lavfi: rewrite nullsrc using the code in vsrc_testsrc.c

Factorize code, extend the functionality of the filter, and make it
return empty buffers. This is useful for filters which ignore the input
frames content.

This is also changing the syntax of the nullsrc source, and dropping the
framerate expression evaluation, which does not look particularly useful.

1 files changed, 13 insertions, 18 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index 7d9a6fc779..76266af37b 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -2586,22 +2586,6 @@ testsrc=t=dc_luma
 
 will generate a "dc_luma" test pattern.
 
-@section nullsrc
-
-Null video source, never return images. It is mainly useful as a
-template and to be employed in analysis / debugging tools.
-
-It accepts as optional parameter a string of the form
-@var{width}:@var{height}:@var{timebase}.
-
-@var{width} and @var{height} specify the size of the configured
-source. The default values of @var{width} and @var{height} are
-respectively 352 and 288 (corresponding to the CIF size format).
-
-@var{timebase} specifies an arithmetic expression representing a
-timebase. The expression can contain the constant
-"AVTB" (the default timebase), and defaults to the value "AVTB".
-
 @section frei0r_src
 
 Provide a frei0r source.
@@ -2629,7 +2613,11 @@ Some examples follow:
 frei0r_src=200x200:10:partik0l=1234 [overlay]; [in][overlay] overlay
 @end example
 
-@section rgbtestsrc, testsrc
+@section nullsrc, rgbtestsrc, testsrc
+
+The @code{nullsrc} source returns unprocessed video frames. It is
+mainly useful to be employed in analysis / debugging tools, or as the
+source for filters which ignore the input data.
 
 The @code{rgbtestsrc} source generates an RGB test pattern useful for
 detecting RGB vs BGR issues. You should see a red, green and blue
@@ -2639,7 +2627,7 @@ The @code{testsrc} source generates a test video pattern, showing a
 color pattern, a scrolling gradient and a timestamp. This is mainly
 intended for testing purposes.
 
-Both sources accept an optional sequence of @var{key}=@var{value} pairs,
+These sources accept an optional sequence of @var{key}=@var{value} pairs,
 separated by ":". The description of the accepted options follows.
 
 @table @option
@@ -2679,6 +2667,13 @@ testsrc=duration=5.3:size=qcif:rate=10
 will generate a video with a duration of 5.3 seconds, with size
 176x144 and a framerate of 10 frames per second.
 
+If the input content is to be ignored, @code{nullsrc} can be used. The
+following command generates noise in the luminance plane by employing
+the @code{mp=geq} filter:
+@example
+nullsrc=s=256x256, mp=geq=random(1)*255:128:128
+@end example
+
 @c man end VIDEO SOURCES
 
 @chapter Video Sinks