[FFmpeg-user] How add expression in drawtext's text

[Jim DeLaHunt]

On 2018-06-03 20:54, qw wrote:
> Hi,
>
>
> I can use the following command to add frame number on video:
>
>
> /usr/local/bin/ffmpeg -t 180 -i input.mp4 -acodec libfdk_aac -ac 2 -b:a 48k -vcodec libx264 -b:v 500k -g 25 \
> -vf "fps=fps=25, scale=w=320:h=240, drawtext='box=1:fontcolor=red:fontsize=20:fontfile=OpenSans-Regular.ttf:text=%{frame_num}'" \
> -f mp4 out.mp4
>
>
> But the width of the text box will increase as frame number increases. I want to make the width of the text box unchanged. If the largest frame number is 99999, I want to make the width of the text is of 5 numbers. How to do it?
As Gyan pointed out, the %{eif...} expression will give you padding of 
zeroes on the left. Specifically, here is your example modified to use 
this feature:

/usr/local/bin/ffmpeg -t 180 -i input.mp4 -acodec libfdk_aac -ac 2 -b:a 48k -vcodec libx264 -b:v 500k -g 25 \
-vf "fps=fps=25, scale=w=320:h=240, drawtext='box=1:fontcolor=red:fontsize=20:fontfile=OpenSans-Regular.ttf:text=%{eif\:frame_num\:d\:5}'" \
-f mp4 out.mp4

I don't see a direct way to have the drawtext filter provide blank space 
padding to the left of your expression. But, you can draw the background 
box with a drawbox filter, and then draw the text without a box using 
the drawtext filter. You will have to adjust some numbers by trial and 
error to make the result look right.

Here is your example modified to give the appearance of a frame number 
left-padded with blanks:

/usr/local/bin/ffmpeg -t 180 -i input.mp4 -acodec libfdk_aac -ac 2 -b:a 48k -vcodec libx264 -b:v 500k -g 25 \
-vf "fps=fps=25, scale=w=320:h=240, \
drawbox=x=0:y=0:width=60:height=20*0.65:thickness=fill:color=white, \
drawtext='x=0+60-(20*0.6)*if(gt(n,9999),5,if(gt(n,999),4,if(gt(n,99),3,if(gt(n,9),2,1)))):'\
'y=0:box=0:fontcolor=red:fontsize=20:fontfile=OpenSans-Regular.ttf:text=%{frame_num}'" \
-f mp4 out.mp4

Here is an explanation of the special numbers I used. They all depend on 
the proportions of the glyphs drawn by the font which you choose.

Drawbox's "height" of 20*0.65 is the fontsize you use in drawtext, 
multiplied by a number based on the size of box which drawtext would 
have drawn. Increase it a little bit to give a margin for error. 0.65 
worked for the font I tested with.

Drawbox's "x" and "y" values are set to the upper-left corner of the box 
on which the text is drawn.

Drawtext's "x" value can be thought of as "leftside location"+"width of 
box" - "width of one digit" * "number of digits".

The "leftside location" is the same number as drawbox's "x" value.
The "width of box" value is drawbox's "width" value.
The "width of one digit expression", here "(20*0.6)", is drawbox's 
fontsize value times a constant based on the width of the font's 
digits.  0.6 worked for the font I tested with.
The "number of digits" expressions is a chain of "if n>9999 then 4 else 
if ..." expressions. It returns 5, 4, 3, 2, or 1, depending on the value 
of "n". Note that "n" and "frame_num" are different names for the same 
thing, but in an expression for "x", you can only use the name "n".

Drawtext's "y" value is set to the same number as drawbox's "y" value.

Now I tested these examples with my video and my font. They may not work 
exactly the same in your system. You may have to adjust the numbers. You 
may also have to fix any silly mistakes I made. But I hope this helps 
you solve your problem.

-- 
     --Jim DeLaHunt, jdlh at jdlh.com     http://blog.jdlh.com/ (http://jdlh.com/)
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