CMediaType Class

When filters are connected, they typically negotiate a type between them. This type describes the format of the data to be exchanged; if the filters do not agree on a media type, they cannot connect. Microsoft® DirectShow™ describes types through the media type structure, which contains two conceptual parts. The first is a list of members that describes generic attributes of a data stream. An example of this is a member that declares whether the data will be passed in fixed-size buffers. The second part of the structure is a variable-length block of data. How large the block of data should be and what it will contain depend on the type of data stream. For example, if the data stream is digital video, the format block is a VIDEOINFOHEADER structure. If, on the other hand, it is digital audio, the format block is a Microsoft Win32® WAVEFORMATEX structure.

A data stream type (for example, digital video) is set with a combination of two globally unique identifiers (GUIDs), called a major type and a subtype. The major type describes the overall class of data, examples of which might be digital video, digital audio, MIDI, or text captions. The subtype should supply a more specific description of the data type. In the case of digital video, for example, the subtype could be RGB8, RGB16, or RGB32 (among others). By having these two types in a generic structure (AM_MEDIA_TYPE), a component, such as a filter graph, can connect filters without any knowledge that is type specific.

The distinction between what goes in the major type and the subtype is somewhat arbitrary. However, as a general rule, transformations between major types (for example, video to audio or video to MIDI) should be relatively rare. Such a rare exception might be a transformation between audio and MIDI. As for the subtype, the more information promoted from the type-specific format block into the subtype, the better the design.

As an example of promoting type-specific information to the subtype, video in DirectShow uses a VIDEOINFOHEADER structure for the type-specific format block. This contains a Win32 BITMAPINFOHEADER structure that defines the video stream. BITMAPINFOHEADER contains the bit depth of the video, such as 8-bit, 16-bit, or 24-bit. This information is duplicated in the subtype field, because a subtype of RGB8 directly infers a bit count of 8.

DirectShow defines a number of major types. The most important of these are a video type that uses VIDEOINFOHEADER for the variable-length format block, and an audio that uses WAVEFORMATEX. However, it is insufficient to have a major type (such as digital video) inferring the contents of the format block (in this case, VIDEOINFOHEADER). The principal reason for this is extensibility: the format block type must be able to be updated without changing the less-specific major type. Therefore, what the format block actually contains is inferred by another GUID called the format type. If the format block contains VIDEOINFOHEADER, the format type GUID will be FORMAT_VideoInfo.

The principal use of the CMediaType class is to manage a media type structure in a simple way. At the same time, the class provides some extra helper functions (such as format-block copying and allocation). The class can be cast to an AM_MEDIA_TYPE structure when an interface method requires one to be passed to it.

The CMediaType class contains a pointer to a block of memory. When copying a CMediaType object, it is insufficient to simply copy the pointer. In C++, a data copy is required, which actually allocates a new block of memory and copies the data into it. This is the purpose of the copy operator.

Similarly, when comparing two CMediaType objects, you must compare the blocks of variable-length data (actually using memcmp) when producing the final result. To make this possible, CMediaType overrides the equivalence operator.

Member Functions

Name	Description
AllocFormatBuffer	Allocates an uninitialized format block in the object.
CMediaType	Constructs a CMediaType object.
Format	Returns the format block for this media type.
FormatLength	Returns the length of the format block of this object.
FormatType	Returns a pointer to the format type.
GetSampleSize	Returns the size of the samples.
InitMediaType	Initializes the media type.
IsFixedSize	Queries whether the samples are fixed in length.
IsPartiallySpecified	Checks if the media type is not completely specified.
IsTemporalCompressed	Queries whether the data stream is compressed temporally.
IsValid	Queries whether the media type is currently valid.
MatchesPartial	Checks whether this media type matches another media type that is only partially specified.
ReallocFormatBuffer	Reallocates the format block, maintaining its current content where possible.
ResetFormatBuffer	Deletes any format block that is currently present.
SetFormat	Sets the format block.
SetFormatType	Sets the type of the format block in the object.
SetSampleSize	Sets the size of the samples.
SetSubtype	Sets the subtype.
SetTemporalCompression	Marks the media type to indicate that samples will be temporally compressed.
SetType	Sets the major type.
SetVariableSize	Marks the media type to indicate that samples will vary in length.
Subtype	Returns a pointer to the subtype.
Type	Returns a pointer to the major type.

Operators

Name	Description
operator =	Performs a copy operation.
operator ==	Tests for equality between CMediaType objects.
operator !=	Tests for inequality between CMediaType objects.

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CMediaType::AllocFormatBuffer

CMediaType Class

Allocates a block of memory for the format block.

BYTE* AllocFormatBuffer(
  ULONG length
  );

Parameters

length

Size required for the format block.

Return Values

Returns a pointer to the new block if successful; otherwise, returns NULL.

