ffmpeg - 移动覆盖/文本命令如何工作? [英] ffmpeg - how does moving overlay / text command work?

问题描述

在 Ffmpeg 中，您可以创建移动文本:

In Ffmpeg you can create moving text:

ffmpeg -y -t 10 -s qcif -f rawvideo -pix_fmt rgb24 -s 1280x720 -i /dev/zero -g 1 -r 24 -vf drawtext="fontfile=~/fonts/Trebuchet_MS.ttf:text='thing crawls':fontsize=155:fontcolor=red:y=h-20*t" wow.mpg

所以这会给我一个黑色的框架，东西爬行"从下往上慢慢移动..

So this will give me a black frame with "thing crawls" slowly going from bottom up..

如果我知道视频的长度(20 秒)并且想要创建东西坠落"，它从时间 0 的屏幕顶部开始，一直到屏幕底部直到 00:00:20、我该怎么做?

If I know the length of the video (20 seconds) and want to, for example create "thing falls" that starts at the top of the screen at time 0 and goes to the bottom of the screen until 00:00:20, how do I do that?

我也可以创建文本从上到下开始但停在屏幕中间的情况吗?

Also can I create the situation where the text will start going from top to bottom, but stop at the middle of the screen?

推荐答案

FFmpeg 文档给出使用 drawtext 过滤器时必须使用的变量的完整列表，但对于移动文本，有一些特别有趣:

The FFmpeg docs give a full listing of the variables that you have to work with when using the drawtext filter, but for mobile text there are a few of particular interest:

‘n’
输入帧数，从0开始

‘n’
the number of input frame, starting from 0

‘t’
时间戳以秒表示，如果输入时间戳未知，则为 NAN

‘t’
timestamp expressed in seconds, NAN if the input timestamp is unknown

通过这些，您可以根据已经看到的帧数设置文本位置.这就是您示例中的 y=h-20*t 表达式所做的.随着 t 的增加，随着 h-20*t 的减少，文本会越来越靠近视频的顶部.

With these, you can set the text position in relation to how many frames have already been seen. That's what the y=h-20*t expression in your example is doing. As t increases, the text moves closer to the top of the video as h-20*t decreases.

为了让你的例子东西倒下"，你需要一个像 20*t 这样的术语.因为y位置从视频顶部的0开始，随着t的增加，它会向下移动屏幕.

To make your example "thing falls", you'd want a term like 20*t instead. Because the y position starts from 0 at the top of the video, as t increases, it will move down the screen.

对于停在屏幕中间的文本，您可能会做一些花哨的数学运算，或者只使用 FFmpeg 的 丰富的逻辑功能.类似 y=t*20*lte(t*20,h/2) + h/2*gt(t*20,h/2) 之类的东西，它在 t* 的同时移动文本20 小于高度的一半，一旦 t*20 大于一半，则将 y 位置保持在 h/2高度.

For text that stops in the middle of the screen, you could probably do some fancy math, or just use FFmpeg's rich set of logical functions. Something like y=t*20*lte(t*20,h/2) + h/2*gt(t*20,h/2) which moves the text while t*20 is less than half the height, then keeps the y position at h/2 once t*20 is greater than half the height.

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