Dissolve

The word is an editing word. A cut replaces one shot with the next on a single frame; a dissolve spends time on the replacement, and that time is what tells the audience the two shots belong to each other. Interfaces borrowed the vocabulary along with the effect, and they borrowed it loosely: on the web a dissolve is any change of content delivered by opacity alone, with nothing sliding, scaling, or arriving from a direction. That is the whole grammar of it. Movement would say where the new content came from, and a dissolve is for the changes where the answer is nowhere, because the new thing occupies exactly the place and the job the old thing had.

The mechanism has two shapes, and they are worth keeping apart. A crossfade is two elements sharing one span, both partly visible at the midpoint, which is what a film editor means by a cross dissolve. The other shape is one element whose contents are exchanged at the low point of a single fade: down to zero, swap, back up. Its midpoint shows neither side, which is why an editor would call it a dip, and why it is the only version available when there is genuinely one thing (an image element whose src changes, a slot painted by a canvas) rather than two. The single-slot version is more forgiving when the two sides are different shapes, since nothing is ever legible on top of anything else, and it costs a beat of emptiness that a crossfade does not.

Timing decides whether it reads as one thing becoming another or as two separate events. Under roughly 200 milliseconds a dissolve is close enough to a cut that it stops carrying any relationship at all; over roughly 600 it reads as a scene change. Linear opacity is usually better than an eased curve, because the eye is more sensitive to change at the dark end and easing both halves tends to sag visibly in the middle. If the swap crosses the midpoint, do the content change exactly there, at zero opacity, so nobody ever sees the seam.

Reach for it when the change has no direction to report: a photo replaced by the next in a gallery, a chart’s range swapped, a status word updated in place. Reach for something else when it does. Direction belongs to a slide transition, arrival belongs to an entrance animation, and a plain fade is the case where there is nothing on the other side at all, only presence becoming absence.