Tutorials

• Intro
• C
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Getting started with libmapper

Since libmapper uses GNU autoconf, getting started with the library is the same as any other library on Linux; use ./configure and then make to compile it. On Mac OS X, we provide a precompiled Framework bundle for 32- and 64-bit Intel platforms, so using it with XCode should be a matter of including it in your project.

Overview of the API structure

If you take a look at the API documentation, there is a section called "modules". This is divided into the following main sections:

• Graphs
• Objects
  • Devices
  • Signals and Signal Instances
  • Maps
• Lists

For this tutorial, the only sections to pay attention to are Devices and Signals. The other sections are mostly used when building user interfaces for designing mapping configurations. Functions and types from each module are prefixed with mpr_<module>_, in order to avoid namespace clashing.

Devices

Creating a device

To create a libmapper device, it is necessary to provide two arguments to the function mpr_dev_new:

mpr_dev mpr_dev_new(const char *name_prefix, mpr_graph graph);

Every device on the network needs a descriptive name – the name does not have to be unique since during initialization a unique ordinal will be appended to the device name. This allows multiple devices with the same name to exist on the network.

The second argument is an optional graph instance. It is not necessary to provide this, but can be used to specify different networking parameters, such as specifying the name of the network interface to use. For this tutorial we will let libmapper choose a default interface.

An example of creating a device:

mpr_dev my_dev = mpr_dev_new("test", 0);

Polling the device

The device lifecycle looks like this:

In other words, after a device is created, it must be continuously polled during its lifetime, and then explicitly freed when it is no longer needed.

The polling is necessary for several reasons: to respond to requests on the admin bus; to check for incoming signals; to update outgoing signals. Therefore even a device that does not have signals must be polled. The user program must organize to have a timer or idle handler which can poll the device often enough. Polling interval is not extremely sensitive, but should be 100 ms or less. The faster it is polled, the faster it can handle incoming and outgoing signals.

The mpr_dev_poll function can be blocking or non-blocking, depending on how you want your application to behave. It takes a number of milliseconds during which it should do some work, or 0 if it should check for any immediate actions and then return without waiting:

int mpr_dev_poll(mpr_dev dev, int block_ms);

An example of calling it with non-blocking behaviour:

mpr_dev_poll(my_dev, 0);

If your polling is in the middle of a processing function or in response to a GUI event for example, non-blocking behaviour is desired. On the other hand if you put it in the middle of a loop which reads incoming data at intervals or steps through a simulation for example, you can use mpr_dev_poll as your "sleep" function, so that it will react to network activity while waiting.

It returns the number of messages handled, so optionally you could continue to call it until there are no more messages waiting. The same effect can be acheived by passing a negative value for the block_ms argument. Of course, you should be careful doing that without limiting the time it will loop for, since if the incoming stream is fast enough you might never get anything else done!

Note that an important difference between blocking and non-blocking polling is that during the blocking period, messages will be handled immediately as they are received. On the other hand, if you use your own sleep, messages will be queued up until you can call poll(); stated differently, it will "time-quantize" the message handling. This is not necessarily bad, but you should be aware of this effect.

If your code has updated signal values and will not be calling poll() immediately, you may wish to call the function update_maps(). This will immediately cause any outgoing maps to be processed and send their updates to the destination signal.

Initialization

Since there is a delay before the device is completely initialized, it is sometimes useful to be able to determine this using mpr_dev_ready. Only when mpr_dev_ready returns non-zero is it valid to use the device's name.

Freeing the device

It is necessary to explicitly free the device at the end of your program. This not only frees memory, but also sends some messages to "politely" remove itself from the network.

An example of freeing a device:

mpr_dev_free(my_dev);

Signals

Now that we know how to create a device, poll it, and free it, we only need to know how to add signals in order to give our program some input/output functionality. While libmapper enables arbitrary connections between any declared signals, we still find it helpful to distinguish between two type of signals: inputs and outputs.

• outputs signals are sources of data, updated locally by their parent device
• inputs signals are consumers of data and are not generally updated locally by their parent device.

This can become a bit confusing, since the "reverb" parameter of a sound synthesizer might be updated locally through user interaction with a GUI, however the normal use of this signal is as a destination for control data streams so it should be defined as an input signal. Note that this distinction is to help with GUI organization and user-understanding – libmapper enables connections from input signals and to output signals if desired.

