Certain embodiments involve a graphics manipulation application using brushstroke parameters that include a maximum alpha-deposition parameter and a fractional alpha-deposition parameter. For instance, the graphics manipulation application uses an alpha flow increment computed from the maximum alpha-deposition parameter and the fractional alpha-deposition parameter to compute an output canvas color. In some embodiments, if the current canvas opacity exceeds or equals the maximum alpha-deposition parameter, the current canvas opacity is selected as the output canvas opacity. Otherwise, the graphics manipulation application computes the output canvas opacity by increasing the current canvas opacity based on the alpha flow increment. The graphics manipulation application updates a canvas portion affected by a brushstroke input to include the output canvas opacity and the output canvas color.