| Alpha | The alpha channel values of the mask layer image should be used as the mask values. |
| Luminance | The luminance values of the mask layer image should be used as the mask values. |
| MatchSource | If the mask-image property is of type 'mask-source', the luminance values of the mask layer image should be used as the mask values. If it is of type 'image', the alpha values of the mask layer image should be used as the mask values. |