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0.1.1
A SMPTE timecode library for Elixir
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lib/utils/drop_frame.ex
defmodule Vtc.Utils.DropFrame do
@moduledoc false
alias Vtc.Framerate
alias Vtc.Timecode
alias Vtc.Utils.Rational
# 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, %{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 = round(drop_rate * (total_minutes - div(total_minutes, 10)))
{:ok, -adjustment}
end
end
def parse_adjustment(_, _), do: {:ok, 0}
# Validates that the frame value from our string is not a frame that should have been
# skipped on a non-tenth minute.s
@spec parse_adjustment_validate(Timecode.Sections.t(), integer()) ::
:ok | {:error, Timecode.ParseError.t()}
defp parse_adjustment_validate(sections, drop_rate) do
tenth_minute? = rem(sections.minutes, 10) == 0
minute_boundary? = sections.seconds == 0
if sections.frames < drop_rate and not tenth_minute? and minute_boundary? 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, %{ntsc: :drop} = rate) do
framerate = Ratio.to_float(rate.playback)
dropped_per_min = round(framerate * 0.066666)
frames_per_hour = round(framerate * 60 * 60)
frames_per_24_hours = frames_per_hour * 24
frames_per_10_min = round(framerate * 60 * 10)
frames_per_min = round(framerate) * 60 - dropped_per_min
frame_number = rem(frame_number, frames_per_24_hours)
tens_of_mins = div(frame_number, frames_per_10_min)
remaining_mins = rem(frame_number, frames_per_10_min)
tens_of_mins_adjustment = dropped_per_min * 9 * tens_of_mins
if remaining_mins > dropped_per_min do
remaining_minutes_adjustment =
dropped_per_min * div(remaining_mins - dropped_per_min, frames_per_min)
tens_of_mins_adjustment + remaining_minutes_adjustment
else
tens_of_mins_adjustment
end
end
def frame_num_adjustment(_, _), do: 0
# 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
time_base = rate |> Framerate.timebase() |> Rational.round()
round(time_base * 0.066666)
end
end