Creating Procedures and more Complex Effects with Images

The commands from SEQUENCE TRANSFORMS can be used in Tcl procedure to create more complex effects. For instance, a common video editing operation is assembly, when two or more sequences are connected and written. The following Rivl fragment Preview extracts n seconds off the beginning of each movie in a list of movies. The call to seq_new creates an empty sequence for the return value. The loop repeatedly extracts the first n seconds from each sequence in movieList and appends it to result, which is returned.

proc preview {movieList n} {
	#Create an empty sequence
	set result [seq_new]
	foreach movie $movieList {
		#Grab the first n seconds off the next movie
		set chunk [seq_crop $movie 0.0 $n]
		#Concatenate the segment onto the result
		seq_concat! result $chunk
	}
	return $result
}

Probably the most important single command when creating procedures applied to sequences is seq_map.Seq_map executes a given script for each image of a sequence and assembles the resulting images into a new sequence. Seq_map is similar to "mapcar" in Lisp. Consider the command

seq_map $clip {im_fade %1 0.5}

Sometimes, rather than applying the exact same effect to each image, it is desirable to vary the effect over time. For example, consider the fade-to-black effect. This effect can be achieved by calling to im_fade on each image in the sequence with a paramer that decrease over time. In this case, seq_map must call a procedure with a parameter that indicates the time of the image being modified. To this end, seq_map performs the following additional subtitutions:

seq_map $clip {im_fade %l [expr 1-%p]}

im_fade image1 1.00
im_fade image2 0.95
im_fade image3 0.90
...
im_fade image20 0.05
im_fade image21 0.00

proc Transition {transition movieA movieB duration} {
	set lengthA [seq_length $movieA]
	set lengthB [seq_length $movieB]

	# Untouched parts of first and second movie
	set begin [seq_crop $movieA 0.0 [expr $lengthA-$duration]]
	set end [seq_crop $movieB $duration $lengthB]

	# Apply timed effect to end of first movie; overlay with
	# beginning of second movie
	set mid1 [seq_crop $movieA [expr $lengthA-$duration] $lengthA]
	set mid2 [seq_crop $movieB 0.0 $duration]
	set middle [seq_overlay [seq_map $mid1 $transition] $mid2]

	seq_concat $begin $middle $end
	}