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Ronald S. Bultje
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docs/random/mimetypes
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Mimetypes in GStreamer
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======================
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1) What is a mimetype
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---------------------
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A mimetype is a combination of two (short) strings (words), the content
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type and the content subtype, that make up a pair that describes a file
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content type. In multimedia, mime types are used to describe the media
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streamtype . In GStreamer, obsiously, we use mimetypes in the same way.
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They are part of a GstCaps, that describes a media stream. Besides a
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mimetype, a GstCaps also contains stream properties (GstProps), which
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are combinations of key/value pairs, and a name.
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An example of a mimetype is 'video/mpeg'. A corresponding GstCaps could
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be created using:
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GstCaps *caps = gst_caps_new("video_mpeg_type",
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"video/mpeg",
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gst_props_new("width", GST_PROPS_INT(384),
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"height", GST_PROPS_INT(288),
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NULL));
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Obviously, mimetypes and their corresponding properties are of major
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importance in GStreamer for uniquely identifying media streams.
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2) The problems
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---------------
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Some streams may have mimetypes or GstCaps that do not fully describe
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the stream. In most cases, this is not a problem, though. For a stream
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that contains Ogg/Vorbis data, we don't need to know the samplerate of
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the raw audio stream, for example, since we can't play it back anyway.
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The samplerate _is_ important for _raw_ audio, so a decoder would need
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to retrieve the samplerate from the Ogg/Vorbis stream headers (that are
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part of the bytestream) in order to pass it on in the GstCaps that
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belongs to the decoded audio ('audio/raw').
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Another problem is that many media types can be defined in multiple ways.
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For example, MJPEG video can be defined as video/jpeg, video/mjpeg,
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image/jpeg, video/avi with a compression of (fourcc) MJPG, etc. None of
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these is really official, since there isn't an official mimetype for
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encoded MJPEG video.
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The main focus of this document is to propose a standardized set of
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mimetypes and properties that will be used by the GStreamer plugins.
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3) Different types of streams
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-----------------------------
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There are several types of media streams. The most important distinction
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will be container formats, audio codecs and video codecs. Container
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formats are bytestreams that contain one or more substreams inside it,
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and don't provide any direct media data itself. Examples are Quicktime,
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AVI or MPEG (bytestream). They mostly contain of a set of headers that
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define the media stream(s) that is packed inside the container and the
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media data itself.
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Video codecs and audio codecs describe encoded audio or video data.
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Examples are MPEG-1 video, DivX ;-) video, MPEG-1 layer 3 (MP3) audio
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or Ogg/Vorbis audio. Actually, Ogg is a container format too (for Vorbis
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audio), but these are usually used in conjunction with each other.
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3a) Container formats
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1 - AVI (Microsoft RIFF/AVI)
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mimetype: video/x-msvideo
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2 - Quicktime (Apple)
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mimetype: video/x-quicktime
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3 - MPEG (MPEG LA)
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mimetype: video/mpeg
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properties: 'systemstream' = 1 (INT)
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4 - ASF (Microsoft)
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mimetype: video/x-asf
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5 - WAV (PCM)
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mimetype: audio/x-wav
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6 - RealMedia (Real)
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mimetype: video/realmedia
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7 - DV (Digital Video)
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mimetype: video/dv
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properties: 'systemstream' = 1 (INT)
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8 - Ogg
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mimetypes: media/ogg
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3b) Video codecs
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For convenience, the fourcc codes used in the AVI container format will be
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listed along with the mimetype and optional properties. All video codecs
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share the properties 'width' and 'height', both INT, which define the size
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of the frame (in pixels).
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1a - Raw Video (YUV/YCbCr)
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mimetype: video/raw
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properties: 'format' = 'XXXX' (fourcc)
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known fourccs: YUY2, IYUV/I420, Y41P, etc.
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1b - Raw Video (RGB)
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mimetype: video/raw
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properties: 'format' = 'RGB ' (fourcc)
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'endianness' = 1234/4321 (INT) <- endianness
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'r_mask' = bitmask (0x..) (INT) <- red pixel mask
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'g_mask' = bitmask (0x..) (INT) <- green pixel mask
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'b_mask' = bitmask (0x..) (INT) <- blue pixel mask
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'depth' = 15/16/24/32 (INT) <- bits per pixel (depth)
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'bpp' = 16/24/32 (INT) <- bits per pixel (in memory)
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2 - MPEG-1, -2 and -4 video (ISO/LA MPEG)
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mimetype: video/mpeg
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properties: 'systemstream' = 0 (INT)
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'mpegversion' = 1/2/4 (INT)
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known fourccs: MPEG, MPGI
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3 - DivX ;-) 3.x, 4.x and 5.x video
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mimetype: video/divx
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properties: 'divxversion' = 3/4/5 (INT)
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known fourccs: DIV3, DIV4, DIV5, DIVX, DX50, DIVX, divx
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4 - Microsoft MPEG 4.1, 4.2 and 4.3
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mimetype: video/x-msmpeg
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properties: 'mpegversion' = 41/42/43 (INT)
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known fourccs: MPG4, MP42, MP43
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5 - Motion-JPEG (official and extended)
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mimetype: video/jpeg
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known fourccs: MJPG (YUY2 MJPEG), JPEG (any), PIXL (Pinnacle/Miro), VIXL
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6 - Sorensen (Quicktime - SVQ1/SVQ3)
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mimetypes: video/x-svq
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properties: 'svqversion' = 1/3 (INT)
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7 - H263 and related codecs
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mimetype: video/h263
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known fourccs: H263, i263, M263, x263, VDOW, VIVO
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8 - RealVideo (Real)
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mimetype: video/realvideo
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9 - Digital Video (DV)
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mimetype: video/dv
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properties: 'systemstream' = 0 (INT)
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known fourccs: DVSD, dvsd
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10 - Windows Media Video 1 and 2 (WMV)
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mimetype: video/wmv
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properties: 'wmvversion' = 1/2 (INT)
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11 - XviD
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mimetype: video/xvid
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known fourccs: xvid, XVID
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12 - 3IVX
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mimetype: video/3ivx
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known fourccs: 3IV1, 3IV2
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13 - Ogg/Tarkin
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mimetype: video/x-ogg-tarkin
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14 - Ogg/Theora
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mimetype: video/x-ogg-theora
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3c) Audio Codecs
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for convenience, the two-byte hexcodes (as are being used for identification
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in AVI files) are also given
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1 - Raw Audio
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mimetype: audio/raw
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properties: 'rate' = X (INT) <- samplerate
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'width' = X (INT) <- audio bitsize
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'depth' = X (INT) <- same?
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'law' = 0/1/2 (INT) <- no law (0), alaw (1) or mulaw (2)
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'signedness' = X (BOOLEAN)
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'channels' = X (INT) <- number of audio channels
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'endianness' = 1234/4321 <- endianness of audio stream
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2 - MPEG-1 layer 1/2/3 audio
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mimetype: audio/mpeg
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properties: 'mpegversion' = 1/2/3 (INT)
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3 - Ogg/Vorbis
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mimetype: application/x-ogg
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4 - Windows Media Audio 1 and 2 (WMA)
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mimetype: audio/wma
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5 - AC3
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mimetype: audio/ac3
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4 - Status of this document
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---------------------------
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This document is currently under construction and the types listed in here
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are purely imaginary. Don't take this as your starting point (yet).
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Blame Ronald Bultje <rbultje@ronald.bitfreak.net> aka BBB for any mistakes
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in this document.
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