I'm writing a shell script to concatenate some images, and I'm using the vips command-line because it has low memory requirements (and I intend to work with extremely large images).
However, if one image is RGB and the other is RGBA, it fails:
$ vips insert foo3.png foo4.png foo.png 64 0 --expand --background '0 0 0 0'
insert: images must have the same number of bands, or one must be single-band
$ file foo?.png
foo3.png: PNG image data, 1 x 1, 8-bit/color RGB, non-interlaced
foo4.png: PNG image data, 1 x 1, 8-bit/color RGBA, non-interlaced
How can I convert the input image to RGBA when using vips? I've been searching through the documentation, but I can't find it.
I'm also willing to use nip2 tool, which also uses libvips. I'm not willing to use ImageMagick (or similar tools) because the script will end up working with images as large as 20k×20k pixels, which take several gigabytes of RAM, more than I have now.
vips bandjoin_const source.png dest.png 255
255 for opaque, 0 for transparent.