative effect of the brush stroke. The per-bristle opacity value may be calculated based on the brush orientation or position, direction of the stroke, number of bristles, type or shape of the brush tool, and/or pressure applied to the brush tool.