My secret.
It's really two stereograms.
Layer 1 is the background and the inside of the heart with the floating smaller heart. It uses the bright pink pattern.
Layer 2 is the front of the large heart. The pattern has small white objects (the object is not important--it can be any small object or text, symbols etc.) on a black background.
The second stereogram is placed over the first and Medhi Eraser Genuine (or Classic) is used to make the black transparent.
The two depth image hearts both start with the same color and then I used a Contour to create the concave and convex versions. If the central value of gray is not the same, the two hearts will not appear as one.
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