why xvid_decoder not export yuv file

[tim__huang]

in xvid_decraw.c,xvid_decraw call dec_main to decoder one frame and get one originality image,

static int
dec_main(unsigned char *istream,
unsigned char *ostream,
int istream_size,
xvid_dec_stats_t *xvid_dec_stats)

and than it save in file used function write_image
static int write_image(char *prefix, unsigned char *image)

and we can see that write_image only save image as png pgm or tga file format .

in the function write_image,the second param ,image,is yuv file format?and why it save it as yuv file format?