182 lines
8.9 KiB
Plaintext
182 lines
8.9 KiB
Plaintext
AVIFile: it's not just an API set, it's a way of life.
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So, people are confused about how to approach the AVIFile APIs.
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Let's try to help by making some things clear.
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1. AVIFile isn't just for AVI files.
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Perhaps this would be clearer if these functions had some different
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name, but let's live with AVIFile for now. The important thing to
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keep in mind is that we aren't talking about .AVI files, but about
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some ideal time-based file format which supports the operations we'd
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like it to support, like reading and writing from multiple
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time-stamped streams. If the APIs conform closely to what's actually
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in .AVI files, it's just because I made up both the .AVI format and
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the AVIFile APIs.
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The guiding principle is that any other type of file containing this
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time-stamped sort of data, be it a QuickTime file, an MPEG file, a
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.WAV file, or whatever, should be accessible in one simple way.
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2. AVIFile isn't just for files.
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Having a uniform API to read these different sorts of files is
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convenient, but we can go beyond that. Once you have the abstract
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concept of a "file" which is a bundle of "streams" (each of which has
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a particular data type and format, along with a start time and
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duration, and can be read from or written to at various points in
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time), it is clear that not every "file" needs to correspond to an
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actual disk file, and not every "stream" has to come from a "file" at
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all.
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Useful kinds of streams to think about:
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Entirely synthetic streams, like the bouncing ball example; in this
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case, a "stream" reduces to a function GiveMeVideoFrame(x).
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Streams which are essentially filters applied to other streams; for
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example, given a stream of video (that is, some abstract object that
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can hand you frame 4 if you want it), you can build from that a
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"compressed" stream, which is simply a stream that when asked for
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frame 4 goes and asks the original stream for its frame 4, then
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compresses it and hands it back to you, saying "here's frame 4, all
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compressed like you wanted it."
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3. AVIFile tries to conform to the Component Object model.
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Since AVIFile is intended to be extensible to support additional file
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formats, it has to have some way of transparently linking to DLLs
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containing routines that understand those file formats. In the past,
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we in MMSys have done this sort of thing using installable drivers
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and a message-based scheme. (See MCI, ICM, and more....) With the
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release of OLE 2.0, we have a new standard for this sort of thing.
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It's a little scary at first, requires either C++ or some C++-like
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thinking, and involves header files you haven't seen before. In any
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case, we're all going to have to live with it.
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4. Don't be intimidated by talk of C++ and "objects".
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From a C point of view, all an "object pointer" like a PAVISTREAM is
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is a pointer to a structure whose first member happens to be another
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pointer to a table of functions. This means that in addition to
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carrying around data like "how long is this video sequence", the
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structure also contains pointers to code that knows how to actually
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get things done. All C++ does for you is provide a nicer syntax for
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doing this sort of thing.
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You can make yourself a PAVISTREAM by hand. All you have to do
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is allocate room for a structure big enough to contain the pointer
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to the function table and any other data you need to keep around.
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Then, you make a function table with the Read, Write, and other
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functions to operate on your type of stream, and make sure your
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structure points to your table. Just like that, you have a bona fide
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PAVISTREAM which you can pass to AVISave, AVIStreamRead, and so on.
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5. Luckily, you can usually ignore all of the "Component Object model"
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stuff.
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Unless you're doing something complicated, you should be able to just
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call the various AVIFileXXXX and AVIStreamXXXX APIs and pretty much
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ignore all of the talk of ISomethingOrOther and CLSID_Confusing.
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If you are trying to make a DLL that will add support for reading and
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writing a new file format, you will in fact have to learn something
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about what a "class factory" is, but cross that bridge when you come
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to it.
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6. AVISave() is just a helper function.
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All the AVISave function does is copy some streams into a new file.
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It doesn't do anything that you couldn't do yourself by calling the
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AVIFileXXXX and AVIStreamXXXX APIs. It does, however, make your life
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easier by: calling AVIMakeCompressedStream for you according to the
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options you pass in; calling AVIFileOpen and AVIFileCreateStream
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appropriately to make the new streams in the new file you're making;
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and finally, looping through all of the streams from start to end
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copying the data from the old streams into the new file, and doing it
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in the right order so that things come out nicely interleaved.
