Ichthyostega
255fc82a1b
...extract these functions and the associated test
from the low-level C time handling library and
document them with a dedicated C++ header and test.
''This is unfinished work'' —
the extracted functions provide only the low level computations;
actually, a specialised time quantisation or time code would be required.
------------
Note though,
after extracting these functions, the rest of the plain-C test
can be removed, since equivalent functionality is covered in
much more detail by the tests of the C++ time handling framework.
Notably this allows to get rid of the direct component accessor functions.
------------
__Remark__: the base implementation of many time conversion functions
and especially NTSC drop-frame was provided by Stefan Kangas
See:
6a44134833
79 lines
3.3 KiB
C++
79 lines
3.3 KiB
C++
/*
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DROPFRAME.hpp - drop-frame timecode conversions
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Copyright (C)
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2010, Stefan Kangas <skangas@skangas.se>
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**Lumiera** is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by the
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Free Software Foundation; either version 2 of the License, or (at your
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option) any later version. See the file COPYING for further details.
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*/
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/** @file dropframe.hpp
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** Calculations to support mapping into NTSC drop frame timecode.
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** This is a scheme for mapping frame numbers to a time display in SMPTE format,
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** i.e. with hours, minutes, seconds and a frame count. It was introduced in USA
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** at the time when colour television with the NTSC standard was introduced.
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** Since the existing black and white television broadcast system based on analogue
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** signal processing had initially be fixed to the same frequency as the power grid,
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** which runs with 60 Hz in USA, the addition of the additional carriers for the
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** colour information was confronted with problems of signal cross-talk, due to
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** some overtones of the used carriers falling into the same frequency band.
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** As a pragmatic workaround, the frame rate was reduced to 29.97 fps.
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**
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** This fix of one problem however caused secondary problems for the production
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** of content for television broadcast, since at this frame rate, one hour of
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** content would not sum up cleanly to a simple number of frames. Since the
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** purpose of time code is to label each frame with a time stamp, as a solution,
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** the mapping of time stamps to frame numbers is manipulated:
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** - every minute, there is one second which is mapped to 28 frames only
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** - with the exception of every 10 minutes, where no such gap is applied
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** This scheme adds up to allocating 108 frames less per hour, as would be used
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** with a full frame rate of 30 fps. It should be noted though that applying such
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** a »drop event« every minute does not mean to _omit actual content frames_ —
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** rather it is the dime display in the time stamp which suddenly skips ahead by
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** +2 frames every minute (but not every 10th minute).
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**
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** @todo 2025 this header provides some calculation functions to implement such
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** a mapping scheme (thereby documenting the know-how). But the project
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** never got to the point of integrating this functionality into an
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** actual timecode implementation. Doing so will be future work.
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** ///////////////////////////////////////////////////TICKET #751
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**
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** @see time-dropframe-test.cpp
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** @see timequant.hpp
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** @see lib::time::Time
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** @see TimeValue_test
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**
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*/
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#ifndef LUMIERA_TIME_DROPFRAME_H
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#define LUMIERA_TIME_DROPFRAME_H
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#include "lib/time/timevalue.hpp"
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namespace lib {
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namespace time {
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/**
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* Compute the consecutive frame number from a given time,
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* which is interpreted at the NTSC drop frame timecode grid.
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*/
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int64_t calculate_ntsc_drop_frame_number (raw_time_64 time);
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/**
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* Build effective time from a NTSC drop frame timecode.
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* @warning take care not to specify time codes that are illegal NTSC drop-frame times.
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*/
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raw_time_64 build_time_from_ntsc_drop_frame (uint frames, uint secs, uint mins, uint hours);
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}} // lib::time
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#endif /*LUMIERA_TIME_DROPFRAME_H*/
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