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0.1.1
A SMPTE timecode library for Elixir
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lib/drop_frame.ex
defmodule Private.Drop do
@moduledoc false
use Ratio
# 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(Vtc.Timecode.Sections.t(), Vtc.Framerate.t()) ::
{:ok, integer} | {:error, Vtc.Timecode.ParseError.t()}
def parse_adjustment(sections, %Vtc.Framerate{ntsc: ntsc} = rate) when ntsc == :Drop do
drop_frames = get_drop_frames(rate)
# We have a bad frame value if our 'frames' place is less than the drop_frames we
# skip on minutes not divisible by 10.
has_bad_frame = sections.frames < drop_frames
{_, remainder} = Private.Rat.divmod(sections.minutes, 10)
is_tenth_minute = remainder == 0
if has_bad_frame and not is_tenth_minute do
{:error, %Vtc.Timecode.ParseError{reason: :bad_drop_frames}}
else
total_minutes = 60 * sections.hours + sections.minutes
adjustment = drop_frames * (total_minutes - div(total_minutes, 10))
{:ok, -adjustment}
end
end
# If this is not an NTSC framerate, there is not adjustment
def parse_adjustment(_sections, %Vtc.Framerate{ntsc: ntsc} = _rate) when ntsc != :Drop do
{:ok, 0}
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, Vtc.Framerate.t()) :: integer
def frame_num_adjustment(frame_number, rate) do
timebase = Vtc.Framerate.timebase(rate)
drop_frames = get_drop_frames(rate)
# Get the number frames-per-minute at the whole-frame rate
frames_per_minute_whole = 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_frames
# 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} = Private.Rat.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_frames * 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 = frames - timebase
adjustment = adjustment + drop_frames
# 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_frames
# Return our original frame number adjusted by our calculated adjustment.
frame_number + adjustment
end
end
@spec get_drop_frames(Vtc.Framerate.t()) :: integer
def get_drop_frames(rate) do
timebase = Vtc.Framerate.timebase(rate)
Private.Rat.round_ratio?(timebase * Ratio.new(0.066666, 1))
end
end