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<h4>Source Images</h4>

<p>The symbols <samp>u</samp> and <samp>v</samp> refer to the first and second images, respectively, in the current image sequence.  Refer to a particular image in a sequence by appending its index to any image reference (usually <samp>u</samp>), with a zero index for the beginning of the sequence. A negative index counts from the end.  For example, <samp>u[0]</samp> is the first image in the sequence, <samp>u[2]</samp> is the third, <samp>u[-1]</samp> is the last image, and <s