16. The XFree86 X Video Extension (Xv) Device Dependent Layer

XFree86 offers the X Video Extension which allows clients to treat video as any another primitive and ``Put'' video into drawables. By default, the extension reports no video adaptors as being available since the DDX layer has not been initialized. The driver can initialize the DDX layer by filling out one or more XF86VideoAdaptorRecs as described later in this document and passing a list of XF86VideoAdaptorPtr pointers to the following function:

Bool xf86XVScreenInit(
          ScreenPtr pScreen,
          XF86VideoAdaptorPtr *adaptPtrs,
          int num)

After doing this, the extension will report video adaptors as being available, providing the data in their respective XF86VideoAdaptorRecs was valid. xf86XVScreenInit() copies data from the structure passed to it so the driver may free it after the initialization. At the moment, the DDX only supports rendering into Window drawables. Pixmap rendering will be supported after a sufficient survey of suitable hardware is completed.

The XF86VideoAdaptorRec:

typedef struct {
	unsigned int type; 
	int flags;
	char *name;
	int nEncodings;
	XF86VideoEncodingPtr pEncodings;  
	int nFormats;
	XF86VideoFormatPtr pFormats;  
	int nPorts;
	DevUnion *pPortPrivates;
	int nAttributes;
	XF86AttributePtr pAttributes;
	int nImages;
	XF86ImagePtr pImages;
	PutVideoFuncPtr PutVideo;
	PutStillFuncPtr PutStill;
	GetVideoFuncPtr GetVideo;
	GetStillFuncPtr GetStill;
	StopVideoFuncPtr StopVideo;
	SetPortAttributeFuncPtr SetPortAttribute;
	GetPortAttributeFuncPtr GetPortAttribute;
	QueryBestSizeFuncPtr QueryBestSize;
	PutImageFuncPtr PutImage;
	QueryImageAttributesFuncPtr QueryImageAttributes;
} XF86VideoAdaptorRec, *XF86VideoAdaptorPtr;

Each adaptor will have its own XF86VideoAdaptorRec. The fields are as follows:

type

This can be any of the following flags OR'd together.

XvInputMask XvOutputMask

These refer to the target drawable and are similar to a Window's class. XvInputMask indicates that the adaptor can put video into a drawable. XvOutputMask indicates that the adaptor can get video from a drawable.

XvVideoMask XvStillMask XvImageMask

These indicate that the adaptor supports video, still or image primitives respectively.

XvWindowMask XvPixmapMask

These indicate the types of drawables the adaptor is capable of rendering into. At the moment, Pixmap rendering is not supported and the XvPixmapMask flag is ignored.

flags

Currently, the following flags are defined:

VIDEO_NO_CLIPPING

This indicates that the video adaptor does not support clipping. The driver will never receive ``Put'' requests where less than the entire area determined by drw_x, drw_y, drw_w and drw_h is visible. This flag does not apply to ``Get'' requests. Hardware that is incapable of clipping ``Gets'' may punt or get the extents of the clipping region passed to it.

VIDEO_INVERT_CLIPLIST

This indicates that the video driver requires the clip list to contain the regions which are obscured rather than the regions which are are visible.

VIDEO_OVERLAID_STILLS

Implementing PutStill for hardware that does video as an overlay can be awkward since it's unclear how long to leave the video up for. When this flag is set, StopVideo will be called whenever the destination gets clipped or moved so that the still can be left up until then.

VIDEO_OVERLAID_IMAGES

Same as VIDEO_OVERLAID_STILLS but for images.

