Explicitly state stream order in FFmpeg's guide

[allan-kg]

AFAIK, the FFmpeg standard is : first input is the reference/input, second input is the distorted/output.

• It applies for PSNR and SSIM, for example.
  • One could, for example, do like [reference][distorted]ssim or just ssim when applying filters.
    • Assuming that reference and distorted were properly set.

VMAF is the very only tool that states the reverse order : [distorted][reference]libvmaf.

• VMAF's documentation regarding FFmpeg
  • IMPORTANT : the documentation does NOT explicitly state the order, it just implies it by example.
• This is counterintuitive for FFmpeg's context.

I've noticed that the results do differ when you change the order

• Using the example from the page
  • [distorted][reference] : VMAF score: 76.668905
    • According to the guide this would be the correct order.
      • But counterintuitive in the context of FFmpeg.
  • [reference][distorted] : VMAF score: 83.783316

So, now for the issue:

1. The best scenario for FFmpeg would be to have it ensured to be [reference][distorted]
   • It is not about me, it is about a context that always considers the first input as the reference/input.
2. At least a very explicit ordering documentation is necessary.

[allan-kg]

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