@as1ama1and looping the video through a set of 3 shader actors does quite a good job.
Freeze shader: grabs a frame at the start of each cycle and holds it. It does this through a feedback loop (output → pass-through shader → back into video in 2), because Isadora's GLSL actor has no persistent buffer.
Pass shader: an empty shader that just closes the loop.
Mix shader: lays the frozen frame over the live image and fades it out over time. blend mode decides whether the clone shows only where it's darker or lighter than the live image, so the live video is never covered.
A Wave Generator (sawtooth, 0–100) drives the clock input. One cycle equals one snapshot, so freq sets the cycle time.
Here's the patch: echo trails
Best Michel