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• Basic Video Palette Conversion

  20 août 2011, par Multimedia Mike — General, Python

  How do you take a 24-bit RGB image and convert it to an 8-bit paletted image for the purpose of compression using a codec that requires 8-bit input images ? Seems simple enough and that’s what I’m tackling in this post.

  Ask FFmpeg/Libav To Do It
  Ideally, FFmpeg / Libav should be able to handle this automatically. Indeed, FFmpeg used to be able to, at least at the time I wrote this post about ZMBV and was unhappy with FFmpeg’s default results. Somewhere along the line, FFmpeg and Libav lost the ability to do this. I suspect it got removed during some swscale refactoring.

  Still, there’s no telling if the old system would have computed palettes correctly for QuickTime files.

  Distance Approach
  When I started writing my SMC video encoder, I needed to convert RGB (from PNG files) to PAL8 colorspace. The path of least resistance was to match the pixels in the input image to the default 256-color palette that QuickTime assumes (and is hardcoded into FFmpeg/Libav).

  How to perform the matching ? Find the palette entry that is closest to a given input pixel, where "closest" is the minimum distance as computed by the usual distance formula (square root of the sum of the squares of the diffs of all the components).

  
  
  

  That means for each pixel in an image, check the pixel against 256 palette entries (early termination is possible if an acceptable threshold is met). As you might imagine, this can be a bit time-consuming. I wondered about a faster approach...

  Lookup Table
  I think this is the approach that FFmpeg used to use, but I went and derived it for myself after studying the default QuickTime palette table. There’s a pattern there— all of the RGB entries are comprised of combinations of 6 values — 0x00, 0x33, 0x66, 0x99, 0xCC, and 0xFF. If you mix and match these for red, green, and blue values, you come up with 6 * 6 * 6 = 216 different colors. This happens to be identical to the web-safe color palette.

  The first (0th) entry in the table is (FF, FF, FF), followed by (FF, FF, CC), (FF, FF, 99), and on down to (FF, FF, 00) when the green component gets knocked down and step and the next color is (FF, CC, FF). The first 36 palette entries in the table all have a red component of 0xFF. Thus, if an input RGB pixel has a red color closest to 0xFF, it must map to one of those first 36 entries.

  I created a table which maps indices 0..215 to values from 5..0. Each of the R, G, and B components of an input pixel are used to index into this table and derive 3 indices ri, gi, and bi. Finally, the index into the palette table is given by :

    index = ri * 36 + gi * 6 + bi
  

  For example, the pixel (0xFE, 0xFE, 0x01) would yield ri, gi, and bi values of 0, 0, and 5. Therefore :

    index = 0 * 36 + 0 * 6 + 5
  

  The palette index is 5, which maps to color (0xFF, 0xFF, 0x00).

  Validation
  So I was pretty pleased with myself for coming up with that. Now, ideally, swapping out one algorithm for another in my SMC encoder should yield identical results. That wasn’t the case, initially.

  One problem is that the regulation QuickTime palette actually has 40 more entries above and beyond the typical 216-entry color cube (rounding out the grand total of 256 colors). Thus, using the distance approach with the full default table provides for a little more accuracy.

  However, there still seems to be a problem. Let’s check our old standby, the Big Buck Bunny logo image :

  
  
  Distance approach using the full 256-color QuickTime default palette
  
  
  Distance approach using the 216-color palette
  
  
  Table lookup approach using the 216-color palette
  

  I can’t quite account for that big red splotch there. That’s the most notable difference between images 1 and 2 and the only visible difference between images 2 and 3.

  To prove to myself that the distance approach is equivalent to the table approach, I wrote a Python script to iterate through all possible RGB combinations and verify the equivalence. If you’re not up on your base 2 math, that’s 2²⁴ or 16,777,216 colors to run through. I used Python’s multiprocessing module to great effect and really maximized a Core i7 CPU with 8 hardware threads.

  So I’m confident that the palette conversion techniques are sound. The red spot is probably attributable to a bug in my WIP SMC encoder.