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7. AVIStreamReadData, AVIStreamWriteData, AVIFileReadData, and
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AVIFileWriteData are not actually for reading and writing real data.
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I know, these routines have strange names and they're just confusing
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everybody.
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What they're not for: These routines are not what you use to read and
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write audio samples and video frames and that sort of thing. For
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that, you must use AVIStreamRead and AVIStreamWrite. (If you have a
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PAVIFile, call AVIFileGetStream and then call AVIStreamRead.)
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What they're for: Reading and writing copyright information, the
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author's name, and other stuff like that that's kept in INFO chunks
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in RIFF files.
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8. AVIStreamGetFrame is just another helper function.
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AVIStreamGetFrame (and AVIStreamGetFrameOpen/Close) are just
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functions which handle the relatively simple task of getting a
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decompressed video frame out of a stream. This involves finding the
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right ICM decompressor for the task at hand, figuring out where the
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last key frame was, and decompressing frames as necessary.
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Perhaps this should all be done automatically for you; perhaps you
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should do this instead by opening a "decompressed" stream based on
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the compressed stream you want to read. It's not either of those
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ways now, so use AVIStreamGetFrame.
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8a. What in heck is AVIMakeCompressedStream() for?
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AVIMakeCompressedStream makes a new stream pointer for you that acts
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just like the old stream you already had, but is compressed or
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decompressed. That isn't very clear, so I'll try again: If you have
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a PAVISTREAM which, when you read it, gives you back 8-bit RGB frames
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of "Gone With the Wind", you can use AVIMakeCompressedStream to make
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a stream that will return the same pictures, but compressed in, say,
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CRAM format. The reverse is also true: given a compressed stream,
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AVIMakeCompressedStream can make you an uncompressed version of that
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stream.
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To use it, just fill out an AVICOMPRESSOPTIONS structure. (Or pass
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NULL, which will decompress.)
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In theory, you can either read from the compressed stream or write to
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it. (Right now, this still doesn't work for audio compression....)
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However, you can only do one at a time.
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9. What in the world is an HRESULT?
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For some reason, all OLE 2.0 functions return HRESULTs which could
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potentially contain extra information about an error that occurred.
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For now, they are essentially just the same as a plain error code.
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To convert one of the AVIERR_XXXX codes to an HRESULT, call
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ResultFromScode() on it. To convert back, use GetScode()....
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10. In its current state, AVIFile is not finished.
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Some things which are bad about the current AVIFile APIs:
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AVISaveOptions() puts up a nasty big nested dialog which isn't
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user-friendly.
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AVISave() doesn't handle palette changes correctly.
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Almost nothing handles the concept of, say, a wave format changing
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halfway through a stream.
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T here is no way to find when the next palette change happens, short
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of asking for the video format on every frame until you find a place
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it changes.
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Dealing with compressed and uncompressed streams is kind of screwy.
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If you know exactly what you have and what you want, you can call
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AVIMakeCompressedStream() to do what you need.
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Right now, the handler for compressing streams only supports reading,
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not writing. There is no reason this should be true, and maybe I'll
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fix it.
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There are no status callbacks for anything. If something takes a
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really long time, you wait.
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Error returns are generally not good, if they're there at all.
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There is no way to find what a specific handler can do short of
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trying to do it and seeing what works.
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The documentation is still primitive.
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11. Miscellaneous hints:
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Be sure to call AVIStreamInit() and AVIStreamExit(). If you don't,
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the component object DLL won't get initialized and nothing will work.
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Be sure the right information is in your registration database. It
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should all get put there automatically, but isn't too resistant to
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tampering.
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Some functions take pointers to long variables which need to be initialized
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to the size of your buffer before you call them. You need to use code like:
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cbSize = cbBuffer; AVIStreamReadFormat(pstream, 0, lpBuffer, &cbSize);
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After this, cbSize will contain the actual correct size of the format, which
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you can then compare to the size you passed in to see if your buffer
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was large enough.
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