Purple fringing, or bichrominance as it's more correctly termed, is a form of chromatic aberration caused by the over-excitation of the pixels on the sensor in the camera. The effect can occur anywhere within an image but it's most common at the edges of high contrast areas, especially when a dark element is strongly backlit, such as branches silhouetted against a blue sky. The Defringe filter selectively adjusts these areas to remove the colour fringing.
Using the Defringe filter to remove purple fringing from the wall edge.
On selecting the Defringe filter, it will default to settings which remove the purple fringing, as this is the most common type. This can be manually adjusted as necessary on the context toolbar, or by sampling the fringe colour on the page to remove it.
Settings
The following settings can be adjusted on the context toolbar:
Hue—defines the colour of the fringe to be desaturated. Drag the slider to define the hue.
Complementary—also removes fringing matching the chosen hue's complementary colour (e.g., choosing purple would also remove yellow fringing).
Radius—defines the radius of the pixels affected around the fringed areas.
Tolerance—defines how close the fringe colour must be to the defined colour before it is affected.
Threshold—defines the amount of contrast needed between the fringed areas before affecting the pixels.