Creating a signal

We'll start with creating a "sender", so we will first talk about how to update output signals. A signal requires a bit more information than a device, much of which is optional:

mpr_sig mpr_sig_new(mpr_dev parent, mpr_dir dir, const char *name, int length,
                    mpr_type type, const char *unit, void *min, void *max,
                    int *num_inst, mpr_sig_handler *h, int events);

The only required parameters here are the signal direction, name, vector length and data type. Signals are assumed to be vectors of values, so for usual single-valued signals, a length of 1 should be specified. Finally, supported types are currently MPR_INT32, MPR_FLT, or MPR_DBL for integer, float, and double values, respectively.

The other parameters are not strictly required, but the more information you provide, the more libmapper can do some things automatically. For example, if min and max are provided, it will be possible to create linear-scaled connections very quickly. If unit is provided, libmapper will be able to similarly figure out a linear scaling based on unit conversion (from cm to inches for example). Currently automatic unit-based scaling is not a supported feature, but will be added in the future. You can take advantage of this future development by simply providing unit information whenever it is available. It is also helpful documentation for users.

Notice that these optional values are provided as void* pointers. This is because a signal can either be int, float, or double, and your maximum and minimum values should correspond in type and length. So you should pass in a int*, float*, or double* by taking the address of a local variable.

Lastly, it is usually necessary to be informed when input signal values change. This is done by providing a function to be called whenever its value is modified by an incoming message. It is passed in the handler parameter, along with an events parameter specifying which types of events should trigger the handler.

An example of creating a "barebones" int scalar output signal with no unit, minimum, or maximum information and no callback handler:

mpr_sig outA = mpr_sig_new(dev, MPR_DIR_OUT, "outA",
                           1, MPR_INT32, 0, 0, 0, 0, 0, 0);

An example of a float signal where some more information is provided:

float min = 0.0f;
float max = 5.0f;
mpr_sig s1 = mpr_sig_new(dev, MPR_DIR_OUT, "sensor1", 1, MPR_FLT,
                         "V", &min, &max, 0, 0, 0);

So far we know how to create a device and to specify an output signal for it. To recap, let's review the code so far:

mpr_dev dev = mpr_dev_new("my_device", 0, 0);
mpr_sig sig = mpr_sig_new(dev, MPR_DIR_OUT, "sensor1", 1, MPR_FLT,
                          "V", &min, &max, 0, 0, 0);

while (!done) {
    mpr_dev_poll(dev, 50);
    ... do stuff ...
    ... update signals ...
}

mpr_dev_free(dev);

Note that although you have a pointer to the mpr_sig structure (which was returned by mpr_sig_new()), its memory is "owned" by the device. In other words, you should not worry about freeing its memory - this will happen automatically when the device is destroyed. It is possible to retrieve a device's inputs or outputs by name or by index at a later time using the functions mpr_dev_get_sigs().

Updating signals

We can imagine the above program getting sensor information in a loop. It could be running on an network-enable ARM device and reading the ADC register directly, or it could be running on a computer and reading data from an Arduino over a USB serial port, or it could just be a mouse-controlled GUI slider. However it's getting the data, it must provide it to libmapper so that it will be sent to other devices if that signal is mapped.

This is accomplished by the function mpr_sig_set_value():

void mpr_sig_set_value(mpr_sig sig, mpr_id inst, int length,
                       mpr_type type, void *value);

As you can see, a void* pointer must be provided, which must point to a data structure matching the length and type arguments. This data structure is not required to match the length and type of the signal itself—libmapper will perform type coercion if necessary.

So in the "sensor 1" example, assuming in do_stuff() we have some code which reads sensor 1's value into a float variable called v1, the loop becomes:

while (!done) {
    mpr_dev_poll(my_dev, 50);

    // call hypothetical user function that reads a sensor
    float v1 = do_stuff();
    mpr_sig_set_value(sig, 0, 1, MPR_FLT, &v1);
}

This is about all that is needed to expose sensor 1's value to the network as a mappable parameter. The libmapper GUI can now map this value to a receiver, where it could control a synthesizer parameter or change the brightness of an LED, or whatever else you want to do.

