Mask

Use the mask filter to make the transparent pixels a specific colour and make a transparent mask from the non-transparent pixels. You simply specify the colour in RGB format. You can mask the invisible portion of an picture and create a solid outline of it (or, if you do it twice, you can mask the mask, to get a silhouette of your original picture). The mask filter's only property is the color that you want to mask to be,

For example:

If you want to see a working example, you can look at the Filter Effects page. This HTML page shows the effects of all the visual filters. For each filter, there is an example. Underneath the example, there is a box for each parameter. You can change the values for the parameter and click on the Change button to see the effect. The Reset button restores the example to the initial state.