Output is to stdout (but feel free to have the shell redirect to a file).
Input is read from stdin, like all other YUV4MPEG2 filters and tools. YUV4MPEG2 streams contain frames using the Y'CbCr colorspace (ITU-R BT.601). For 4:4:4 (color) streams, y4mtopnm will convert each pixel to the usual R'G'B' colorspace used for computer graphics, and produce PPM images. "MONO" (luma-only) streams will be converted to full-range [0,255] grayscale and output as PGM images.
YUV4MPEG2 streams may (often!) have subsampled chroma planes, but y4mtopnm will not process them (except in the special 'flatten' mode; see below). Subsampled streams will need to be converted to 4:4:4 using a tool such as y4mscaler.
If y4mtopnm is given a "444ALPHA" stream (4:4:4 video with an 8-bit alpha channel), the results depend on whether or not the "-P" option is specified. With "-P", it will produce PAM images with a TUPLTYPE of RGB_ALPHA, containing both color and alpha channel data. Without "-P", it will produce pairs of PPM and PGM images: a PPM for the color pixels followed by a PGM for the alpha channel data. (Such a sequence of interspersed PPM and PGM images cannot be later re-processed by pnmtoy4m. This feature is mostly for quick and easy stream debugging.)
If multiple output images are generated, they are simply output one after another. If you want to turn such a "multi-image" stream/file into individual files, use pnmsplit. (Some PNM filters can process multi-image files/streams; however, many written before June 2000 will only process the first image.)
y4mtopnm and pnmtoy4m are inverses of each other; you can typically pipe the output of one into the other, and vice-versa (except for the "444ALPHA" case noted above and the "mixed-mode" case noted below). Note that the colorspace operations can be lossy in both directions (due to range-compression and/or quantization). And, when converting to PNM, information on interlacing and sample aspect ratio of a stream is lost (but can be reconstructed by supplying command-line arguments to pnmtoy4m).
If the input stream is interlaced, the field-images are output in temporal order, i.e. the first image of a bottom-field-first stream will be the bottom-field. If the stream is progressive or has "mixed-mode" interlacing, then the temporal order is ambiguous, and the top-field will always be output first.
Note that it will not be possible to later use pnmtoy4m to reconstruct a "mixed-mode" stream from a sequence of PNM/PAM images.
+-----+ +-----+ 4:4:4 | Y | 4:4:4 w/alpha | Y | +-----+ +-----+ | Cb | | Cb | +-----+ +-----+ | Cr | | Cr | +-----+ +-----+ | A | +-----+ +-----+ 4:2:2, | Y | 4:2:0 +--+--+ +-----+--+--+ |Cb|Cr| 4:1:1 | Y |Cb|Cr| +--+--+ +-----+--+--+ +-----+ MONO | Y | +-----+In this mode, any YUV4MPEG2 chroma format is accepted, but no upsampling or colorspace conversion is performed. This is essentially just a reformatting/permutation of the original data from the input stream into a PGM image container. All the other command-line flags continue to have the same effect on output.