The tools in the Edit Phase make use of several parameters with common meanings: Size, Hardness, and Flow, as shown in Figure (A).
Size determines the spatial extent of the brush, indicated by a circle with a cross inside it. The size is the diameter of the brush circle.
Hardness determines the sharpness of the brush in terms of its effect on the image. For all Hardness values, the center pixel is modified by 100% times the Flow rate, and a pixel on the boundary of the brush circle is not modified at all. The effect of the brush at the lowest Hardness value drops off linearly from the center of the brush to the edge. At the highest hardness value, the brush produces its maximum effect from the center to the edge of the brush.
Flow is a general multiplier on the effect of the brush. Whatever change the brush will make to the image, given the Size and Hardness values, is multiplied by the Flow, which represents a percentage modifier. A low Flow makes small changes as the user moves the brush. A high Flow value makes more changes as the user moves the brush.
Figures (B) and (C) demonstrate two cases using the Paintbrush tool: low hardness and low flow, and high hardness and high flow. Note the feathered soft effect with the low values and the sharp crisp effect with the high values.