1. A method, comprising: obtaining a sketch image comprising one or more strokes indicating intended directions of flow for an instance of a directional texture element, wherein a directional texture element is a texture that has a directional flow characteristic; dividing a specified target region of the sketch image into a plurality of target region patches; matching each target region patch to one of a plurality of directional texture element sample patches according to similarity of one or more directional texture element features; and replacing each target region patch with its matched sample patch to synthesize the directional texture element instance.