  Source Code
  Update August 23, 2011 : Here’s the Python code I used for proving equivalence between the 2 approaches. In terms of leveraging multiple CPUs, it’s possibly the best program I have written to date.

  PLAIN TEXT

  PYTHON :

  1. # !/usr/bin/python
  2.  
  3. from multiprocessing import Pool
  4.  
  5. palette = []
  6. pal8_table = []
  7.  
  8. def process_r(r) :
  9.  counts = []
  10.  
  11.  for i in xrange(216) :
  12.   counts.append(0)
  13.  
  14.  print "r = %d" % (r)
  15.  for g in xrange(256) :
  16.   for b in xrange(256) :
  17.    min_dsqrd = 0xFFFFFFFF
  18.    best_index = 0
  19.    for i in xrange(len(palette)) :
  20.     dr = palette[i][0] - r
  21.     dg = palette[i][1] - g
  22.     db = palette[i][2] - b
  23.     dsqrd = dr * dr + dg * dg + db * db
  24.     if dsqrd <min_dsqrd :
  25.      min_dsqrd = dsqrd
  26.      best_index = i
  27.    counts[best_index] += 1
  28.  
  29.    # check if the distance approach deviates from the table-based approach
  30.    i = best_index
  31.    r = palette[i][0]
  32.    g = palette[i][1]
  33.    b = palette[i][2]
  34.    ri = pal8_table[r]
  35.    gi = pal8_table[g]
  36.    bi = pal8_table[b]
  37.    table_index = ri * 36 + gi * 6 + bi ;
  38.    if table_index != best_index :
  39.     print "(0x%02X 0x%02X 0x%02X) : distance index = %d, table index = %d" % (r, g, b, best_index, table_index)
  40.  
  41.  return counts
  42.  
  43. if __name__ == ’__main__’ :
  44.  counts = []
  45.  for i in xrange(216) :
  46.   counts.append(0)
  47.  
  48.  # initialize reference palette
  49.  color_steps = [ 0xFF, 0xCC, 0x99, 0x66, 0x33, 0x00 ]
  50.  for r in color_steps :
  51.   for g in color_steps :
  52.    for b in color_steps :
  53.     palette.append([r, g, b])
  54.  
  55.  # initialize palette conversion table
  56.  for i in range(0, 26) :
  57.   pal8_table.append(5)
  58.  for i in range(26, 77) :
  59.   pal8_table.append(4)
  60.  for i in range(77, 128) :
  61.   pal8_table.append(3)
  62.  for i in range(128, 179) :
  63.   pal8_table.append(2)
  64.  for i in range(179, 230) :
  65.   pal8_table.append(1)
  66.  for i in range(230, 256) :
  67.   pal8_table.append(0)
  68.  
  69.  # create a pool of worker threads and break up the overall job
  70.  pool = Pool()
  71.  it = pool.imap_unordered(process_r, range(256))
  72.  try :
  73.   while 1 :
  74.    partial_counts = it.next()
  75.    for i in xrange(216) :
  76.     counts[i] += partial_counts[i]
  77.  except StopIteration :
  78.   pass
  79.  
  80.  print "index, count, red, green, blue"
  81.  for i in xrange(len(counts)) :
  82.   print "%d, %d, %d, %d, %d" % (i, counts[i], palette[i][0], palette[i][1], palette[i][2])

• Revision 28930 : on bouge

  31 mai 2009, par ben.spip@… — Log

  on bouge

• FFMPEG is not working in app service post deployment

  1er mars 2024, par Tushar Gupta

  I using the Xabe.FFmpeg package to generate clips from a video. The code is basically converting a video to multiple clips and it's working fine in my local but whenever I am uploading the code to app service the code is not working.

