How to stack frames of a video together using lighten blending mode?

Question

I want each frame of video to get added on top of the previous frames. I don't want any of them to fade out. I am shooting locked down with a black background. A model moves through the frame with a light. I want to stack the frames one on top of the others so that the light moves through the frame and the last frame shows the path taken as a light trail.

What is the easiest way to add each new frame on top of the rest? I would like to avoid putting a separate layer for each frame manually.

Answer 1

Use "Echo" effect. I believe it will suite you.

You need adjust it. Make number of echos equal to your frame count.

Answer 2

Copy this script into a text file and save it as stackinator2000.jsx then in AE go to File>Scripts>Run Script File and load the file you saved. Then select a layer and run it.

NOTE

• It will take a long time if the comp is long.
• It will create a layer for every frame. I hope you've got a fast machine.
• save your project before trying it. I've tested it and it works here, but all care, no responsibility, yadda yadda.
• It is important that the script is saved as stackinator2000.jsx—trust me on this one. If that's unacceptable then calling it The_Mighty_Stackatron.jsx or stackMeister_77.jsx is a possible workaround, but it will be 25% less awesome.

app.beginUndoGroup('stackinator2000');
var theComp = app.project.activeItem
if (!theComp) {
  //no comp selected, silly rabbit
  alert('choose a layer in a comp');
} else {
  var theLayer = theComp.selectedLayers[0];
  if (!theLayer) {
    //no layer selected
    alert('choose a layer');
  } else {
    var firstFrame = Math.max(theLayer.inPoint / theComp.frameDuration, 0);
    var lastFrame = Math.min(theComp.duration, theLayer.outPoint) / theComp.frameDuration
    var frame;
    //set the blending mode while we're at it
    theLayer.blendingMode = BlendingMode.LIGHTEN;
    //turn on time remapping
    theLayer.timeRemapEnabled = true;
    //delete the last key to make it a freeze frame - this will remove any other time remapping set
    while (theLayer.timeRemap.numKeys > 1) {
      theLayer.timeRemap.removeKey(2)
    }
    //step forward one frame at a time
    for (frame = firstFrame; frame +1 < lastFrame; frame++) {
      //duplicate the layer and work with the new layer
      theLayer = theLayer.duplicate();
      //offset the inpoint by one frame
      theLayer.inPoint = (frame + 1) * theComp.frameDuration;
      //prevents weirdness with the out point that I don't understand
      theLayer.outPoint = lastFrame * theComp.frameDuration;
      //set the time remapping for the first frame of the layer
      theLayer.timeRemap.setValueAtTime(theLayer.inPoint, theLayer.inPoint - theLayer.startTime);
      //delete the old key which should be before the current one
      while (theLayer.timeRemap.numKeys > 1) {
        theLayer.timeRemap.removeKey(1);
      }
    }
  }
}
app.endUndoGroup();