Remarks

Any previous format block is deleted and a new block is allocated and installed. The size required must be nonzero.

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CMediaType::CMediaType

CMediaType Class

Constructs a CMediaType object.

CMediaType( );
CMediaType(
  const GUID * majortype
  );
CMediaType(
  const AM_MEDIA_TYPE& mtype
  );
CMediaType(
  const CMediaType& cmtype
  );

Parameters

majortype

Major type GUID.

mtype

AM_MEDIA_TYPE structure.

cmtype

CMediaType object from which this object is constructed.

Return Values

No return value.

Remarks

A CMediaType object can be constructed in a number of different ways. The class provides a default constructor that takes no parameters. It can also be constructed based on an AM_MEDIA_TYPE structure or another CMediaType object. In both cases, it takes a data copy of the format block before returning.

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CMediaType::Format

CMediaType Class

Returns a pointer to the variable-length format block of the object.

BYTE* Format( ) const;

Return Values

Returns the format block of the object whose content is type-specific.

Remarks

If no format block has been allocated, it might return NULL.

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CMediaType::FormatLength

CMediaType Class

Returns the size, in bytes, of the format block that the object contains.

ULONG FormatLength( ) const;

Return Values

Returns the length of the format block, or NULL if no format block is present.

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CMediaType::FormatType

CMediaType Class

Retrieves the format type.

const GUID *FormatType( ) const;

Return Values

Returns a pointer to the format type.

Remarks

The format GUID describes the content of the variable-length format block. Examples of format types are FORMAT_VideoInfo and FORMAT_WaveFormatEx.

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CMediaType::GetSampleSize

CMediaType Class

Returns the maximum sample size for the data stream.

ULONG GetSampleSize( ) const;

Return Values

Returns the maximum size of any sample to be sent, or zero to indicate that the sample size is variable.

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CMediaType::InitMediaType

CMediaType Class

Initializes the sample.

void InitMediaType( );

Return Values

No return value.

Remarks

This member function clears memory, sets the fixed sample size property, and sets the sample size to 1.

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CMediaType::IsFixedSize

CMediaType Class

Determines if the samples for the stream will be fixed or variable size.

BOOL IsFixedSize( ) const;

Return Values

Returns one of the following values.

Value	Meaning
TRUE	Samples will be fixed size.
FALSE	Samples will be variable length.

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CMediaType::IsPartiallySpecified

CMediaType Class

Determines if the media type is only partially defined. This is the case if the major type or format type is GUID_NULL.

BOOL IsPartiallySpecified( ) const;

Return Values

Returns one of the following values.

Value	Meaning
TRUE	Media type is partially specified.
FALSE	Media type is completely specified.

Remarks

This function does not check the sub type.

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CMediaType::IsTemporalCompressed

CMediaType Class

Asks if the stream will be compressed temporally.

BOOL IsTemporalCompressed( ) const;

Return Values

Returns one of the following values.

Value	Meaning
TRUE	Stream will have temporal compression.
FALSE	Stream will have no temporal compression.

Remarks

Some data streams, such as compressed video, have temporal dependencies between successive samples. Other data streams do not have temporal dependencies between their samples; that is, each sample can be treated as an independent unit; for example, MIDI.

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CMediaType::IsValid

CMediaType Class

Queries whether the object has a valid major type.

BOOL IsValid( ) const;

Return Values

Returns one of the following values.

Value	Meaning
TRUE	CMediaType object has a valid major type.
FALSE	CMediaType object does not have a valid major type.

Remarks

When CMediaType objects are constructed, their GUIDs are initialized with GUID_NULL (unless they are constructed based on another AM_MEDIA_TYPE structure or CMediaType object). This member function is useful for discovering if the object has been correctly initialized.

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CMediaType::MatchesPartial

CMediaType Class

Determines if this media type matches the media type pointed to by the ppartial parameter.

BOOL MatchesPartial(
  const CMediaType *ppartial
  ) const;

Parameters

ppartial

Pointer to the media type to match.

Return Values

Returns one of the following values.

Value	Meaning
TRUE	Media types match for the parts that are defined.
FALSE	Media types do not match.

Remarks

The matching applies only for the parts of ppartial that are defined. That is, this only matches the major type, subtype, or format type of the media type if these are not defined as GUID_NULL.

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CMediaType::ReallocFormatBuffer

CMediaType Class

Reallocates the format block to a new size.

BYTE* ReallocFormatBuffer(
  ULONG length
  );

Parameters

length

New size required for the format block.

Return Values

Returns a pointer to the new block if successful; otherwise, returns NULL.

Remarks

Any current format block will be copied into the newly allocated block up to its maximum size. Any excess will be lost when the new block is smaller than the old one. When the new block is larger, the excess is not filled with zeros.

