ffmpeg - 移动overlay / text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：

  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 crawl'：fontsize = 155：fontcolor = red：y = h-20 * twow.mpg 
  

所以这将给我一个黑色的框架，爬行慢慢去从下到上..

如果我知道视频的长度（20秒），并想要，例如创建事情下降，从顶部开始屏幕在时间0，并且到屏幕的底部直到00:00:20，我该怎么做？

还可以创建文本的情况开始从上到下，但是在屏幕中间停止？

解决方案

FFmpeg文档给出了使用drawtext过滤器时必须使用的变量的完整列表，但是移动文本有一些特别的兴趣：

'n'

输入框的数量，从0

't'

以秒为单位的时间戳，如果输入时间戳未知，则为NAN

通过这些，您可以设置与已经看到的帧数相关的文本位置。这就是你的例子中的 y = h-20 * t 表达式。随着 t 增加，文本将更接近视频的顶部，因为 h-20 * t 减少。 p>

为了使您的例子事情落空，您需要一个类似 20 * t 的术语。因为 y 位置从视频顶部的0开始，因为 t 增加，它将向下移动屏幕

对于在屏幕中间停止的文本，您可能会做一些奇特的数学，或者只是使用FFmpeg的丰富的逻辑功能集。像 y = t * 20 * lte（t * 20，h / 2）+ h / 2 * gt（t * 20，h / 2）而 t * 20 不到高度的一半，然后将 y 的位置保持在 h / 2 一次 t * 20 大于身高的一半。

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

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

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?

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

解决方案

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’
the number of input frame, starting from 0

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

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.

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.

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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