Combining 2 GraphicsMagick commands into 1 command, like in ImageMagick - php

Question

I was using ImageMagick to create a new resized image with watermark, with this single command (in PHP):
exec("convert -filter Lanczos {$original_image} -thumbnail {$max_width}x{$max_height} -quality 90 {$watermark} -gravity center -unsharp 2x0.5+0.7+0 -composite {$cached}");
Now I switched to GM and am looking for a way to run 1 command to do the same task. The only way I found was to split it to 2 separate commands:
//create the resized image
exec("gm convert -filter Lanczos {$original_image} -thumbnail {$max_width}x{$max_height} -quality 90 -unsharp 2x0.5+0.7+0 {$cached}");
//apply the watermark and recreate the watermarked image, overwriting the previously resized image
exec("gm composite -quality 90 -dissolve 100 -gravity center {$watermark} {$cached} {$cached}");
Is there a way to combine them into 1 single command and by that maybe also reduce resources & drive usage?

Answer 1

I have received the following reply on this from Bob Friesenhahn, GraphicsMagick Maintainer:
You did not say what version of GraphicsMagick you are using. Modern
versions support a '-compose' option which may be put on the command
line after the input file name to remember the composition algorithm
to use. This composition algorithm is then used if the -mosaic or
-extent operators are used to do a composition. You can also use a
-page option after the input file name to locate the image when it
is composited with prior images in the list. Due to a weakness in
GM's convert command processing, the -mosaic or -extent operators must
be the last command prior to saving the output file. I believe that
ImageMagick's -composite must be a version of -mosaic which adds more
features (e.g. -mosaic might not support gravity but -composite does).
It seems like GraphicsMagick should implement something completely
compatible with ImageMagick's -composite.
Regardless, there is an effective workaround available if you need to
use your existing GM commands.
If you have a modern GraphicsMagick which supports 'gm batch', then
you can use the 'mpr' coder ("Magick Persistent Registry") to remember
intermediate images between commands and you can easily adapt your two
commands to execute with full efficiency using the existing command
lines. This Unix shell example should give you some ideas:
{
echo convert seaworld.jpg mpr:temporary
echo convert mpr:temporary crap.jpg
} | gm batch -prompt off -echo on
convert seaworld.jpg mpr:temporary
convert mpr:temporary crap.jpg
Notice that the output of the first command was saved (as an image
handle as natively used within GraphicsMagick) into 'mpr:temporary'
and then the second command took input from 'mpr:temporary' and wrote
the final output file. You can use arbitrary string arguments to
'mpr:' so you can have several images "in flight".
With this approach you can use 'gm convert' and 'gm composite' in the
same batch command.
I am not sure how one would best access this batch facility from PHP
but if PHP can stream commands to it from a pipe, then it can run for
quite a long time as a co-process to PHP and save considerable compute
time and overhead.