Any object can be used to mask another object. The opacity and lightness of the mask determines the opacity of the masked objects. A masked object will be fully opaque only at places that are: inside the mask path, where the mask has maximum lightness (i.e., white), and the mask has maximum Alpha. So to summarize:
Regions with minimum lightness (i.e., black) will be fully transparent.
Regions with minimum Alpha (zero alpha) will be fully transparent.
Regions outside the mask will be fully transparent.
Masking has a few problems in v0.47 and v0.48. First, masks should use luminosity values; instead they use a linear combination of red, green, and blue values. Second, the mask value should be calculated in linear RGB color space; instead it is calculated in sRGB color space. Third, export via Cairo to PDF and PostScript is broken for regions outside the nominal mask area. The first and third problems are fixed in v0.48.1.
To mask an object (or Group), select both the object to be masked and the object to be used as a mask. The mask must be above the object to be masked in z-order. Then use the → → command. To unmask a masked object, select the object and use the → → command. The mask is then restored as a regular object and is placed just above the formerly masked object in z-order.
More than one object (or Group) can be masked at the same time. Just follow the above instructions but include all the objects to be masked in the selection (with the mask on top). Inkscape will store one copy of the mask in the <defs> section of the SVG file for each masked object; thus, the masked objects can be edited separately.
When a masked object is selected, the Status Bar will display the type of object clipped along with the word “masked.” The bounding box of the masked object is the same as that of the unmasked object.
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