How to create "curtain" ("barn doors") and "circle wipe" effects in FFmpeg?

I was trying to create two effects by using FFmpeg: "curtain" ("barn doors") and "circle wipe" but had no success. The basic idea is to start from black screen (100% of black padding) and during 1 second (or more) display the entire video (the black padding on the top and on the bottom should get less and less).

How can I do this?

Answer

Generally, there are two methods to achieve such effects.

#1

Create a grayscale animated mask and attach that as the alpha channel to the video. Perform an overlay of the video over a black canvas.

Pros: one-time op to generate the mask. Much faster than method 2.
Cons: Animation parameters like size, speed, duration are fixed. These can be tweaked within ffmpeg but results may not be smooth. Mask file needs to be kept and accessible.

#2

Use the blend filter with a blank canvas as bottom layer and set expressions to carry out the effect.

Pros: customizable. May need some trial and error to achieve exact effect.
Cons: much slower than method 1. At smaller resolutions, result is coarser than a properly anti-aliased mask. If you don't remember expressions or how they work, expressions have to be saved as well and tweaking may be hard.

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That said, here are the two effects using the blend filter. Both effects start at 2 seconds and happen over 3 seconds.

Curtains, unveiled vertically

ffmpeg -i in.mp4 -filter_complex \
         "[0]format=yuv444p,split=2[bg][v];[bg]drawbox=t=fill[bg]; \
          [v][bg]blend=all_expr='if(lte(2*abs(Y-H/2)/H,(T-2)/3),A,B)',\
             format=yuv420p" \
 out.mp4

A faster method can be adapted from first command here. That command does it horizontally.

Circle Wipe, expanding

ffmpeg -i in.mp4 -filter_complex \
         "[0]format=yuv444p,split=2[bg][v];[bg]drawbox=t=fill[bg]; \
          [v][bg]blend=all_expr='if(lte(sqrt(pow(X-W/2,2)+pow(Y-H/2,2))/sqrt(pow(W,2)+pow(H,2)),(T-2)/3)/2,A,B)',\
             format=yuv420p" \
 out.mp4