The Be Book - Classes And Methods

BParameter

Derived From:	–
Mix-in Classes:	–
Declared In:	`media/Parameter.h`
Library:	`libmedia.so`
Allocation:	Constructor only

Class Overview

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 Code	Description
`B_OK`	The item was successfully added to the list.
`B_ERROR`	The item couldn't be added to the list.

CountChannels(), SetChannelCount()

int32 CountChannels();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();int32 CountOutputs();

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::GetParameterValue() and BControllable::SetParameterValue() functions.

Return Code	Description
`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.	See Ports in the The Kernel Kit

Group()

BParameterGroup* Group() const;

Returns the BParameterGroup that most directly contains the BParameter.

ID()

int32 ID() 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() 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 SetMediaType(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() 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).

Type()

media_parameter_type Type() 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() 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() = 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() const;

Returns the BParameterWeb that contains the BParameter.

Constants

Parameter Kinds

Declared in: 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.

The B_VIDEO_FORMAT kind has specific video format values associated with it:

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

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_SIMPLE_TRANSPORT

A discrete parameter indicating one of five states: rewinding, stopped, playing, paused, and fast-forwarding.

B_GENERIC

The parameter's kind isn't one of the above.

Indicates the kind of parameter represented by a BParameter object.

media_parameter_type

Declared in: 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.

Note

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.