The Media Kit: BParameter

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# BParameter

Derived from: BFlattenable

Declared in: be/media/ParameterWeb.h

Library: libmedia.so

Allocation: Constructor only

###### Summary

The BParameter class is the superclass from which all controls representing configurable parameters within a BControllable node are derived.

The word "parameter," in terms of the Media Kit, refers to a point at which the data manipulation performed by a BControllable node can be configured, and doesn't actually refer to a user interface object. BParameter-derived objects don't implement a user interface. See the BMediaTheme class for information on how to create a user interface for a BControllable node.

The BParameter class is an abstract class; you'll normally use the `MakeNullParameter()`, `MakeDiscreteParameter()`, and `MakeContinuousParameter()` functions in the BParameterGroup class to actually create parameters and add them toa BParameterWeb. For more detailed information on parameters and how to create a BParameterWeb, see the BParameterGroup and BParameterWeb classes.

## Hook Function

- `ValueType()

  `

## Constructor and Destructor

BParameter objects may only be created by using the appropriate functions in the BParameterGroup class.

## Member Functions

### AddInput() , AddOutput()

status_t `AddInput(`

BParameter *input`)

`

status_t `AddOutput(`BParameter *output`) `

`AddInput()` adds the BParameter specified by input to the set of BParameters that transmit data into the BParameter. The specified input parameter's `AddOutput()` function is called to let it know that it's sending data to this parameter.

`AddOutput()` adds the BParameter specified by output to the set of BParameters to which the BParameter outputs data. The specified output parameter's `AddInput()` function is called to let it know that it's receiving data from this parameter.

RETURN CODES

`B_OK`. The item was successfully added to the list.

- `B_ERROR`. The item couldn't be added to the list.

### CountChannels() , SetChannelCount()

int32 `Count` `Channels(`void`) `
void `SetChannelCount``(`int32 numChannels`) `

Some BParameters have more than one channel of the same type. For example, a stereo volume parameter with independent left and right gain would have two channels, while a single parameter value that affects both the left and right by the same amount would have only one channel. `CountChannels()` returns the number of channels manipulated by the BParameter.

The default is one channel; if your node provides more channels, call `SetChannelCount()` after creating the BParameter. `SetChannelCount()` specifies the number of channels the parameter manipulates.

### CountInputs() , CountOutputs()

int32 `CountInputs(`void`) `
int32 `CountOutputs(`void`) `

These two functions return the number of inputs or outputs currently connected to the BParameter. The number of inputs or outputs may be different from the number of channels, since an input or output might be comprised of multiple channels (a single stereo sound input or output might consist of a left channel and a right channel, for example).

### GetValue() , SetValue()

status_t `GetValue(`void *buffer, size_t *ioSize, bigtime_t *lastChange`) `
status_t `SetValue(`const void *buffer, size_t size, bigtime_t changeWhen`) `

`GetValue()` returns the current value of the BParameter. On entry, buffer points to ioSize bytes of memory, indicating where the current value of the BParameter should be stored. On return, ioSize indicates the actual number of bytes returned in the buffer, and lastChange indicates the time at which the BParameter's value last changed.

`SetValue()` changes the value of the BParameter. The new value, which is size bytes long, is obtained from the specified buffer. The change occurs at the performance time specified by changeWhen.

In either case, if the parameter is a multichannel parameter, the buffer should be large enough to contain the values for all the channels, and ioSize or size should indicate the total size of the buffer.

These calls simply dispatch to the `BControllable::GetControlValue()` and `BControllable::SetControlValue()` functions.

RETURN CODES

`B_OK`. No errors.

- `B_NO_MEMORY`. The buffer is too small (`GetValue()`).

- `B_BAD_VALUE`. The parameter doesn't have a value (`GetValue()`).

- Port errors. <<<insert reference to Kernel Kit here>>>

### Group()

BParameterGroup *`Group(`void`)` const

Returns the BParameterGroup that most directly contains the BParameter.

### ID()

int32 `ID(`void`)` const

Returns the BParameter's ID number. This ID is used by the node to route value change requests to the right place. This value should be unique within the BParameterWeb containing the BParameter.

### InputAt() , OutputAt()

BParameter *`InputAt(`int32 index`) `
BParameter *`OutputAt(`int32 index`) `

`InputAt()` returns the BParameter that feeds data into the input number specified by index, which can range from zero to `CountInputs()`-1.

`OutputAt()` returns the BParameter that receives data from the output number specified by index, which can range from zero to `CountOutputs()`-1.

Both functions return `NULL` if an invalid index is specified.

### Kind()

const char *`Kind(`void`)` const

Returns the BParameter's kind. The kind isn't necessarily user-readable, but it can be used to figure out what kind of BControl to create to visually represent the configuration option provided by the BParameter.

See the `BParameterGroup::MakeNullParameter()`, `BParameterGroup::MakeDiscreteParameter()`, and `BParameterGroup::MakeContinuousParameter()` functions for more detailed information about the available kinds.

### MediaType() , SetMediaType()

media_type `MediaType(`void`) `
void `Set` `MediaType(`media_type type`) `

`MediaType()` returns the type of media data that flows through the BParameter.

`SetMediaType()` changes the recorded type of media data. The default, if it hasn't been changed, is `B_MEDIA_NO_TYPE`.

The media_type of a BParameter is used to inform interested parties as to the format of the data; it's an informational setting, and doesn't alter the data flowing through the BParameter in any way.

### Name()

const char *`Name(`void`)` const

Returns the BParameter's name, which is suitable for display to the user (for example, it can be used as the label in a user interface object).

