title: FFmpeg fontcolor_expr to dynamically change fontcolor over time
body: <p>I'm using Fluent-FFmpeg with Node.JS to create videos with text overlaid on them and I'm wanting to change the color of the text as the video progresses.</p>
  
  <p>My filters follow this pattern:</p>
  
  <pre><code>drawtext=enable='between(t,18.93,20.28)':fontfile=fonts/cousine-bold.ttf:fontsize=144:fontcolor_expr=%{expr\\\: if(between(t\\, 0\\, 20)\\, 888888\\, 111111)}:x=82:y=288:text='PROGRAMMING'
  </code></pre>
  
  <p>And it prints out this stuff:</p>
  
  <pre><code>ffmpeg version 3.3.2 Copyright (c) 2000-2017 the FFmpeg developers
    built with Apple LLVM version 8.1.0 (clang-802.0.42)
    configuration: --prefix=/usr/local/Cellar/ffmpeg/3.3.2 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libfreetype --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-opencl --disable-lzma --enable-vda
    libavutil      55. 58.100 / 55. 58.100
    libavcodec     57. 89.100 / 57. 89.100
    libavformat    57. 71.100 / 57. 71.100
    libavdevice    57.  6.100 / 57.  6.100
    libavfilter     6. 82.100 /  6. 82.100
    libavresample   3.  5.  0 /  3.  5.  0
    libswscale      4.  6.100 /  4.  6.100
    libswresample   2.  7.100 /  2.  7.100
    libpostproc    54.  5.100 / 54.  5.100
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'uploads/0c719e39820366bc62c8fd66a6327828':
    Metadata:
      major_brand     : qt  
      minor_version   : 0
      compatible_brands: qt  
      creation_time   : 2017-06-23T00:19:46.000000Z
      com.apple.quicktime.make: Apple
      com.apple.quicktime.model: MacBookPro9,2
      com.apple.quicktime.software: Mac OS X 10.12.5 (16F73)
      com.apple.quicktime.creationdate: 2017-06-22T17:18:42-0700
    Duration: 00:00:22.00, start: 0.000000, bitrate: 3408 kb/s
      Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, bt709, progressive), 1280x720 [SAR 1:1 DAR 16:9], 3091 kb/s, 15 fps, 15 tbr, 30k tbn, 50 tbc (default)
      Metadata:
        creation_time   : 2017-06-23T00:19:46.000000Z
        handler_name    : Core Media Data Handler
        encoder         : H.264
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 316 kb/s (default)
      Metadata:
        creation_time   : 2017-06-23T00:19:46.000000Z
        handler_name    : Core Media Data Handler
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (h264 (native) -&gt; h264 (libx264))
    Stream #0:1 -&gt; #0:1 (aac (native) -&gt; aac (native))
  Press [q] to stop, [?] for help
  [libx264 @ 0x7ff72e810a00] using SAR=1/1
  [libx264 @ 0x7ff72e810a00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
  [libx264 @ 0x7ff72e810a00] profile High, level 3.1
  [libx264 @ 0x7ff72e810a00] 264 - core 148 r2748 97eaef2 - H.264/MPEG-4 AVC codec - Copyleft 2003-2016 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0     direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=15 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
  Output #0, mp4, to './outputFile.mp4':
    Metadata:
      major_brand     : qt  
      minor_version   : 0
      compatible_brands: qt  
      com.apple.quicktime.creationdate: 2017-06-22T17:18:42-0700
      com.apple.quicktime.make: Apple
      com.apple.quicktime.model: MacBookPro9,2
      com.apple.quicktime.software: Mac OS X 10.12.5 (16F73)
      encoder         : Lavf57.71.100
      Stream #0:0(und): Video: h264 (libx264) ([33][0][0][0] / 0x0021), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=-1--1, 15 fps, 15360 tbn, 15 tbc (default)
      Metadata:
        creation_time   : 2017-06-23T00:19:46.000000Z
        handler_name    : Core Media Data Handler
        encoder         : Lavc57.89.100 libx264
      Side data:
        cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
      Stream #0:1(und): Audio: aac (LC) ([64][0][0][0] / 0x0040), 44100 Hz, stereo, fltp, 128 kb/s (default)
      Metadata:
        creation_time   : 2017-06-23T00:19:46.000000Z
        handler_name    : Core Media Data Handler
        encoder         : Lavc57.89.100 aac
  frame=   52 fps=0.0 q=27.0 size=      37kB time=00:00:04.66 bitrate=  65.7kbits/s speed=8.74x    
  frame=   72 fps= 70 q=27.0 size=     187kB time=00:00:06.03 bitrate= 253.4kbits/s speed=5.83x    
  frame=   95 fps= 61 q=27.0 size=     358kB time=00:00:07.52 bitrate= 390.4kbits/s speed=4.82x    
  frame=  117 fps= 57 q=27.0 size=     569kB time=00:00:08.98 bitrate= 518.9kbits/s speed=4.35x    
  frame=  137 fps= 53 q=27.0 size=     732kB time=00:00:10.32 bitrate= 580.2kbits/s speed=4.03x    
  frame=  159 fps= 52 q=27.0 size=     942kB time=00:00:11.79 bitrate= 654.7kbits/s speed=3.84x    
  frame=  180 fps= 50 q=27.0 size=    1110kB time=00:00:13.18 bitrate= 689.5kbits/s speed=3.67x    
  frame=  204 fps= 50 q=27.0 size=    1331kB time=00:00:14.78 bitrate= 737.2kbits/s speed= 3.6x    
  frame=  223 fps= 48 q=27.0 size=    1486kB time=00:00:16.06 bitrate= 757.9kbits/s speed=3.49x    
  frame=  248 fps= 48 q=27.0 size=    1708kB time=00:00:17.71 bitrate= 789.7kbits/s speed=3.46x    
  frame=  266 fps= 47 q=27.0 size=    1859kB time=00:00:18.92 bitrate= 805.0kbits/s speed=3.35x    
  frame=  285 fps= 46 q=27.0 size=    2013kB time=00:00:20.19 bitrate= 816.4kbits/s speed=3.28x    
  [drawtext @ 0x7ff72d84bc00] Cannot find color '888888.000000'
      Last message repeated 9 times
  frame=  308 fps= 46 q=27.0 size=    2224kB time=00:00:21.75 bitrate= 837.5kbits/s speed=3.27x    
  frame=  330 fps= 46 q=27.0 size=    2401kB time=00:00:21.96 bitrate= 895.5kbits/s speed=3.06x    
  frame=  330 fps= 41 q=-1.0 Lsize=    2834kB time=00:00:22.00 bitrate=1055.0kbits/s speed=2.73x    
  video:2484kB audio:338kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.441003%
  [libx264 @ 0x7ff72e810a00] frame I:22    Avg QP:13.64  size: 47604
  [libx264 @ 0x7ff72e810a00] frame P:154   Avg QP:16.76  size:  8574
  [libx264 @ 0x7ff72e810a00] frame B:154   Avg QP:19.42  size:  1138
  [libx264 @ 0x7ff72e810a00] consecutive B-frames:  6.7% 93.3%  0.0%  0.0%
  [libx264 @ 0x7ff72e810a00] mb I  I16..4: 85.1% 13.3%  1.6%
  [libx264 @ 0x7ff72e810a00] mb P  I16..4:  5.5%  3.1%  0.0%  P16..4: 41.5%  4.0%  5.5%  0.0%  0.0%    skip:40.4%
  [libx264 @ 0x7ff72e810a00] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8: 14.7%  0.1%  0.0%  direct: 4.7%  skip:80.4%  L0:28.0% L1:71.4% BI: 0.7%
  [libx264 @ 0x7ff72e810a00] 8x8 transform intra:21.7% inter:84.8%
  [libx264 @ 0x7ff72e810a00] coded y,uvDC,uvAC intra: 60.5% 98.6% 92.9% inter: 6.0% 24.3% 3.1%
  [libx264 @ 0x7ff72e810a00] i16 v,h,dc,p: 35% 19% 43%  3%
  [libx264 @ 0x7ff72e810a00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 10%  7% 80%  1%  1%  1%  1%  0%  0%
  [libx264 @ 0x7ff72e810a00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 33% 20% 45%  0%  0%  0%  0%  0%  0%
  [libx264 @ 0x7ff72e810a00] i8c dc,h,v,p: 77%  9% 10%  4%
  [libx264 @ 0x7ff72e810a00] Weighted P-Frames: Y:0.0% UV:0.0%
  [libx264 @ 0x7ff72e810a00] ref P L0: 75.3%  0.3% 17.1%  7.3%
  [libx264 @ 0x7ff72e810a00] ref B L0: 80.0% 20.0%
  [libx264 @ 0x7ff72e810a00] kb/s:924.66
  [aac @ 0x7ff72e812200] Qavg: 2159.960
  </code></pre>
  
