y4mtopnm(1)

🎀 y4mtopnm, section 1 of the MJPEG tools manual 💖

okay so bestie, buckle up — this is y4mtopnm from the MJPEG tools manual, and it's genuinely one of the most useful little gremlins in the whole toolchain. it turns your YUV4MPEG2 video stream into a whole pile of images. let's get into it ✨

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🌸 NAME 🌸

y4mtopnm — convert a YUV4MPEG2 stream to PNM images 🦋

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💅 SYNOPSIS 💅

y4mtopnm [options]

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✨ DESCRIPTION ✨

so like, y4mtopnm takes a YUV4MPEG2 stream and converts it into a sequence of raw PPM, PGM, or PAM images. and yeah, technically "pnm" is the umbrella over PPM, PGM and PBM — but PAM is a fourth format that can effectively contain any of the other three, and more. so it's pnm's cooler older sister basically 💅

Output goes to stdout (but feel free to have the shell redirect it into a file, no judgement, that's literally what it's for).

Input gets read from stdin, same as literally every other YUV4MPEG2 filter and tool. that's the vibe. 🌈

YUV4MPEG2 streams contain frames using the Y'CbCr colorspace (ITU-R BT.601). For 4:4:4 (color) streams, y4mtopnm converts each pixel to the usual R'G'B' colorspace that computer graphics lives in, and spits out PPM images. And "MONO" (luma-only) streams? Those get converted to full-range [0,255] grayscale and output as PGM images. slay.

⚠️ okay important one: YUV4MPEG2 streams may (often!) have subsampled chroma planes, but y4mtopnm will not process them — except in the special 'flatten' mode, see below. Subsampled streams need to be converted to 4:4:4 first using a tool such as y4mscaler. no cap.

Now. If you hand y4mtopnm a "444ALPHA" stream — that's 4:4:4 video with an 8-bit alpha channel — the results depend on whether or not you passed the -P option, and this is genuinely the plot twist:

• With -P — it produces PAM images with a TUPLTYPE of RGB_ALPHA, containing both color and alpha channel data. gorgeous, complete, iconic.
• Without -P — it produces pairs</> of PPM and PGM images: a PPM for the color pixels followed by a PGM for the alpha channel data.

😭 heads up: 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. it's giving "look at it, don't try to reverse it."

If multiple output images are generated, they're just output one after another, back to back, no ceremony. 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. old ones are not passing the vibe check, sorry.)

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). so write that down somewhere bestie.

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👑 OPTIONS 👑

y4mtopnm accepts the following options:

• -P — Produce PAM format output, instead of PPM and/or PGM. PAM can encode a superset of PNM, but is not yet understood by as many graphics programs. (Indeed, surprisingly few of the NetPBM tools seem to understand it. embarrassing for them honestly.)
• -D — Produce a separate output image for each field of each frame. (Otherwise, a single image per frame, containing two interleaved fields, is created.)

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. it's a one-way door, bestie.

• -f — Produce a special 'flattened' output, most useful for analyzing and debugging streams. All the planes of a frame/field are tiled together into an enlarged PGM (or grayscale PAM) output image as follows:

               +-----+                    +-----+
       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. ✨

• -v [0,1,2] — Set verbosity level. 🗣️
  • 0 = warnings and errors only.
  • 1 = add informative messages, too.
  • 2 = add chatty debugging message, too.

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🌈 EXAMPLES 🌈

To turn the first 15 frames of an (MJPEG or DV) AVI file into individual PPM files:

lav2yuv -f 15 your-video.avi | y4mscaler -O chromass=444 | y4mtopnm
| pnmsplit - "your-video-%d.ppm"

look at that pipeline. beautiful. no notes. 🔥

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💜 AUTHOR 💜

This manual page was written by Matt Marjanovic.

If you have questions, remarks, problems or you just want to contact the developers, the main mailing list for the MJPEG-tools is:

mjpeg-users@lists.sourceforge.net

For more info, see our website at

http://mjpeg.sourceforge.net/

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🦋 SEE ALSO 🦋

pam(5), pgm(5), pnm(5), ppm(5), pnmtoy4m(1), mjpegtools(1), mpeg2enc(1), lav2yuv(1), pnmsplit(1), y4mscaler(1)

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and that's the whole page bestie 💖 this one was brought to you by the MJPEG Tools Team, dated 28 April 2004 — oldie but a goodie, and yes she still serves. go convert something. periodt. 👑✨