### OutputAt() see InputAt()

### Channels()

### SetMediaType() see MediaType()

### SetValue() see GetValue()

### Type()

media_parameter_type `Type(`void`)` const

Returns the kind of parameter the object represents.

Constant | Description
--- | ---
`B_NULL_PARAMETER` | The BParameter is a BNullParameter.
`B_DISCRETE_PARAMETER` | The BParameter is a BDiscreteParameter
`B_CONTINUOUS_PARAMETER` | The BParameter is a BContinuousParameter

### Unit()

const char *`Unit(`void`)` const

Returns the unit of measurement used by the BParameter. This might be, for example, "dB," "kHz," or "fps." It should be human-readable, as it may be displayed in a user interface.

### ValueType()

virtual type_code `ValueType(`void`)` = 0

Returns a type code indicating the type of data type of the BParameter's value. This is usually `B_INT32_TYPE` for selectors or `B_FLOAT_TYPE` for sliders.

### Web()

BParameterWeb *`Web(`void`)` const

Returns the BParameterWeb that contains the BParameter.

## Constants

### Parameter Kinds

Declared in: <be/media/ParameterWeb.h>

Constant | Description
--- | ---
`B_MASTER_GAIN` | The parameter represents the main volume control.
`B_GAIN` | The parameter represents a gain control.
`B_BALANCE` | The parameter represents a balance control.
`B_FREQUENCY` | The parameter represents a frequency, like a radio tuner.
`B_LEVEL` | The parameter represents a level, as in EQ and effects.
`B_SHUTTLE_SPEED` | The parameter represents playback speed. A value of 1.0 indicates normal speed; less than 1.0 is slower, greater than 1.0 is faster.
`B_CROSSFADE` | The parameter indicates a crossfade for mixing audio or video; 0 indicates that the the first of a pair of streams should be presented, 100 indicates that the other should be presented, and values in between indicate the degree of mixing that should occur.
`B_COMPRESSION` | The parameter indicates compression ratio; the value is 0 for no compression, 99 for 100:1 compression.
`B_QUALITY` | The parameter indicates quality level; the value is 0 for maximum compression, 100 for no compression.
`B_BITRATE` | The parameter indicates bit rate in bits per second.
`B_GOP_SIZE` | The parameter indicates a "group of pictures" such as how many frames apart a keyframe should be inserted into a video stream.
`B_MUTE` | The parameter represents a mute control; a value of 0 passes data through unchanged, while a value of 1 mutes the data.
`B_ENABLE` | The parameter represents an enable toggle; a value of 0 disables the function while a value of 1 enables it.
`B_INPUT_MUX` | The parameter represents an input MUX. The value specifies which input should pass through.
`B_OUTPUT_MUX` | The parameter represents an output MUX; the value specifies which output should receive the incoming data.
`B_TUNER_CHANNEL` | The parameter represents a channel tuner (like a TV channel); the value indicates the channel number.
`B_TRACK` | The parameter's value indicates a track number.
`B_RECSTATE` | The parameter indicates whether the node is silent (0), playing (1), or recording (2).
`B_SHUTTLE_MODE` | The parameter indicates performance mode: -1 for backwards, 0 for stop, 1 for playing, and 2 for paused.
`B_RESOLUTION` | The parameter indicates video or audio resolution.
`B_COLOR_SPACE` | The parameter indicates video color space.
`B_FRAME_RATE` | The parameter represents a selector for picking frame rate.
`B_VIDEO_FORMAT` | The parameter represents a selector for choosing a video format.
`B_WEB_PHYSICAL_INPUT` | The parameter represents a physical input (such as a microphone jack or line input jack).
`B_WEB_PHYSICAL_OUTPUT` | The parameter represents a physical output (such as a line output jack or headphone jack).
`B_WEB_LOGICAL_INPUT` | The parameter represents a point at which bits of data are transferred between the computer and the A/V input hardware.
`B_WEB_LOGICAL_OUTPUT` | The parameter represents a point at which bits of data are transferred between the computer and the A/V output hardware.
`B_WEB_ADC_CONVERTER` | The parameter represents an analog to digital converter.
`B_WEB_DAC_CONVERTER` | The parameter represents a digital to analog converter.
`B_WEB_BUFFER_INPUT` | The parameter represents a media_input.
`B_WEB_BUFFER_OUTPUT` | The parameter represents a media_output.
`B_GENERIC` | The parameter's kind isn't one of the above.
`B_SIMPLE_TRANSPORT` | A discrete parameter indicating one of five states: rewinding, stopped, playing, paused, and fast-forwarding.

Indicates the kind of parameter represented by a BParameter object.

The `B_VIDEO_FORMAT` kind has specific values associated with it:

Value | Video Format
--- | ---
1 | NTSC-M
2 | NTSC-J
3 | PAL-BDGHI
4 | PAL-M
5 | PAL-N
6 | SECAM
7 | MPEG-1
8 | MPEG-2

### media_parameter_type

Declared in: <be/media/ParameterWeb.h>

Constant | Description
--- | ---
`B_NULL_PARAMETER` | The BParameter is a BNullParameter.
`B_DISCRETE_PARAMETER` | The BParameter is a BDiscreteParameter
`B_CONTINUOUS_PARAMETER` | The BParameter is a BContinuousParameter

These are the possible parameter types, which are returned by the `Type()` function. They indicate what subclass of BParameter the object is.

| Keep in mind that because these constants are members of the BParameter class, if you need to reference them from outside a BParameter, you need to preface the reference with "BParameter::", such as `BParameter::B_NULL_PARAMETER`.

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