Sub-sampling coupled with intensity-tapering may suppress, to some extent, cluttering and convey the flow orientation (the positive direction of the field) while a rainbow color map can be adopted to show the velocity magnitude. As an alternative, colorwheel maps flow vectors to Hue-Saturation-Value (HSV) colors through the use of a legend where there are a one-to-one correspondence between normalized vectors and circularly distributed radial lines of hue and a certain map between magnitude and radially varying brightness or value. Arrows are a little bit more complex in geometry than intensity-tapering line segments and hence require sparser sub-sampling of the field to alleviate the cluttering problem. Arrow-size mapping, in addition to the aforementioned two color map schemes, can be employed to depict the velocity magnitude. Sometimes arrows are placed at jittered grid points in a way to combat artifacts that would otherwise result from the regular pattern of the underlying grid. However, cavities might occur near flow boundaries.