Answers.Body: <p><code>[0:v][v0]</code> and <code>[2:v][v2]</code> don't contain any filter declaration, and thus ffmpeg complains. Insert whichever filter is desired in between the input and output pads.</p>
  <p>I suppose you meant to scale the heights of the 3 inputs to match. In which case,</p>
  <p><code>ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \ -filter_complex &quot;[0:v]scale=-1:1440[v0];[1:v]scale=-1:1440[v1];[2:v]scale=-1:1440[v2];[v0][v1][v2]hstack=inputs=3&quot; -map &quot;[v]&quot; -map 1:a output.mp4</code></p>
  <p>on input processing with -filter_complex, you use this pattern:</p>
  <p><code>[input_number:v]operation[tmp_output_name]</code></p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-12-04T07:46:26.510Z
Answers.Text: I suggest an edit, please take a look.
Answers.UserDisplayName: Adi Prasetyo
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-12-02T05:11:49.197Z
Answers.LastActivityDate: 2020-12-04T08:04:32.437Z
Answers.LastEditDate: 2020-12-04T08:04:32.437Z
Answers.UserDisplayName: Gyan
Body: <p>the desired output is combining 3 vids, and using scale or pad (do some processing on the input).</p>
  <pre><code>ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \
  -filter_complex &quot;[0:v][v0];[1:v]scale=-1:1440[v1];[2:v][v2],[v0][v1][v2]hstack=inputs=3&quot;
   -map &quot;[v]&quot; -map 1:a output.mp4
  </code></pre>
  <p>give:</p>
  <blockquote>
  <p>[AVFilterGraph @ 0xec0a80] No such filter: ''
  Error initializing complex filters.
  Invalid argument</p>
  </blockquote>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-11-29T05:28:31.903Z
Comments.Text: What are `[0:v][v0];` and `[2:v][v2],` meant to do?
Comments.UserDisplayName: Gyan
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-12-01T20:37:00.653Z
Comments.Text: there is the confusing part, the pattern on filtergraph, [in]opt_preprocess[out]?
Comments.UserDisplayName: Adi Prasetyo
Tags: <ffmpeg>
Title: no such filter, when using filter complex with pre-scale
UserDisplayName: Adi Prasetyo
ThreadText: # no such filter, when using filter complex with pre-scale
  
  (Asked by: username_0 on 2020-11-28)
  
  the desired output is combining 3 vids, and using scale or pad (do some processing on the input).
  
  ```
  ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \
  -filter_complex "[0:v][v0];[1:v]scale=-1:1440[v1];[2:v][v2],[v0][v1][v2]hstack=inputs=3"
   -map "[v]" -map 1:a output.mp4
  
  ```
  
  give:
  
  > [AVFilterGraph @ 0xec0a80] No such filter: ''
  > Error initializing complex filters.
  > Invalid argument
  
  **Tags:** ffmpeg
  
  > **Comments:**
  > **username_1**: What are `[0:v][v0];` and `[2:v][v2],` meant to do?  
  > **username_0**: there is the confusing part, the pattern on filtergraph, [in]opt_preprocess[out]?  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_1 on 2020-12-02. Score: 2 | Accepted answer
  
  `[0:v][v0]` and `[2:v][v2]` don't contain any filter declaration, and thus ffmpeg complains. Insert whichever filter is desired in between the input and output pads.
  
  I suppose you meant to scale the heights of the 3 inputs to match. In which case,
  
  `ffmpeg -i in.mp4 -i in2.mkv -i in3.mkv \ -filter_complex "[0:v]scale=-1:1440[v0];[1:v]scale=-1:1440[v1];[2:v]scale=-1:1440[v2];[v0][v1][v2]hstack=inputs=3" -map "[v]" -map 1:a output.mp4`
  
  on input processing with -filter\_complex, you use this pattern:
  
  `[input_number:v]operation[tmp_output_name]`
  
  > **Comments:**
  > **username_0**: I suggest an edit, please take a look.  
  
  ---
  
  