annotate vamp-sdk/hostext/PluginChannelAdapter.h @ 74:64d45f526afc

* strengthen imprecations against doing heavy lifting in plugin constructor

/* -*- c-basic-offset: 4 indent-tabs-mode: nil -*-  vi:set ts=8 sts=4 sw=4: */

/*
    Vamp

    An API for audio analysis and feature extraction plugins.

    Centre for Digital Music, Queen Mary, University of London.
    Copyright 2006-2007 Chris Cannam and QMUL.
  
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*/

#ifndef _VAMP_PLUGIN_CHANNEL_ADAPTER_H_
#define _VAMP_PLUGIN_CHANNEL_ADAPTER_H_

#include "PluginWrapper.h"

namespace Vamp {

namespace HostExt {

/**
 * PluginChannelAdapter is a Vamp plugin adapter that implements a
 * policy for management of plugins that expect a different number of
 * input channels from the number actually available in the source
 * audio data.
 *
 * A host using PluginChannelAdapter may ignore the getMinChannelCount
 * and getMaxChannelCount reported by the plugin, and still expect the
 * plugin to run.
 *
 * PluginChannelAdapter implements the following policy:
 *
 * -- If the plugin supports the provided number of channels directly,
 * PluginChannelAdapter will just run the plugin as normal.
 *
 * -- If the plugin only supports exactly one channel but more than
 * one channel is provided, PluginChannelAdapter will use the mean of
 * the channels.  This ensures that the resulting values remain within
 * the same magnitude range as expected for mono data.
 *
 * -- If the plugin requires more than one channel but exactly one is
 * provided, the provided channel will be duplicated across all the
 * plugin input channels.
 *
 * If none of the above apply:
 * 
 * -- If the plugin requires more channels than are provided, the
 * minimum acceptable number of channels will be produced by adding
 * empty (zero valued) channels to those provided.
 *
 * -- If the plugin requires fewer channels than are provided, the
 * maximum acceptable number of channels will be produced by
 * discarding the excess channels.
 *
 * Hosts requiring a different channel policy from the above will need
 * to implement it themselves, instead of using PluginChannelAdapter.
 *
 * Note that PluginChannelAdapter does not override the minimum and
 * maximum channel counts returned by the wrapped plugin.  The host
 * will need to be aware that it is using a PluginChannelAdapter, and
 * be prepared to ignore these counts as necessary.  (This contrasts
 * with the approach used in PluginInputDomainAdapter, which aims to
 * make the host completely unaware of which underlying input domain
 * is in fact in use.)
 * 
 * (The rationale for this is that a host may wish to use the
 * PluginChannelAdapter but still discriminate in some way on the
 * basis of the number of channels actually supported.  For example, a
 * simple stereo audio host may prefer to reject plugins that require
 * more than two channels on the grounds that doesn't actually
 * understand what they are for, rather than allow the channel adapter
 * to make a potentially meaningless channel conversion for them.)
 *
 * In every respect other than its management of channels, the
 * PluginChannelAdapter behaves identically to the plugin that it
 * wraps.  The wrapped plugin will be deleted when the wrapper is
 * deleted.
 */

class PluginChannelAdapter : public PluginWrapper
{
public:
    PluginChannelAdapter(Plugin *plugin); // I take ownership of plugin
    virtual ~PluginChannelAdapter();

    bool initialise(size_t channels, size_t stepSize, size_t blockSize);

    FeatureSet process(const float *const *inputBuffers, RealTime timestamp);

protected:
    class Impl;
    Impl *m_impl;
};

}

}

#endif