XvQueryEncodings - return list of encodings for an adaptor
#include <X11/extensions/Xvlib.h>
int XvQueryEncodings(Display *dpy, XvPortID port,
                     unsigned int *p_num_encodings,
                     XvEncodingInfo **pp_encoding_info);
  - dpy
- Specifies the connection to the X server.
- port
- Specifies the port whose adaptor is to be queried for its list of
      encodings.
- p_num_encodings
- A pointer to where the number of encodings supported by the adaptor is
      written.
- pp_encoding_info
- A pointer to where the list of returned encoding information is returned.
      XvEncodingInfo has the following structure:
    
    typedef struct {
      XvEncodingID encoding_id;
      char *name;
      unsigned long width;
      unsigned long height;
      XvRational rate;
      unsigned long num_encodings;
    } XvEncodingInfo;
    
- encoding_id
- Specifies the encoding-id of the encoding. The encoding-id is used to
      identify an encoding when a port's encoding attribute is changed.
- name
- A pointer to a formatted string that identifies the encoding. The string
      has the format "timing-signaltype". For example
      "ntsc-composite".
- width,height
- The width and height, in pixels, of the decoded video image.
- rate
- The field rate of the decoded video.
The XvRational structure is used to specify a fractional number.
    It has the following structure:
    typedef struct {
      int numerator;
      int denominator;
    } XvRational;
The numerator and denominator fields specify the appropriate parts
  of a fractional number.
XvQueryEncodings(3) returns encoding information about an adaptor. Each
  encoding is described by the XvEncodingInfo structure described above. The
  encodings are identified by an encoding-id, which can be used to set or get
  the encoding attribute of a port.
  - [Success]
- Returned if XvQueryEncodings(3) completed successfully.
- [XvBadExtension]
- Returned if the Xv extension is unavailable.
- [XvBadAlloc]
- Returned if XvQueryEncodings(3) failed to allocate memory to
      process the request.
  - [XvBadPort]
- Generated if the requested port does not exist.