Delta file (difference file)

Problem

Compare two 8-bit video files by using a delta file (difference file).

Solution

ffmpeg                                     \
    -i input_file_1                        \
    -i input_file_2                        \
    -filter_complex                        \
        "[1] format=yuva444p,              \
             lut=c3=128,                   \
             negate [1_with_alpha];        \
         [0][1_with_alpha] overlay [out]"  \
    -map "[out]"                           \
    output_file

Command syntax

ffmpeg

starts the command

-i input_file_1

path, name and extension of the first input files

-i input_file_2

path, name and extension of the second input files

-filter_complex

A complex filter graph is needed:

• three filters are applied successively to the second input file (this is a filter chain):
  • an alpha channel is added,
  • the alpha channel is set to “medium grey” and
  • the colours are inverted;
• the overlay between the first input file and the colour-inverted second input file gives the difference.

-map "[out]"

maps the [out] stream to the output

output_file

path, name and extension of the output file

Discussion

A delta file is useful for comparing files with little differences, while for comparing files with big differences a split screen may be a better choice.

The different filters of a filter chain are divided by a comma (,). The different filter chains of a filter graph are divided by a semi-colon (;). If the whole filter graph is put into quotation marks, then spaces can be added after , and ; and also elsewhere for readability reasons.

The mapping is not necessary in this case, because it’s implicit.

Exercises

• alpha channel set to another value than “medium grey”
• delta file of input files with 10-bit per colour channel

2024-11-29