DM6437 . Why video_preview() cannot be used as a usual funcution be called in main() ?

In dvsdk_1_01_00_15 or  dvsdk_1_11_00_00,   viedo_preview is a demo.

In this demo, there is a function called video_preview(), which is showed at the last of the post. And this function is called in the PreviewTask of DSP/BIOS, working well.

I modified this demo. Now calling  video_preview() function manual in main() function, but no display.

I feel puzzle, Is there any wrong ?

/*
* ======== video_preview ========
*/
void video_preview(void) {

FVID_Frame *frameBuffTable[FRAME_BUFF_CNT];
FVID_Frame *frameBuffPtr;
GIO_Handle hGioVpfeCcdc;
GIO_Handle hGioVpbeVid0;
GIO_Handle hGioVpbeVenc;
int status = 0;
int result;
int i;
int standard;
int width;
int height;

/* Set video display/capture driver params to defaults */
PSP_VPFE_TVP5146_ConfigParams tvp5146Params = 
VID_PARAMS_TVP5146_DEFAULT;
PSP_VPFECcdcConfigParams vpfeCcdcConfigParams = 
VID_PARAMS_CCDC_DEFAULT_D1;
PSP_VPBEOsdConfigParams vpbeOsdConfigParams = 
VID_PARAMS_OSD_DEFAULT_D1;
PSP_VPBEVencConfigParams vpbeVencConfigParams;

standard = read_JP1();

/* Update display/capture params based on video standard (PAL/NTSC) */
if (standard == STANDARD_PAL) {
width = 720;
height = 576;
vpbeVencConfigParams.displayStandard = PSP_VPBE_DISPLAY_PAL_INTERLACED_COMPOSITE;
}
else {
width = 720;
height = 480;
vpbeVencConfigParams.displayStandard = PSP_VPBE_DISPLAY_NTSC_INTERLACED_COMPOSITE;
}
vpfeCcdcConfigParams.height = vpbeOsdConfigParams.height = height;
vpfeCcdcConfigParams.width = vpbeOsdConfigParams.width = width;
vpfeCcdcConfigParams.pitch = vpbeOsdConfigParams.pitch = width * 2;

/* init the frame buffer table */
for (i=0; i<FRAME_BUFF_CNT; i++) {
frameBuffTable[i] = NULL;
}

/* create video input channel */
if (status == 0) {
PSP_VPFEChannelParams vpfeChannelParams;
vpfeChannelParams.id = PSP_VPFE_CCDC;
vpfeChannelParams.params = (PSP_VPFECcdcConfigParams*)&vpfeCcdcConfigParams;
hGioVpfeCcdc = FVID_create("/VPFE0",IOM_INOUT,NULL,&vpfeChannelParams,NULL);
status = (hGioVpfeCcdc == NULL ? -1 : 0);
}

/* create video output channel, plane 0 */
if (status == 0) {
PSP_VPBEChannelParams vpbeChannelParams;
vpbeChannelParams.id = PSP_VPBE_VIDEO_0;
vpbeChannelParams.params = (PSP_VPBEOsdConfigParams*)&vpbeOsdConfigParams;
hGioVpbeVid0 = FVID_create("/VPBE0",IOM_INOUT,NULL,&vpbeChannelParams,NULL);
status = (hGioVpbeVid0 == NULL ? -1 : 0);
}

/* create video output channel, venc */
if (status == 0) {
PSP_VPBEChannelParams vpbeChannelParams;
vpbeChannelParams.id = PSP_VPBE_VENC;
vpbeChannelParams.params = (PSP_VPBEVencConfigParams *)&vpbeVencConfigParams;
hGioVpbeVenc = FVID_create("/VPBE0",IOM_INOUT,NULL,&vpbeChannelParams,NULL);
status = (hGioVpbeVenc == NULL ? -1 : 0);
}

/* configure the TVP5146 video decoder */
if (status == 0) {
result = FVID_control(hGioVpfeCcdc, VPFE_ExtVD_BASE+PSP_VPSS_EXT_VIDEO_DECODER_CONFIG, &tvp5146Params);
status = (result == IOM_COMPLETED ? 0 : -1);
}

/* allocate some frame buffers */
if (status == 0) {
for (i=0; i<FRAME_BUFF_CNT && status == 0; i++) {
result = FVID_allocBuffer(hGioVpfeCcdc, &frameBuffTable[i]);
status = (result == IOM_COMPLETED && frameBuffTable[i] != NULL ? 0 : -1);
}
}

/* prime up the video capture channel */
if (status == 0) {
FVID_queue(hGioVpfeCcdc, frameBuffTable[0]);
FVID_queue(hGioVpfeCcdc, frameBuffTable[1]);
FVID_queue(hGioVpfeCcdc, frameBuffTable[2]);
}

/* prime up the video display channel */
if (status == 0) {
FVID_queue(hGioVpbeVid0, frameBuffTable[3]);
FVID_queue(hGioVpbeVid0, frameBuffTable[4]);
FVID_queue(hGioVpbeVid0, frameBuffTable[5]);
}

/* grab first buffer from input queue */
if (status == 0) {
FVID_dequeue(hGioVpfeCcdc, &frameBuffPtr);
}

/* loop forever performing video capture and display */
while ( status == 0 ) {

/* grab a fresh video input frame */
FVID_exchange(hGioVpfeCcdc, &frameBuffPtr);

/* display the video frame */
FVID_exchange(hGioVpbeVid0, &frameBuffPtr);

}