Understanding +[-2,-1], etc. in source code

Software G'MIC
1

I’m reading through the G’MIC source code to understand it better. I keep coming across lines like +[-2,-1] and I’m not exactly sure how to process this. At first I thought it meant “add”, but add only takes one param.

Let’s take Retro Fade for example:

fx_retrofade :
  repeat $! l[$>] split_opacity l[0]
    +f 0
    repeat $1
      +noise[0] $3 c. 0,255 autoindex. $2,0,0
      +[-2,-1]
      progress {$>*100/$1}
    done
    k. n 0,255
    progress 100
  done a c done done

From what I can tell, the meat of this means:

(1) fill the image with black
(2) for each iteration, apply noise to the original, clamp the noisy image and autoindex it
(3) ??? this is where I’m confused

Then repeat and keep the last image.

Can someone help me figure out this missing piece?

2

+[-2,-1] means " add images [-2] and [1] together, and merge the result into a single image."

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3

@groose Welcome!

+ in front of command means make new copy of image(s) and do command

  • hint: + means add and ++ means make new copy and then add
  • arithmetic operators can be in two forms: add[-2,-1] or add[-2] [-1]
    – the first one adds the two images and the result ends up in location [-2] (which is the same as [-1])
    – the second one adds [-1] to [-2] and the result is found in [-2] but [-1] still exists

[...] and ... is the selection syntax

  • in the case of [-2,-1], it involves the second last and last images in the image sequence
  • and a single . means [-1]

Together, the command does the following:

  • Repeat through all input images separately
  • Only work on RGB, separating RGBA and combining it at the end
  • Create a new 0 image
  • Create another copy of [0] and then denoise, clip and index it
  • Add this to the 0'ed image (i.e., [1], which updates each time there is a repetition)
  • Iterate the above and then keep the result after the loop ends
  • Normalize image to range
  • progress shows the progress in the terminal/console
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4

Thanks the the both of you! I really appreciate the full breakdown, this has been very helpful!

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