Hello,

Here is a tutorial to learn the "GEGL graph" filter.
EDIT: For the motivations, you can read this thread first.
EDIT (thanks to rich2005): Warning: this filter doesn't make a single effect, but it enables you to program almost all effects and combine them.

1) First steps:

• Open an image
• Go to "Filters->Generic->GEGL graph
  
  Attachment:
  
  menu.jpg [ 42.83 KiB | Viewed 41491 times ]
  
• Click the "pipeline" box
  
  Attachment:
  
  pipeline.jpg [ 53.31 KiB | Viewed 41491 times ]
  
• Hit "ctrl-a" and the "delete" key
  
  Attachment:
  
  emptyPipeline.jpg [ 31.1 KiB | Viewed 41491 times ]
  
• Write anything (for example, "sdfg")
• ...to be continued in the next post...

• Look at below the "pipeline" box
  
  Attachment:
  
  info.jpg [ 39.25 KiB | Viewed 41493 times ]
  
  This part will be the most useful one (It's the info zone)
  but, for now, it's not very useful : it just indicate that the "sdfg" operation doesn't exist
  .
• Delete "sdfg" and write "e"
  
  Attachment:
  
  info2.jpg [ 55.76 KiB | Viewed 41493 times ]
  
  Now, it's more useful, it offers suggestions
  .
• We choose "gegl:edge". Note: you don't need to write "gegl:"
  So, continue to write to obtain "edge" and look at the info zone for each character you type
  
  Attachment:
  
  info3.jpg [ 100.13 KiB | Viewed 41493 times ]
  
  The info zone has disappeared and the "edge" effect is done
  .
• Congratulations, you have written your first GEGL graph.

Note: Some operations don't belong to GEGL. For these ones, you have to prefix the operation with "gimp:" (for example, "gimp:desaturate").

Have fun!

To be continued...

2) How to add parameters:

You wish to add parameters but you don't know which exist : no problem .

• After "edge" write a blank space and write "aa=" (without any spaces between "aa" and "=").
  
  Attachment:
  
  param1.jpg [ 42.31 KiB | Viewed 41410 times ]
  
  The info zone indicates all possible parameters (It calls them "properties")
  .
• We choose "algorithm". So replace "aa" by "algorithm"
  
  Attachment:
  
  param2.jpg [ 43.18 KiB | Viewed 41410 times ]
  
  The info zone indicates all possible value for the "algorithm" parameter
  .
• We choose "differential". So, just after "=", write "differential" (without any spaces between "=" and "differential").
  
  Attachment:
  
  param3.jpg [ 34.14 KiB | Viewed 41410 times ]
  
  The info zone has disappeared and the "edge algorithm=differential" effect is done

.
CAUTION: "id=blahblah" and "ref=blahblah" look like parameters and are written like parameters but they are NOT, they are special operations
(We will see later their use).
(If you are very curious and cannot wait, "id=blahblah" is "save" and "ref=blahblah" is "retrieve")

Have fun!

To be continued...

I like the way you're teaching, by showing that the cat can walk on our keyboard and the result will be that we won't break anything but get help, is very nice

Thanks for your valuable feedback!

3) How to chain multiple effects:

You simply list all the effects each in one line like this:
Note: You don't have to put them into multiple lines. You may put them into a single line but separated by spaces (It's just less clear ).
Note: We will see later that it's possible to get all intermediate stages back.

Have fun!

To be continued...

Thanks for doing this tutorial. I find it very helpful.

Thanks, nelo, for your valuable feedback!

4) How to save and retrieve any intermediate stage:

To save an intermediate stage, simply add the special operation "id=chosenName" where the intermediate stage occurs.
For example:
To retrieve an intermediate stage, simply add the special operation "ref=nameOfStageToRetrieve" where you want it to come back.
For example:
This is a crazy example (since I discard "invert-gamma" result ), but we will see useful examples later.

Have fun!

To be continued...

I'm curious as to what this plugin does.

Excellent!
Thanks cli345 (out of curiosity, does cli in your nickname stand for "command line interface"?)

Maybe, but I'm guessing it's derived from Claude and the French city of Lyon.
This looks interesting

Thank you, mackenzieh, for your question.
Rich2005 has answered here (Thanks, rich2005).

This diagram is aimed to explain my last (crazy) example:

(EDIT: I've changed the diagram)

I think I get it now.

5) How to combine multiple effects:

Some operations (for example, "multiply", "subtract") enable you to combine two results.
These operations accept a special parameter : "aux=[ ... ]" (No space around "=", a mandatory space after "[" and before "]").

For example, one of these operations is "subtract" (this operation does the same job as "subtract" layer mode).
In "GEGL graph", write:

Notes:

• (When you've typed "aux=", the info zone says "aux" parameter doesn't exist (It's not true). But, when you've typed "[", the error disappears.)
  
• This GEGL graph does the same effect as this process:
  
  • duplicate the layer
  • apply a gaussian blur onto the upper layer
  • set the mode of upper layer to "subtract"
  • do "merge down" on the upper layer
• ("aux=[ blahblah ]" seems always accepted but, for most operations, it does not change anything.)
  
• This GEGL graph can be compacted to "id=myStart subtract aux=[ ref=myStart gaussian-blur ]"

Have fun!

To be continued...

I was just scratching my blockhead if it's possible to change layer modes somehow, having workflows that include stress, fattal and mantiuk filters with changed layer modes. This is fantastic information, now I can convert some of them into GEGL Graph one-clickers.
Thank you Claude.

great stuff, thanks!

Thanks, for your feedback!

This diagram is aimed to explain my last example:

The last diagram can be summarized by:

6) Practice

Code this "GEGL graph":

Here are loose fragments for easy copy-pastes:


Warning: At each letter typed, the whole effect is recomputed. So, it's better to write in a text editor and copy-paste onto the "GEGL graph" window.

Have fun!

To be continued...

Good hint, I disable preview when testing longer chain of effects and use a small image.
Few times when editing a longer chain somewhere in between, it got stuck and I had to click the GEGL Graph exit button which resulted in crash.

Practice result