ppmsave produces files jpeg-archive can't handle

[chregu]

I'm using https://github.com/danielgtaylor/jpeg-archive for further compressing jpeg while keeping quality. For that I use jpeg-compare/jpeg-recompress from jpeg-archive. Those two commands take a ppm file as input. Which works perfectly fine when the ppm is produced by imagemagick, but not when I use ppmsave from vips, eg.

$image = \Jcupitt\Vips\Image::newFromFile("f.jpg");
$image->ppmsave("f.ppm");

jpeg-compare then reports

$  /usr/local/bin/jpeg-compare -r -m ssim f.ppm f.jpg
Unsupported bit depth 640!

(the bit depth number is always different)

Are should I report that to jpeg-archive? I see that someone actually started maintaining it again this month

[jcupitt]

Hello @chregu, sorry, this issue dropped off my radar in the xmas build up and I've only just found it again :(

I tried here and it seems to work:

$ vips copy ~/pics/k2.jpg x.ppm
$ jpeg-compare -r -m ssim x.ppm ~/pics/k2.jpg
SSIM: 0.999998

The error Unsupported bit depth 640! seems to suggest a 16-bit PPM. If I look at the head of my x.ppm I see:

P6
#vips2ppm - Sat Mar 10 18:12:54 2018

1450 2048
255
[binary data]

ie. a regular JPG should just have 255 for the max value.

Anyway, my guess would be that you are doing some other processing which is making a larger bit-depth PPM.