VIDEO_CLIP_TO_VIEWPORT

Indicates that the clip region passed to the driver functions should be clipped to the visible portion of the screen in the case where the viewport is smaller than the virtual desktop.

name

The name of the adaptor.

nEncodings
pEncodings

The number of encodings the adaptor is capable of and pointer to the XF86VideoEncodingRec array. The XF86VideoEncodingRec is described later on. For drivers that only support XvImages there should be an encoding named "XV_IMAGE" and the width and height should specify the maximum size source image supported.

nFormats
pFormats

The number of formats the adaptor is capable of and pointer to the XF86VideoFormatRec array. The XF86VideoFormatRec is described later on.

nPorts
pPortPrivates

The number of ports is the number of separate data streams which the adaptor can handle simultaneously. If you have more than one port, the adaptor is expected to be able to render into more than one window at a time. pPortPrivates is an array of pointers or ints - one for each port. A port's private data will be passed to the driver any time the port is requested to do something like put the video or stop the video. In the case where there may be many ports, this enables the driver to know which port the request is intended for. Most commonly, this will contain a pointer to the data structure containing information about the port. In Xv, all ports on a particular adaptor are expected to be identical in their functionality.

nAttributes
pAttributes

The number of attributes recognized by the adaptor and a pointer to the array of XF86AttributeRecs. The XF86AttributeRec is described later on.

nImages
pImages

The number of XF86ImageRecs supported by the adaptor and a pointer to the array of XF86ImageRecs. The XF86ImageRec is described later on.

PutVideo PutStill GetVideo GetStill StopVideo SetPortAttribute GetPortAttribute QueryBestSize PutImage QueryImageAttributes

These functions define the DDX->driver interface. In each case, the pointer data is passed to the driver. This is the port private for that port as described above. All fields are required except under the following conditions:

1. PutVideo, PutStill and the image routines PutImage and QueryImageAttributes are not required when the adaptor type does not contain XvInputMask.
2. GetVideo and GetStill are not required when the adaptor type does not contain XvOutputMask.
3. GetVideo and PutVideo are not required when the adaptor type does not contain XvVideoMask.
4. GetStill and PutStill are not required when the adaptor type does not contain XvStillMask.
5. PutImage and QueryImageAttributes are not required when the adaptor type does not contain XvImageMask.

With the exception of QueryImageAttributes, these functions should return Success if the operation was completed successfully. They can return XvBadAlloc otherwise. QueryImageAttributes returns the size of the XvImage queried.

If the VIDEO_NO_CLIPPING flag is set, the clipBoxes may be ignored by the driver. ClipBoxes is an X-Y banded region identical to those used throughout the server. The clipBoxes represent the visible portions of the area determined by drw_x, drw_y, drw_w and drw_h in the Get/Put function. The boxes are in screen coordinates, are guaranteed not to overlap and an empty region will never be passed. If the driver has specified VIDEO_INVERT_CLIPLIST, clipBoxes will indicate the areas of the primitive which are obscured rather than the areas visible.

typedef int (* PutVideoFuncPtr)( ScrnInfoPtr pScrn,
          short vid_x, short vid_y, short drw_x, short drw_y,
          short vid_w, short vid_h, short drw_w, short drw_h,
          RegionPtr clipBoxes, pointer data )

This indicates that the driver should take a subsection vid_w by vid_h at location (vid_x,vid_y) from the video stream and direct it into the rectangle drw_w by drw_h at location (drw_x,drw_y) on the screen, scaling as necessary. Due to the large variations in capabilities of the various hardware expected to be used with this extension, it is not expected that all hardware will be able to do this exactly as described. In that case the driver should just do ``the best it can,'' scaling as closely to the target rectangle as it can without rendering outside of it. In the worst case, the driver can opt to just not turn on the video.

typedef int (* PutStillFuncPtr)( ScrnInfoPtr pScrn,
          short vid_x, short vid_y, short drw_x, short drw_y,
          short vid_w, short vid_h, short drw_w, short drw_h,
          RegionPtr clipBoxes, pointer data )

This is same as PutVideo except that the driver should place only one frame from the stream on the screen.

typedef int (* GetVideoFuncPtr)( ScrnInfoPtr pScrn,
          short vid_x, short vid_y, short drw_x, short drw_y,
          short vid_w, short vid_h, short drw_w, short drw_h,
          RegionPtr clipBoxes, pointer data )