  &#xA;

  Stack : .net&#xA;App Service OS : Windows

  &#xA;

  Error :

  &#xA;

  2024-02-13 08:57:34.092 +00:00 [Error] Microsoft.AspNetCore.Server.IIS.Core.IISHttpServer : Connection ID "16573246629528734243", Request ID "40000a24-0000-e600-b63f-84710c7967bb" : An unhandled exception was thrown by the application.System.ComponentModel.Win32Exception (193) : An error occurred trying to start process 'C :\home\site\wwwroot\Controllers\ffmpeg\bin\ffmpeg.exe' with working directory 'C :\home\site\wwwroot'. The specified executable is not a valid application for this OS platform.at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)at System.Diagnostics.Process.Start()at Xabe.FFmpeg.FFmpeg.RunProcess(String args, String processPath, Nullable1 priority, Boolean standardInput, Boolean standardOutput, Boolean standardError)at Xabe.FFmpeg.FFmpegWrapper.&lt;>c__DisplayClass14_0.<runprocess>b__0()at System.Threading.Tasks.Task</runprocess>1.InnerInvoke()at System.Threading.Tasks.Task.<>c.<.cctor>b__281_0(Object obj)at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)--- End of stack trace from previous location ---at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)--- End of stack trace from previous location ---at Xabe.FFmpeg.Conversion.Start(String parameters, CancellationToken cancellationToken)at ExtractResponseAPI.Controllers.HomeController.PrepareVideoClips(CloudBlockBlob sourceVideoBlob, TimeSpan startTime, TimeSpan endTime) in C :\Users\tushar.h.gupta\source\repos\ExtractResponseAPI\Controllers\HomeController.cs:line 164at ExtractResponseAPI.Controllers.HomeController.GetIntervalsAsync(String query) in C :\Users\tushar.h.gupta\source\repos\ExtractResponseAPI\Controllers\HomeController.cs:line 60at ExtractResponseAPI.Controllers.HomeController.Get(String query) in C :\Users\tushar.h.gupta\source\repos\ExtractResponseAPI\Controllers\HomeController.cs:line 24at lambda_method4(Closure, Object)at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(ActionContext actionContext, IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.g__Awaited|12_0(ControllerActionInvoker invoker, ValueTask1 actionResultValueTask)at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<invokenextactionfilterasync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State&amp; next, Scope&amp; scope, Object&amp; state, Boolean&amp; isCompleted)at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<invokeinnerfilterasync>g__Awaited|13_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<invokefilterpipelineasync>g__Awaited|20_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<invokeasync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<invokeasync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)at Microsoft.AspNetCore.Server.IIS.Core.IISHttpContextOfT</invokeasync></invokeasync></invokefilterpipelineasync></invokeinnerfilterasync></invokenextactionfilterasync>1.ProcessRequestAsync()

  &#xA;

  Code :

  &#xA;

  private async Task> PrepareVideoClips(CloudBlockBlob sourceVideoBlob, TimeSpan startTime, TimeSpan endTime)&#xA;{&#xA;    string tempDirectory = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());&#xA;&#xA;    // Create a temporary directory to store clips locally&#xA;    Directory.CreateDirectory(tempDirectory);&#xA;&#xA;    // Download the source video&#xA;    string sourceVideoPath = Path.Combine(tempDirectory, "sourceVideo.mp4");&#xA;    await sourceVideoBlob.DownloadToFileAsync(sourceVideoPath, FileMode.Create);&#xA;    FFmpeg.SetExecutablesPath("Controllers\\ffmpeg\\bin\\");&#xA;    // Use FFmpegCore to trim the video&#xA;    string outputVideoPath = Path.Combine(tempDirectory, "output.mp4");&#xA;&#xA;    await FFmpeg.Conversions.New()&#xA;        .AddParameter($"-ss {startTime.TotalSeconds}") // Start time&#xA;        .AddParameter($"-i {sourceVideoPath}")&#xA;        .AddParameter($"-to {(endTime - startTime).TotalSeconds}") // Duration&#xA;        .SetOutput(outputVideoPath)&#xA;        .Start();&#xA;&#xA;    // Return a list of file paths for the clips&#xA;    return new List<string> { outputVideoPath };&#xA;}&#xA;</string>

  &#xA;

  I tried deploying the app service using different OS but still facing the same issue, I also tried using different packages but the result is same.

  &#xA;