How to use and identify inputs for filter effect primitives.
The value chosen for a filter effect primitive can be either one of
six keywords or can be a string which matches a previous
attribute value within the same filter effect container. If no value
is provided and this is the first filter effect primitive, then the
input will be
SourceGraphic. If no value is provided and this
is a subsequent filter effect primitive, then this filter effect
primitive will use the result from the previous filter primitive as
If the value for
result appears multiple times within a given
filter container, then a reference to that result will use the closest
preceding filter primitive with the given value for the
results. Forward references to results are an error and will not draw.
Definitions for the seven possible options:
SourceGraphic This keyword represents the appearance of
grobs before they are being filtered. For raster effects filter
primitives, the grobs will be rasterized into an initially clear
RGBA raster in image space. Pixels left untouched by the original
graphic will be left clear. The image is specified to be rendered in
linear RGBA pixels. The alpha channel of this image captures any
anti-aliasing specified by SVG. (Since the raster is linear, the
alpha channel of this image will represent the exact percent
coverage of each pixel.)
SourceAlpha This keyword represents the appearance of
grobs before they are being filtered.
SourceAlpha has all of
the same rules as
SourceGraphic except that only the alpha
channel is used. The input image is an RGBA image consisting of
implicitly black color values for the RGB channels, but whose alpha
channel is the same as
SourceGraphic. If this option is used,
then some implementations might need to rasterize the graphics
elements in order to extract the alpha channel.
BackgroundImage This keyword represents an image
snapshot of the canvas under the filter region at the time that the
referring grob is being filtered.
BackgroundAlpha Same as
only the alpha channel is used.
FillPaint This keyword represents the value of the
fill property on the grob being filtered. The
FillPaint image has conceptually infinite extent. Frequently
this image is opaque everywhere, but it might not be if the "paint"
itself has alpha, as in the case of a gradient or pattern which
itself includes transparent or semi-transparent parts.
StrokePaint This keyword represents the value of the
col property on the grob being filtered. The
StrokePaint image has conceptually infinite
extent. Frequently this image is opaque everywhere, but it might not
be if the "paint" itself has alpha, as in the case of a gradient or
pattern which itself includes transparent or semi-transparent parts.
The result of any filter effect operation. This is the name
that has been given to the
result argument of a filter
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