  <p>In particular this annoying bit:</p>
  
  <pre><code>[drawtext @ 0x7ff72d84bc00] Cannot find color '888888.000000'
  </code></pre>
  
  <p>However, it would be really nice to do something like this:</p>
  
  <pre><code>fontcolor_expr=%{expr\\\\: if(between(t\\, 0\\, 20)\\, pink\\, white)}
  </code></pre>
  
  <p>Which prints out this error:</p>
  
  <pre><code>[Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  frame=  287 fps= 46 q=27.0 size=    2023kB time=00:00:20.31 bitrate= 815.9kbits/s speed=3.27x    
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Undefined constant or missing '(' in 'pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] [Eval @ 0x7fff5f7760c0] Missing ')' or too many args in 'if(between(t,0,20),pink,white)'
  [Parsed_drawtext_0 @ 0x7ff3cf523300] Expression 'if(between(t, 0, 20), pink, white)' for the expr text expansion function is not valid
  </code></pre>
  
  <p>There aren't a lot of examples of fontcolor_expr, so I wanted to see if anyone had experience with this. Any tips on changing fonrcolors dynamically without creating multiple filters?</p>
  
  <p>Please note that <code>fontcolor_expr=white</code> and <code>fontcolor_expr=888888</code> are valid and create videos with a fixed font color.</p>