Reveal animation

The content is already there. It has been laid out, it is sitting at its final position, and the only thing animating is the shape of the window you are allowed to see it through. Android names this precisely: ViewAnimationUtils.createCircularReveal takes a view, a centre point, a starting radius and a final one, and animates a clipping circle between them. On the web the same move is a clip-path interpolated from a small circle() to one large enough to cover the farthest corner, or a mask-image whose gradient stop travels. Nothing is translated, nothing is faded, nothing is inserted. The box was full the whole time.

That is what makes it feel different from an entrance. A slide says the content came from somewhere; a fade says it materialised. A reveal says it was always here and you are only now being let in, which is why the origin point does so much work. Grow the circle from the button that was pressed and the panel reads as having been opened by that press. Grow it from a corner, or from the middle of the screen, and the causal link is gone and the effect turns decorative. The Material pattern of expanding a floating action button into the sheet it summons is the whole idea in one gesture: the control does not move aside, it becomes the surface.

Two neighbours are easy to confuse with it. A wipe is a directional sweep whose edge crosses the frame, so it has a front and a back and it is usually a transition between two states rather than the arrival of one. A fade changes opacity, which means the content is on screen and merely transparent, so text under it can still be selected mid-animation and a half-faded element reads as unresolved rather than as partly uncovered. A reveal keeps the content fully opaque and simply refuses to paint the part outside the shape, so at any frame what you can see is finished. Exit animation is the same mechanism run backwards, and the pairing is worth stating: a reveal that leaves by fading is a small inconsistency people feel without naming.

Three practical notes. The clip has to start somewhere the reader can accept, so a start radius of exactly zero is often worse than a small one, because a shape appearing from literally nothing has no anchor. The room the content will take must already be reserved, since the element occupies its full box throughout and a layout that only settles when the animation ends was never really doing a reveal. And under a stated reduced-motion preference the correct answer is not a slower circle but no circle at all: the content is what matters and it was never anywhere else.