Signal conditioning

Most synthesizers of course will not know what to do with the value of sensor1--it is an electrical property that has nothing to do with sound or music. This is where libmapper really becomes useful.

Scaling or other signal conditioning can be taken care of before exposing the signal, or it can be performed as part of the mapping. Since the end user can demand any mathematical operation be performed on the signal, they can perform whatever mappings between signals as they wish.

As a developer, it is therefore your job to provide information that will be useful to the end user.

For example, if sensor 1 is a position sensor, instead of publishing "voltage", you could convert it to centimeters or meters based on the known dimensions of the sensor, and publish a "sensor1/position" signal instead, providing the unit information as well.

We call such signals "semantic", because they provide information with more meaning than a relatively uninformative value based on the electrical properties of the sensing technique. Some sensors can benefit from low-pass filtering or other measures to reduce noise. Some sensors may need to be combined in order to derive physical meaning. What you choose to expose as outputs of your device is entirely application-dependent.

Best of all, you can publish both "sensor1/position" and "sensor1/voltage" if desired, in order to expose both processed and raw data. Keep in mind that these will not take up significant processing time, and zero network bandwidth, if they are not mapped.

Receiving signals

Now that we know how to create a sender, it would be useful to also know how to receive signals, so that we can create a sender-receiver pair to test out the provided mapping functionality. The current value and timestamp for a signal can be retrieved at any time by calling mpr_sig_get_value(), however for event-driven applications you may want to be informed of new values as they are received or generated.

As mentioned above, the mpr_sig_new() function takes an optional handler and events. This is a function that will be called whenever the value of that signal changes or instances events occur. To create a receiver for a synthesizer parameter "pulse width" (given as a ratio between 0 and 1), specify a handler when calling mpr_sig_new(). We'll imagine there is some C++ synthesizer implemented as a class Synth which has functions setPulseWidth() which sets the pulse width in a thread-safe manner, and startAudioInBackground() which sets up the audio thread.

Create the handler function, which is fairly simple,

void pw_handler (mpr_sig sig, mpr_int_evt evt, mpr_id inst, int length,
                 mpr_type type, void *value, mpr_time_t *time)
{
    if (!length || !value)
        return;
    Synth *s = (Synth*) mpr_obj_get_prop_as_ptr(sig, MPR_PROP_DATA, 0);
    s->setPulseWidth(*(float*)v);
}

First, the pointer to the Synth instance is extracted from the MPR_PROP_DATA property, then it is dereferenced to set the pulse width according to the value pointed to by v.

Then main() will look like,

void main()
{
    Synth synth;
    synth.startAudioInBackground();

    float min_pw = 0.0f;
    float max_pw = 1.0f;

    mpr_dev synth_dev = mpr_dev_new("synth", 0);

    mpr_sig pw = mpr_sig_new(synth_dev, MPR_DIR_IN, "pulsewidth", 1, 'f', 0,
                             &min_pw, &max_pw, 0, pulsewidth_handler,
                             MPR_SIG_UPDATE);
    mpr_obj_set_prop(pw, MPR_PROP_DATA, 0, 1, MPR_PTR, &synth, 0);

    while (!done)
        mpr_dev_poll(synth_dev, 50);

    mpr_dev_free(synth_dev);
}

Working with timetags

libmapper uses the mpr_time_t data structure to store NTP timestamps. For example, the handler function called when a signal update is received contains a timetag argument. This argument indicates the time at which the source signal was sampled (in the case of sensor signals) or generated (in the case of sequenced or algorithimically-generated signals).

libmapper provides helper functions for getting the current device-time, setting the value of a mpr_time structure from other representations, and comparing or copying timetags. Check the API documentation for more information.

Working with signal instances

libmapper also provides support for signals with multiple instances, for example:

• control parameters for polyphonic synthesizers;
• touches tracked by a multitouch surface;
• "blobs" identified by computer vision systems;
• objects on a tabletop tangible user interface;
• temporal objects such as gestures or trajectories.

The important qualities of signal instances in libmapper are:

• instances are interchangeable: if there are semantics attached to a specific instance it should be represented with separate signals instead.
• instances can be ephemeral: signal instances can be dynamically created and destroyed. libmapper will ensure that linked devices share a common understanding of the relatonships between instances when they are mapped.
• map once for all instances: one mapping connection serves to map all of its instances.

