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Device Feedback

Quadro SDI Output PG-03776-001_v06 | 51

9.2 DETECTING DUPLICATE FRAMES

A duplicate frame will occur on the SDI output when a new frame is not ready in the

queue at the time of the vertical retrace. This will happen when an application’s draw

time exceeds the time period between subsequent vertical retrace events on the outgoing

SDI video signal. When a new frame is not sent prior to the next vertical retrace, one of

two possible scenarios takes place. If, there is a frame already queued, then that frame is

sent and the number of queued buffers is reduced by one. In this case, there will be no

repeated frame on the SDI video output. In the case that the internal buffer queue is

empty, the last sent frame will be scanned out again by the SDI video device. This will

display as a duplicate frame.

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Note: Internally, the Quadro SDI device only displays complete interlaced or

progressively scanned frames. As such, only complete frames are displayed. In the

case of interlace video this behavior prevents the intermixing of fields from

different video frames.

9.2.1 Using the GLX/WGL_video_out Extension

When an application uses the WGL_NV_video_out or GLX_NV_video out extensions,

dropped frames or fields or missed vsync events are detected by catching unexpected

results in the value of the vsync counter

pulCounterOutputVideo returned

wglGetVideoInfoNV() or glXGetVideoInfoNV(). When these functions are called

each time in the draw loop, the value of

pulCounterOutputVideo should increment by

1 in the case of a progressive video format or when rendering fields in an interlaced

video format and by 2 when rendering frames in an interlaced video format. An

example of checking for dropped frames within the draw loop is outlined in Code

Listing 39.

Code Listing 39: Detecting a Dropped/Duplicate Frame

unsigned long l_ulNumVertBlanks;

unsigned long l_ulNumBufs;

static unsigned long l_ulLastNumBufs = 0;

static unsigned long l_ulLastSent = 0;

static unsigned long l_ulLastNumVertBlanks = 0;

wglGetVideoInfoNV(ghpDevList[0], &l_ulNumBufs,

&l_ulNumVertBlanks);

if (gbInterlaced) {

if (!options.field) {

l_iBufType = WGL_VIDEO_OUT_FRAME;

l_iVsyncDiff = 2;

l_bBlock = options.block;

} else {

l_iBufType = l_bField1 ? WGL_VIDEO_OUT_FIELD_1 :

WGL_VIDEO_OUT_FIELD_2;

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