In this case the result must be as though the paint operations had been applied to an intermediate canvas, of a size determined by the rules given in Filter Effects then filtered by the processes defined in Filter Effects . 3.7 Clipping, masking and object opacity SVG allows any painting operation to be limited to a sub-region of the output device by clipping and masking. This is described in Clipping, Masking and Compositing Clipping uses a path to define a region of the output device to which paint can be applied. Any painting operation executed within the scope of the clipping must be rendered such that only those parts of the device that fall within the clipping region are affected by the painting operation. A clipping path can be thought of as a mask wherein those pixels outside the clipping path are black with an alpha value of zero and those pixels inside the clipping path are white with an alpha value of 1.
Clipping is described in Clipping paths .
Masking uses the luminance of the color channels and alpha channel in a referenced SVG element to define a supplemental set of alpha values which are multiplied to the alpha values already present in the graphics to which the mask is applied. Masking is described in Masking . In this case the mask is infinite, with a color of white and an alpha channel of the given opacity value. Then, alpha values on the intermediate canvas are multiplied by the implicit alpha values from the clipping path, the alpha values from the mask, and the alpha values from the 'opacity' property . The resulting canvas is composited into the background using simple alpha blending .