17

I am generating random data and trying to convert it to a PNG image using :

head -c 1MB < /dev/urandom | hexdump -e '16/1 "_x%02X"' | sed 's/_/\\/g; s/\\x  //g; s/.*/    "&"/' | tr -d "\"" | display -depth 8 -size 1000x1000+0 rgb:-

This command always shows a greyish image with some RGB pixels. What am I doing wrong ?

My final goal is to generate at least one image with random data.

Improve this question
4
  • Define "random." The visual average of a bunch of random RGB values will tend toward grey, after all.
    – Wildcard
    Commented Jun 14, 2016 at 2:20
  • That is what I thought, but I could not confirm this theory since every picture looks almost the same.
    – pxoto
    Commented Jun 14, 2016 at 2:22
  • 15 years ago I did something similar in Basic (Chipmunk Basic, to be specific). I had a small graphics window and kept outputting a pixel of random color to random location. The result was a constantly changing picture that still looked essentially the same the whole time—like color static on an old fashioned TV. It's not really grey, but color static.
    – Wildcard
    Commented Jun 14, 2016 at 2:26
  • I have managed to generate some static but the images are mostly still gray.
    – pxoto
    Commented Jun 14, 2016 at 3:15

1 Answer 1

Reset to default
26

Firstly, you need to feed display RGB:- raw bytes, not an encoded hex string like you're building with that hexdump | sed | tr pipeline.

Secondly, you aren't giving it enough bytes: you need 3 bytes per pixel, one for each colour channel.

This does what you want:

mx=320;my=256;head -c "$((3*mx*my))" /dev/urandom | display -depth 8 -size "${mx}x${my}" RGB:-

To save directly to PNG, you can do this:

mx=320;my=256;head -c "$((3*mx*my))" /dev/urandom | convert -depth 8 -size "${mx}x${my}" RGB:- random.png

Here's a typical output image:

RGB image generated from /dev/urandom

If you'd like to make an animation, there's no need to create and save individual frames. You can feed a raw byte stream straight to ffmpeg / avconv, eg 

mx=320; my=256; nframes=100; dd if=/dev/urandom bs="$((mx*my*3))" count="$nframes" | avconv -r 25 -s "${mx}x${my}" -f rawvideo -pix_fmt rgb24 -i - random.mp4
Improve this answer
3
  • 1
    Stream /dev/urandom straight to ffmpeg - a genius idea!
    – Ryan Amaral
    Commented Jul 30, 2021 at 8:03
  • 1
    @Ryan Thanks! It seems to be popular, going by the votes. :) You might like this answer and the linked Myths about /dev/urandom article. BTW, if you want fast, high quality random numbers, please see the PCG family.
    – PM 2Ring
    Commented Jul 30, 2021 at 8:28
  • Thank you for sharing those resources @pm-2ring! A quick note: I found useful changing the output name for something different every time. Changing from random.png ​to random$(date +%Y%m%d%H%M%S).png does the trick for me.
    – Ryan Amaral
    Commented Jul 30, 2021 at 12:27

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .