24 Feb Inside/Outside edge matting
The ‘inside edge’ of an object means a selection inside the object and close to its real edge. And the ‘outside edge’ means a selection covering all the pixels of the object and close to its edge. The area between the inside and the outside is the edge of the object and is the semi-transparent part. The program will calculate the real mask from this user-specified inside/outside edge.
The inside/outside edge is not the precise edge of the object so the user doesn’t always need to spend too much time doing this. The purpose of selection is to build a trimap including known foreground, known background and unknown regions.
- Polygon selection tool: Click to start selection; right click to remove last point; moving forward and click to continue select; double click or move back to the first point and click to end selection. Press ‘shift’ or ‘Ctrl’ for add or subtract mode. Mouse wheel to zoom in/out.
- Injector tool: Inject single point to foreground or background.
- Stroke tools: A quick way to generate selections for objects with clear edges.
Easy way: Create selection by strokes
If the object has a simple and clear edge, you can use ‘Stroke based matting’ to mask it quickly and easily. There are 3 steps to do this,
- Use keep pen in green to stroke inside the object. Use drop pen in blue to stroke on outside the object.
- The program will generate the inside/outside selection based on the strokes automatically. Check the selections, repeat using keep pen and drop pen to define the selection, until it covers most of the area correctly.
- Use unsure pen in red, to make sure the areas between inside and outside covers all semi-transparent area and hair. Polygon tools are also useful to modify the selections.
Each time you are painting with the keep or drop pen the program will calculate the selection again. So make sure the unsure pen is used when you no longer need keep or drop pen. When selections are ready, click ‘Matting’ to start masking the object out.
Normal Way: Create selection by polygon tools
Using polygon tools to create 2 selections, one is inside the object, another outside it.
Close to the border or far away?
If the area has a varied colour or changes, try select precisely and select close to the border; if the colour is simple and slow changing, you can make a coarse selection.
How to deal with a hole?
Using the subtract mode to cut the hole for both inside and outside edge.
How to matting an object with complex mask?
You can make a coarse selection and then matt with it. Checking the result, if the foreground or background turns to semi-transparent, using an ‘add’ or ‘subtract’ mode of polygon tool to add some sample and matting again (click ‘apply’) until you get a good result. The ‘Inject tool’ is also very useful in this case.
What causes defects on result?
Reason 1: If the background and foreground has a similar colour the program may not able to tell the difference and then an error result is outputted.
Reason 2: Unknown colour. If the colour of a pixel has not existed in any known area, including the foreground and the background, it will be treat as a wrong alpha value. To deal with defects, please use ‘General matting tools’.
To deal with defects, please use ‘General matting tools’.