As usual, examine the stdlib: https://raw.githubusercontent.com/dtschump/gmic/master/src/gmic_stdlib.gmic. Contains the code. $ and ${...} indicate the arguments. Sorry, don’t have the time to explain further.
As usual, examine the stdlib: https://raw.githubusercontent.com/dtschump/gmic/master/src/gmic_stdlib.gmic. Contains the code. $ and ${...} indicate the arguments. Sorry, don’t have the time to explain further.