This is same as PutVideo except that the driver gets video from the screen and outputs it. The driver should do the best it can to get the requested dimensions correct without reading from an area larger than requested.

typedef int (* GetStillFuncPtr)( ScrnInfoPtr pScrn,
          short vid_x, short vid_y, short drw_x, short drw_y,
          short vid_w, short vid_h, short drw_w, short drw_h,
          RegionPtr clipBoxes, pointer data )

This is the same as GetVideo except that the driver should place only one frame from the screen into the output stream.

typedef void (* StopVideoFuncPtr)(ScrnInfoPtr pScrn,
          pointer data, Bool cleanup)

This indicates the driver should stop displaying the video. This is used to stop both input and output video. The cleanup field indicates that the video is being stopped because the client requested it to stop or because the server is exiting the current VT. In that case the driver should deallocate any offscreen memory areas (if there are any) being used to put the video to the screen. If cleanup is not set, the video is being stopped temporarily due to clipping or moving of the window, etc... and video will likely be restarted soon so the driver should not deallocate any offscreen areas associated with that port.

typedef int (* SetPortAttributeFuncPtr)(ScrnInfoPtr pScrn,
          Atom attribute,INT32 value, pointer data)

typedef int (* GetPortAttributeFuncPtr)(ScrnInfoPtr pScrn,
          Atom attribute,INT32 *value, pointer data)

A port may have particular attributes such as hue, saturation, brightness or contrast. Xv clients set and get these attribute values by sending attribute strings (Atoms) to the server. Such requests end up at these driver functions. It is recommended that the driver provide at least the following attributes mentioned in the Xv client library docs:

XV_ENCODING
XV_HUE
XV_SATURATION
XV_BRIGHTNESS
XV_CONTRAST

but the driver may recognize as many atoms as it wishes. If a requested attribute is unknown by the driver it should return BadMatch. XV_ENCODING is the attribute intended to let the client specify which video encoding the particular port should be using (see the description of XF86VideoEncodingRec below). If the requested encoding is unsupported, the driver should return XvBadEncoding. If the value lies outside the advertised range BadValue may be returned. Success should be returned otherwise.

typedef void (* QueryBestSizeFuncPtr)(ScrnInfoPtr pScrn,
          Bool motion, short vid_w, short vid_h,
          short drw_w, short drw_h,
          unsigned int *p_w, unsigned int *p_h, pointer data)

QueryBestSize provides the client with a way to query what the destination dimensions would end up being if they were to request that an area vid_w by vid_h from the video stream be scaled to rectangle of drw_w by drw_h on the screen. Since it is not expected that all hardware will be able to get the target dimensions exactly, it is important that the driver provide this function. The returned dimensions must be less than or equal to the requested dimension.

typedef int (* PutImageFuncPtr)( ScrnInfoPtr pScrn,
          short src_x, short src_y, short drw_x, short drw_y,
          short src_w, short src_h, short drw_w, short drw_h,
          int image, char *buf, short width, short height,
          Bool sync, RegionPtr clipBoxes, pointer data )

This is similar to PutStill except that the source of the video is not a port but the data stored in a system memory buffer at buf. The data is in the format indicated by the image descriptor and represents a source of size width by height. If sync is TRUE the driver should not return from this function until it is through reading the data from buf. Returning when sync is TRUE indicates that it is safe for the data at buf to be replaced, freed, or modified.

typedef int (* QueryImageAttributesFuncPtr)( ScrnInfoPtr pScrn,
          int image, short *width, short *height,
          int *pitches, int *offsets)

This function is called to let the driver specify how data for a particular image of size width by height should be stored. Sometimes only the size and corrected width and height are needed. In that case pitches and offsets are NULL. The size of the memory required for the image is returned by this function. The width and height of the requested image can be altered by the driver to reflect format limitations (such as component sampling periods that are larger than one). If pitches and offsets are not NULL, these will be arrays with as many elements in them as there are planes in the image format. The driver should specify the pitch (in bytes) of each scanline in the particular plane as well as the offset to that plane (in bytes) from the beginning of the image.

