ImageMagick: How to batch append 4 parts of images into one (2 rows, 2 columns) (I have 500+ images that need to be combined like this) - powershell

Question

everyone!
I am using ImageMagick-7.0.10-Q16 on Windows 10. I’ve tried Googling for answers, but I’m still left very confused about how to do this. Most of the answers have been for UNIX and not Windows, I have no idea what it means, or given me errors. I don’t have any experience with coding or Windows PowerShell, so forgive my slowness
I have scanned pages of books that have been split into four pieces of jpg files. The images are named after the page number and the orientation of the corresponding piece. BL=Bottom left. BR=Bottom right. TR=Top right. TL=Top left. (BM=Bottom pieces merged. TB=Top pieces merged). So “BL0001.jpg" is the bottomleft piece of page 1. I’m not mentioning their sizes because I don’t want them to be resized or whatever. I just want them to be combined via append like a puzzle like this:
Combined jpg pieces.
The borders and the text-boxes there are just to demonstrate, and are not to be included
So the files are for example like this:
BL0001.jpg
BR0001.jpg
TL0001.jpg
BR0001.jpg
BL0002.jpg
BR0002.jpg
TL0002.jpg
BR0002.jpg
And so on...
This was the last thing I’ve tried in Windows PowerShell:
magick convert B*0001.jpg +append 0001BM.jpg
magick convert T*0001.jpg +append 0001TM.jpg
magick convert 0001*.jpg +swap -append 0001merged.jpg
This combines 4 parts into one image just like I want it to. I found out adding * works like a wildcard and merges all the images like BR and TR together in one go. But I can’t do that for the page number (in this case ‘0001’ in ‘B*0001.jpg’), because that would merge all the files in the folder into the same image, something I don’t want. So what I want to figure out is to how to “batch” run this command for with a sequential numbering system for the different pages. In other words, use a command to batch combine pieces of an image into one image, but with all the scanned pages in jpg in the folder. I know the commands above create addition files with the merged top and bottom parts before the final merge, but I don’t know how to make this command otherwise. I'm willing to try other commands/things too

Answer 1

Using ImageMagick v7 in a simple Windows BAT script you could do something like this...
#echo off
setlocal EnableDelayedExpansion
for /l %%n in ( 1 1 9999 ) do (
set V1=000%%n
set V1=!V1:~-4!
magick *!V1!.jpg +append -crop 2x1# +swap -append +repage !V1!merged.jpg
)
exit /b
That uses a "for" loop to read all four "*0001.jpg" images at a time into an ImageMagick command. The "set V1=" lines are to make sure the variables have the correct number of leading zeros.
The IM command appends, crops, and appends the four images into the properly ordered output, and writes the image as "0001merged.jpg". Then it moves on to process "*0002.jpg" and so on.
I put a top limit on the number of image sets to process with that "9999" in the "for" command to work with the number of leading zeros. Make sure that number is the same or more than the number of image sets you have. It will just print an error for each loop after it goes over the number of image sets, but no harm done.
Note: Using ImageMagick v7 you should just use "magick" because when you use "magick convert" it emulates IMv6 behavior. You probably won't usually want that.