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0.1.1
A SMPTE timecode library for Elixir
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lib/drop_frame.ex
defmodule Vtc.Private.DropFrame do
@moduledoc false
use Ratio, override_math: false, operator: false
alias Vtc.Framerate
alias Vtc.Utils.Rational
alias Vtc.Timecode
# Adjusts the frame number based on drop-frame TC conventions.
#
# Algorithm adapted from:
# https://www.davidheidelberger.com/2010/06/10/drop-frame-timecode/
@spec parse_adjustment(Timecode.Sections.t(), Framerate.t()) ::
{:ok, integer()} | {:error, Timecode.ParseError.t()}
def parse_adjustment(sections, %Framerate{ntsc: :drop} = rate) do
drop_rate = frames_dropped_per_minute(rate)
with :ok <- parse_adjustment_validate(sections, drop_rate) do
total_minutes = 60 * sections.hours + sections.minutes
adjustment = drop_rate * (total_minutes - div(total_minutes, 10))
{:ok, -adjustment}
end
end
def parse_adjustment(_, _), do: {:ok, 0}
@spec parse_adjustment_validate(Timecode.Sections.t(), integer()) ::
:ok | {:error, Timecode.ParseError.t()}
defp parse_adjustment_validate(sections, drop_rate) do
tenth_minute? = match?({_, 0}, Rational.divmod(sections.minutes, 10))
# We have a bad frame value if our frames place is less than the drop_frames we
# are supposed to skip.
if sections.frames < drop_rate and not tenth_minute? do
{:error, %Timecode.ParseError{reason: :bad_drop_frames}}
else
:ok
end
end
# Adjusts the frame number of a timecode so the proper drop-frame value is printed.
#
# Algorithm adapted from:
# https://www.davidheidelberger.com/2010/06/10/drop-frame-timecode/
@spec frame_num_adjustment(integer(), Framerate.t()) :: integer()
def frame_num_adjustment(frame_number, rate) do
timebase = Framerate.timebase(rate)
drop_rate = frames_dropped_per_minute(rate)
# Get the number frames-per-minute at the whole-frame rate
frames_per_minute_whole = Ratio.mult(timebase, 60)
# Get the number of frames are in a minute where we have dropped frames at the
# beginning
frames_per_minute_with_drop = frames_per_minute_whole - drop_rate
# Get the number of actual frames in a 10-minute span for drop frame timecode. Since
# we drop 9 times a minute, it will be 9 drop-minute frame counts + 1 whole-minute
# frame count.
frames_per_10minutes_drop = frames_per_minute_with_drop * 9 + frames_per_minute_whole
# Get the number of 10s of minutes in this count, and the remaining frames.
{tens_of_minutes, frames} = Rational.divmod(frame_number, frames_per_10minutes_drop)
# Create an adjustment for the number of 10s of minutes. It will be 9 times the
# drop value (we drop for the first 9 minutes, then leave the 10th alone).
adjustment = 9 * drop_rate * tens_of_minutes
# If our remaining frames are less than a whole minute, we aren't going to drop
# again. Add the adjustment and return.
if frames < frames_per_minute_whole do
frame_number + adjustment
else
# Remove the first full minute (we don't drop until the next minute) and add the
# drop-rate to the adjustment.
frames = Ratio.sub(frames, timebase)
adjustment = adjustment + drop_rate
# Get the number of remaining drop-minutes present, and add a drop adjustment for
# each.
minutes_drop = floor(div(frames, frames_per_minute_with_drop))
adjustment = adjustment + minutes_drop * drop_rate
# Return our original frame number adjusted by our calculated adjustment.
frame_number + adjustment
end
end
# Get the number of frames that need to be dropped per minute (minus the 10th miute).
@spec frames_dropped_per_minute(Framerate.t()) :: integer()
defp frames_dropped_per_minute(rate) do
rate
|> Framerate.timebase()
|> Ratio.mult(Ratio.new(0.066666, 1))
|> Rational.round()
end
end