Customize a Gradient Wipe transition

You can use a grayscale image as a gradient wipe. In a gradient wipe, image B fills the black area of the grayscale image and then shows through each level of gray as the transition progresses until the white area becomes transparent.

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Gradient wipe source image (far left) and resulting transition

1. In the Effects panel, expand the Video Transitions bin and the Wipe bin inside it.
2. Drag the Gradient Wipe transition from the Wipe bin to an edit point between clips in a Timeline panel.
3. Click Select Image, and then double‑click the file you want to use as the gradient wipe. The image appears in the Gradient Wipe Settings dialog box.
4. Adjust the softness of the transition’s edges by dragging the Softness slider. As you drag the slider to the right, image A increasingly shows through image B. Click OK.
   Note: To change the gradient image or the softness, click Custom in the Effect Controls panel.

Gradient Wipe effect (adobe.com)

 community help (adobe.com)