All signals possess one instance by default. If you would like to reserve more instances you can use:

int mpr_sig_reserve_inst(mpr_sig sig, int num, mpr_id *ids, void **data);

If the ids argument is null libmapper will automatically assign unique ids to the reserved instances.

After reserving instances you can update a specific instance using:

void mpr_sig_set_value(mpr_sig signal, mpr_id instance, int length,
                       mpr_type type, const void *value);

The instance argument does not have to be considered as an array index - it can be any value that is convenient for labelling your instance. libmapper will internally create a map from your id label to one of the preallocated instance structures.

Receiving instances

You might have noticed earlier that the handler function called when a signal update is received has a argument called inst. Here is the function prototype again:

void mpr_sig_handler(mpr_sig sig, mpr_inst_evt evt, mpr_id inst, int len,
                     mpr_type type, const void *value, mpr_time time);

Under normal usage, this argument will have a value (0 <= n <= num_instances) and can be used as an array index. Remember that you will need to reserve instances for your input signal using mpr_sig_reserve_inst() if you want to receive instance updates.

Instance Stealing

For handling cases in which the sender signal has more instances than the receiver signal, the instance allocation mode can be set for an input signal to set an action to take in case all allocated instances are in use and a previously unseen instance id is received. Use the function mpr_obj_set_prop().

The argument mode can have one of the following values:

• MPR_STEAL_NONE Default value, in which no stealing of instances will occur;
• MPR_STEAL_OLDEST Release the oldest active instance and reallocate its resources to the new instance;
• MPR_STEAL_NEWEST Release the newest active instance and reallocate its resources to the new instance;

If you want to use another method for determining which active instance to release (e.g. the sound with the lowest volume), you can create an handler for the signal and write the method yourself:

void my_handler(mpr_sig sig, mpr_int_evt evt, mpr_id inst, int len,
                mpt_type type, const void *value, mpr_time time)
{
    // hypothetical user code chooses which instance to release
    mpr_id release_me = choose_instance_to_release(sig);

    mpr_sig_release_inst(sig, release_me);

    // now an instance is available
}

For this function to be called when instance stealing is necessary, we need to register it for MPR_SIG_INST_OFLW events:

mpr_sig_set_cb(sig, my_handler, MPR_SIG_INST_OFLW);

Publishing metadata

Things like device names, signal units, and ranges, are examples of metadata--information about the data you are exposing on the network.

libmapper also provides the ability to specify arbitrary extra metadata in the form of name-value pairs. These are not interpreted by libmapper in any way, but can be retrieved over the network. This can be used for instance to give a device X and Y information, or to perhaps give a signal some property like "reliability", or some category like "light", "motor", "shaker", etc.

Some GUI could then use this information to display information about the network in an intelligent manner.

Any time there may be extra knowledge about a signal or device, it is a good idea to represent it by adding such properties, which can be of any OSC-compatible type. (So, numbers and strings, etc.)

The property interface is through the functions,

void mpr_obj_set_prop(mpr_obj obj, mpr_prop prop, const char *key,
                      int length, char type, void *value, int publish);

mpr_prop mpr_obj_get_prop_by_idx(mpr_obj obj, mpr_prop prop, const char **key,
                                 int *len, mpr_type *type, const void **val,
                                 int *pub);
mpr_prop mpr_obj_get_prop_by_key(mpr_obj obj, const char *key, int *len,
                                 mpr_type *type, const void **val, int *pub);

The type of the value argument is specified by type: floats are MPR_FLT, 32-bit integers are MPR_INT32, doubles are MPR_DBL, and strings are MPR_STR.

For example, to store a float indicating the X position of a device, you can call it like this:

float x = 12.5;
mpr_obj_set_prop(my_device, 0, "x", 1, MPR_FLT, &x, 1);

char *sensingMethod = "resistive";
mpr_obj_set_prop(sensor1, 0, "sensingMethod", 1, MPR_STR, sensingMethod);

If the parent object (a device and a signal in this case) is local the property change takes place immediately. If the object is remote the property change is only staged and must be pushed out to the network using the functions mpr_obj_push().

The publish argument is used to specify whether the object property should be pushed to the libmapper graph. If set to 0, the property will only be accessible locally.

Reserved keys

You can use any property name not already reserved by libmapper.