FFMpeg Transitions -- Part 3

Our third post concerning video transitions.  Be sure to read our previous posts, I suggest reading them once for knowledge, a second time purely for fun, and periodically thereafter for continued inspiration.

The previous posts focused primarily on the overlay filter.  This post will focus on the blend filter applied dynamically with respect to time or position.

Cross Fade Effect

The blend filter is most typically used in cross fading from one video to another.  Briefly discussed in the first post, the general idea is to provide the filter with two video frames and a fractional value to be applied to each.  For example, a 50/50 split will give an equal weight to each video.  These weights can dynamically change with respect to time.  Cross-fading takes the effect begins by applying a weight of 1.0 to the first video, 0.0 to the second video, then linearly decreasing the weight of the first video while simultaneously increasing the weight of the second.  Finally, ending with a weight of 0.0 for the first video, 1.0 for the second.  Easy, Peasy.

Let's take a look at the full filter example;

$ ffmpeg -i image01.mp4 -i image02.mp4  -filter_complex "[0:v][1:v]blend=all_expr='A*(1-min(T/1,1))+B*(min(T/1,1))'" blend.mp4

Note; like past posts, the denominator in the (T/1) implies that the transition will take 1 second.  Playing with that value will speed up or slow down the morphing.

Location-Based Blend

The previous blend filter is applied uniformly to the entire frame.  Using conditionals and (x,y) locations we can base the blending factors on position.  Consider a simple blend condition where the left-most half of the image uses the first video, the right-most half uses the second video.  Conceptually, the blend would look something like this;

'if(lte(X,H/2),A,B)'

When X is less than the middle of the video frame vertical center, apply A, otherwise B. 

This example, not particularly useful, but imagine applying a moving range rather than simply the middle of the vertical axis.  As that range moved from bottom-to-top the effect would emulate a rising curtain;

$ ffmpeg -i image02.mp4 -i image01.mp4 -filter_complex "[0:v][1:v]blend=all_expr='if(lte(Y,(H-T/1*H)),A,B)'" curtainUp.mp4

Notice rather than (H/2), the range starts at H and progresses to 0 linearly during the 1 second duration.

Similarly, you can perform a curtain down effect like this;

$ ffmpeg -i image01.mp4 -i image02.mp4 -filter_complex "[0:v][1:v]blend=all_expr='if(gte(Y,(T/1*H)),A,B)'" curtainDown.mp4

Using the center point as a start or end point and mirroring the effect on each 1/2 of the frame can open the door to other effects, like;

$ ffmpeg -i image02.mp4 -i image01.mp4 -filter_complex "[0:v][1:v]blend=all_expr='if(between(X,(W/2-T/1*W/2),(W/2+T/1*W/2)),B,A)'" verticalOpen.mp4

$ ffmpeg -i image01.mp4 -i image02.mp4 -filter_complex "[0:v][1:v]blend=all_expr='if(between(Y,(H/2-T/1*H/2),(H/2+T/1*H/2)),B,A)'" horizontalOpen.mp4