forked from mia/Aegisub
137 lines
5 KiB
Text
137 lines
5 KiB
Text
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WARNING: THIS FILE IS HEAVILY OUTDATED!!!!!
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Pre-Rendered Subtitles
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----------------------
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Specifications by Niels Martin Hansen
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Modified and commented by Rodrigo Braz Monteiro
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Introduction
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------------
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Pre-Rendered Subtitles (PRS) is a compact raster data format for subtitles
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to be displayed into video files. It is designed to be simple, fast and small.
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Compared to hardsubs, PRS has the following advantages:
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- Most of the time, it is smaller (hardsubs often require 15 MB in a 24 minutes
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video clip, whereas PRS often requires as little as 3 MB).
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- Can be disabled, for the viewers who preffer to watch the original content.
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- You can have multiple tracks, for multi-language files.
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- PRS subtitles are easier to replace, making the Quality Control process much
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faster.
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Compared to softsubs, it has the following advantages:
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- Decoding is much faster, which allows you to create every sort of advanced
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effects that can still be displayed in realtime.
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- Decoding is much simpler, making it possible to port to a wide range of
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platforms.
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- It is harder to "steal" the subtitles.
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- Does not require the shipment of font files, which might be illegal.
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Other advantages of the PRS format are:
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- It allows effects that are not possible with any current subtitle format,
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such as different blending modes.
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- It makes the task of hardsubbing a complex karaoke much faster.
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- It ensures that the encoder will encode the subtitles exactly as the
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typesetter envisioned them.
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- It allows conversion of video into subtitles, e.g. effects done in programs
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such as Adobe After Effects.
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Overview
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--------
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PRS stores either full PNG images, partial PNG images (through a number of
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blocks) or instructions to do simple modifications (translation, clearing)
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to the current output.
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PRS only uses 32 bit RGBA images.
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The data format is binary, designed for integration into containers such as
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Matroska.
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PRS subtitle files can be generated by specialised software (rendering directly
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to a PRS file or other container supporting PRS) or by a commandline program
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taking a PRS definition file and a number of PNG images as input.
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The PRS definition file is a text file describing the images to be displayed.
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PRS subtitles are frame-based, that is, a PRS subtitle picture is linked to one
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or more frame-numbers in the video.
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All integers are 32 bit unsigned Little Endian unless otherwise specified.
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PRS stand-alone file format
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---------------------------
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The file starts with a 32 bit magic number, formed by the ASCII string
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"PRS".
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The magic is followed by an integer specifying the length in bytes of the
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subpicture stream name. The subpicture stream name follows immediately
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after the length specifier, and is an UTF-8 encoded freeform string. (It may
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contain embedded NUL characters, but this is discouraged. It is encouraged to
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terminate the stream name with a NUL character.)
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The header is followed by a number of image deifnitions and display commands.
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Image definitions and display commands can be mixed in any way, as long as
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no display command refers to an undefined image.
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If an image identifier is defined twice, the later definition overrides any
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earlier ones.
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0x00 "PRS" (zero-terminated)
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0x04 version number (32 bit unsigned)
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0x08 stream name size (32 bit unsigned)
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0x012 stream name string (this won't be writen if the size is 0)
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Image Definition IMG
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This defines an image data block. This image definition starts with the 32 bit
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magic "IMG". This is followed by an integer image identifier, an integer
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image byte size and the image data, encoded in the apropriate format.
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0x00 "IMG" (zero-terminated)
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0x04 block length (32 bit unsigned)
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0x08 image identifier (32 bit unsigned)
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0x0C image format (32 bit unsigned; 1 = PNG, and the only supported format for now)
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0x10 image data length (32 bit unsigned)
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0x14 image data
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Display Command DSP
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This controls when and how images are displayed on the video.
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Start and end frame numbers are both inclusive.
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0x00 "DSP" (zero-terminated)
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0x04 block length (32 bit unsigned)
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0x08 start time in miliseconds (32 bit unsigned)
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0x0C end time in miliseconds (32 bit unsigned)
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0x10 image identifier (32 bit unsigned)
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0x1C layer (16 bit signed)
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0x14 x position (16 bit signed)
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0x18 y position (16 bit signed)
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0x1A alpha multiplier (8 bit unsigned)
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0x1B blend mode (8 bit unsigned)
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Alpha multiplier:
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0x00 is "invisible", 0xFF is "fully visible". This value is multiplied onto
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the alpha value of each pixel in the image to get the actual alpha value for
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that pixel, that is: finalAlpha = pixelAlpha * globalAlpha / 255;
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Blend modes:
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0 none (src*alpha + dest*(1-alpha))
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1 add (src*alpha + dest)
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2 subtract (dest - src*alpha);
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3 inverse subtract (src*alpha - dest) (is this it?)
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4 multiply (src*alpha * dst / 255)
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Values are clipped before they are multiplied with alpha.
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Layers:
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If picture A's layer number is larger than picture B, picture A is displayed
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on top of picture B. If two pictures have the same layer number, no order is
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defined. (It's recommended to use the ordering of the display commands.)
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