The XF86VideoEncodingRec:

typedef struct {
	int id;
	char *name;
	unsigned short width, height;
	XvRationalRec rate;
} XF86VideoEncodingRec, *XF86VideoEncodingPtr;

The XF86VideoEncodingRec specifies what encodings the adaptor can support. Most of this data is just informational and for the client's benefit, and is what will be reported by XvQueryEncodings. The id field is expected to be a unique identifier to allow the client to request a certain encoding via the XV_ENCODING attribute string.

The XF86VideoFormatRec:

typedef struct {
	char  depth;  
	short class;
} XF86VideoFormatRec, *XF86VideoFormatPtr;

This specifies what visuals the video is viewable in. depth is the depth of the visual (not bpp). class is the visual class such as TrueColor, DirectColor or PseudoColor. Initialization of an adaptor will fail if none of the visuals on that screen are supported.

The XF86AttributeRec:

typedef struct {
	int   flags;
	int   min_value;
	int   max_value;
	char  *name;
} XF86AttributeListRec, *XF86AttributeListPtr;

Each adaptor may have an array of these advertising the attributes for its ports. Currently defined flags are XvGettable and XvSettable which may be OR'd together indicating that attribute is ``gettable'' or ``settable'' by the client. The min and max field specify the valid range for the value. Name is a text string describing the attribute by name.

The XF86ImageRec:

typedef struct {
	int id;
	int type;
	int byte_order;
	char guid[16];               
	int bits_per_pixel;
	int format;
	int num_planes;

	/* for RGB formats */
	int depth;
	unsigned int red_mask;       
	unsigned int green_mask;   
	unsigned int blue_mask;   

	/* for YUV formats */
	unsigned int y_sample_bits;
	unsigned int u_sample_bits;
	unsigned int v_sample_bits;   
	unsigned int horz_y_period;
	unsigned int horz_u_period;
	unsigned int horz_v_period;
	unsigned int vert_y_period;
	unsigned int vert_u_period;
	unsigned int vert_v_period;
	char component_order[32];
	int scanline_order;
} XF86ImageRec, *XF86ImagePtr; 

XF86ImageRec describes how video source data is laid out in memory. The fields are as follows:

id

This is a unique descriptor for the format. It is often good to set this value to the FOURCC for the format when applicable.

type

This is XvRGB or XvYUV.

byte_order

This is LSBFirst or MSBFirst.

guid

This is the Globally Unique IDentifier for the format. When not applicable, all characters should be NULL.

bits_per_pixel

The number of bits taken up (but not necessarily used) by each pixel. Note that for some planar formats which have fractional bits per pixel (such as IF09) this number may be rounded _down_.

format

This is XvPlanar or XvPacked.

num_planes

The number of planes in planar formats. This should be set to one for packed formats.

depth

The significant bits per pixel in RGB formats (analgous to the depth of a pixmap format).

red_mask green_mask blue_mask

The red, green and blue bitmasks for packed RGB formats.

y_sample_bits u_sample_bits v_sample_bits

The y, u and v sample sizes (in bits).

horz_y_period horz_u_period horz_v_period

The y, u and v sampling periods in the horizontal direction.

vert_y_period vert_u_period vert_v_period

The y, u and v sampling periods in the vertical direction.

component_order

Uppercase ascii characters representing the order that samples are stored within packed formats. For planar formats this represents the ordering of the planes. Unused characters in the 32 byte string should be set to NULL.

scanline_order

This is XvTopToBottom or XvBottomToTop.

Since some formats (particular some planar YUV formats) may not be completely defined by the parameters above, the guid, when available, should provide the most accurate description of the format.