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\section intro Introduction
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This is the API documentation for the Gmerlin audio video library,
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a library for handling and conversion of uncompressed audio- and video data.
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Click Modules (on top of the page) to get to the main API index.
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Here, you find just some general blabla :)
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\section why Why gavl?
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\subsection problem The problem
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If you start to write some software, which has any kind of audio-, video- or image-support,
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you'll soon be confronted with the fact, that the method for storing audio and image data in
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memory is by no means standardized. In the audio area, we have different ways to store
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multichannel sound (interleaved or all channels separate) and different sample sizes.
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Furthermore, it can happen, that a cheap soundcard isn't happy with the 5.1 channel sound
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and the 48 kHz samplerate, so you'll need to convert it to 44.1 kHz/Stereo.
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In the video and image area, the situation is not better. Images can be planar or packed,
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have different colorspaces, color orderings and chroma subsampling modes.
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So you can learn how to read images in 24 bit RGB and display them with GTK or SDL. Handling
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interleaved 16 bit audio samples is also no major problem. But if your program becomes
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popular, people want to have 24 bit audio, RGB images with 16 bit per color channel
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(or even floating point),
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and they are not longer satisfied with your bilinear video scaler. If you want to support
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a large number of formats, you'll want to convert each format into each other without any
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intermediate conversion (to save time and preserve accuracy).
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Now you can do some simple mathematics to find out, that for N formats, the number of
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conversions from every format to every other is N*(N-1). Assuming, you want to support
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26 pixelformats (like gavl does at the time, this file was written), you end up with
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a theoretical number of 650 conversion routines. After removing redundant conversions,
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you still have to write more than 600 functions. Since these are for pixelformats only,
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and you also want to do audio mixing for all sample formats and lots of other conversions,
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the total number of routines will soon exceed 1000.
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These numbers are probably the reason, why up to now, no universal solution for the
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problems described above, was written.
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\subsection solution The solution
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Programming universal audio/video converters is a painful process. 1000s of conversion
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routines must be written, debugged, tested and optimized.
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For this reason, a decision was made to do this madness once and make it available for other
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programmers who can afford to write GPL software. Gavl handles mainly the following tasks:
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- Definition of audio/video formats. This is done by having several enums and
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structures, which contain all information necessary for unambiguous definition of an
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Audio/Video format. A gavl enabled application will automatically have support for the
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same range of formats as gavl.
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- Containers for audio- and video-frames. These can carry A/V data in all supported
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formats. Basic routines are available to allocate/free/copy frames and some utilities.
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- Conversion among all formats. There are generic audio and video converters available,
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which have a configurable quality level. Higher quality always means lower speed.
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This means, that you can use gavl, if you must handle 100 audio streams simultaneously
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at telephone quality as well as for High Definition video production.
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A development goal of gavl is to have a complete set of conversions. You'll never get
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black images or silent audio frames because of missing conversion routines or speed losses
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because intermediate conversions are invoked. After this goal is reached, sometimes alternate
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routines are implemented, which focus either on more speed or higher accuracy. These can
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be selected by choosing a quality other than 3.
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\subsection advantages Advantages
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Using gavl in your application brings lots of advantages:
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- The most ugly tasks of multimedia programming are already done for you, so you
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can concentrate on your application
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- If you nevertheless find ways to improve the quality and/or speed of gavl, then do so.
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Making and sending a patch will still be less work, than writing such a library from scratch.
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- Without any additional labor, you handle all formats, which are supported by gavl.
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Impress your audience with 32 bit audio or floating point RGB processing.
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- Gavl fits smoothly to all existing Audio/Video and image APIs. You'll find lots of examples
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in the sourcetrees of gmerlin and gmerlin_avdecoder.
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- APIs (for codecs, hardware access, effects etc.), which are based on gavl, can
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nicely interact with each other. You can, for example, easily write an import module for
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gmerlin_avdecoder, which will decode most important Audio/Video formats.
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