What is a trimap?
Posted by Peter Moyer on 04 January 2008 12:23 AM

EZ Mask creates masks by using a trimap--a pre-segmented image consisting of three regions of foreground (what you want to cut out), background and unknown. Partial opacity values are then computed only for pixels inside the unknown region. Two trimap methods can be used: Stroke and Filled. When creating masks, start with Stroke method and move on to the Filled method if the results are not satisfying.

Stroke Trimap
Unlike previous approaches, our trimaps can be relatively sparse consisting of individual foreground and background brush strokes. All pixels left unmarked will be treated as unknown. After processing, if any fine foreground details are missing from the mask, the Unknown brush can be used in these areas to help recover lost detail.

Stroke trimaps require little user input, but when color ambiguity exists between the foreground and background, a Filled trimap may create a more accurate mask. In addition, Stroke trimaps will also take longer to render as they require an extra calculation step.

Filled Trimap
Foreground and background brush strokes are used to mark definite foreground and background pixels while the Unknown brush is used to mark unknown, or mixed regions. Using this method, the entire image is painted/filled with one of the three brushes.

Filled trimaps process faster than Stroke trimaps and can result in extracting a more accurate mask, most notably when color ambiguity exists between the foreground and background. However, they can be tedious to create and tend to fail for images with large portions of semi-transparent foreground where the trimap is difficult to create manually.

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