The size required must be nonzero.

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CMediaType::ResetFormatBuffer

CMediaType Class

Deletes any format block currently held, sets it to NULL, and sets the size of the format block to zero.

void ResetFormatBuffer( );

Return Values

No return value.

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CMediaType::SetFormat

CMediaType Class

Sets the variable-length format block.

BOOL SetFormat(
  BYTE *pFormat,
  ULONG length
  );

Parameters

pFormat

Block of memory containing type-specific information.

length

Overall length of the format block.

Return Values

Returns one of the following values.

Value	Meaning
TRUE	Format block was set.
FALSE	An error occurred; most likely there was no memory available.

Remarks

The function takes a copy of the format block and stores that internally.

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CMediaType::SetFormatType

CMediaType Class

Sets the GUID that describes the content of the format block.

void SetFormatType(
  const GUID * pformattype
  );

Parameters

pformattype

GUID describing the format type.

Return Values

No return value.

Remarks

The format GUID describes what can be expected to be found in the variable-length format block. For example, if the format type is FORMAT_VideoInfo, the format block should contain a VIDEOINFOHEADER structure. The creator of this object is responsible for making them consistent.

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CMediaType::SetSampleSize

CMediaType Class

Sets the maximum sample size for the data stream.

void SetSampleSize(
  ULONG sz
  );

Parameters

sz

Size of the sample.

Return Values

No return value.

Remarks

If the sample size passed is zero, the object is set so that the data stream will send variable-length samples (the CMediaType::GetSampleSize member function will return zero). Otherwise, it will set the maximum size of the sample to the size specified in the sz parameter.

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CMediaType::SetSubtype

CMediaType Class

Sets the subtype for the object.

void SetSubtype(
  const GUID * psubtype
  );

Parameters

psubtype

GUID defining the subtype for the object.

Return Values

No return value.

---

CMediaType::SetTemporalCompression

CMediaType Class

Marks the media type so that the data stream it describes might or might not contain temporal compression (according to the input Boolean flag).

void SetTemporalCompression(
  BOOL bCompressed
  );

Parameters

bCompressed

TRUE to indicate that the stream will contain temporal compression; otherwise, FALSE.

Return Values

No return value.

---

CMediaType::SetType

CMediaType Class

Sets the major type for the object.

void SetType(
  const GUID * ptype
  );

Parameters

ptype

GUID defining the major type for the object.

Return Values

No return value.

---

CMediaType::SetVariableSize

CMediaType Class

Sets the media type to indicate that the data stream will send variable-length samples.

void SetVariableSize( );

Return Values

No return value.

Remarks

Subsequent calls to CMediaType::GetSampleSize will return zero.

---

CMediaType::Subtype

CMediaType Class

Retrieves the subtype.

const GUID *Type( ) const;

Return Values

Returns a pointer to the subtype.

Remarks

The subtype GUID gives finer detail within the major type of data represented by this media type.

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CMediaType::Type

CMediaType Class

Retrieves the major type.

const GUID *Type( ) const;

Return Values

Returns a pointer to the major type.

Remarks

The major type GUID describes the class of data represented by this media type.

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CMediaType::operator =

CMediaType Class

The CMediaType variation of this operator is the copy constructor for a CMediaType object.

The AM_MEDIA_TYPE variation of this operator is the copy constructor for an AM_MEDIA_TYPE object.

CMediaType& operator=(
  const CMediaType& rt
  );
CMediaType& operator=(
  const AM_MEDIA_TYPE& mrt
  );

Parameters

rt

Object to copy during the assignment operation.

mrt

Object to copy during the assignment operation.

Return Values

Returns a reference to this object after the operation.

Remarks

Because the CMediaType class inherits publicly from AM_MEDIA_TYPE, the compiler could generate the copy constructor for the AM_MEDIA_TYPE object itself. However, this could introduce some memory conflicts and leaks in the process because the structure contains a dynamically allocated block (which the AM_MEDIA_TYPE pbFormat member points to), which the compiler's copy constructor will not copy correctly.

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CMediaType::operator ==

CMediaType Class

Tests for equality between CMediaType objects.

inline BOOL operator==(const CMediaType& rt) const;

Parameters

rt

CMediaType object corresponding to the right side of the operator.

Return Values

Returns TRUE if the CMediaType object tested is equal to this object; otherwise, returns FALSE.

Remarks

This object is on the left side of the operator.

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CMediaType::operator !=

CMediaType Class

Tests for inequality between CMediaType objects.

BOOL operator!=(
  const CMediaType& rt
  ) const;

Parameters

rt

CMediaType object corresponding to the right side of the operator.

Return Values

Returns TRUE if the CMediaType object tested is not equal to this object; otherwise, returns FALSE.

Remarks

This object is on the left side of the operator.

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