Curve Merging

From time to time curved digital negatives need some more tweeking. Whith a curvemaking tool like Chartthrob it is easy to run a second pass on an allready curved grayscale chart. But how to merge the two curves in one new curve?

After long search I found a rather simple method in this article by Martin Evening.

“It is based on a tip from Russell Williams at Adobe”, Evening explains, about saving curve data in a photoshop raw image file. A straight curve is represented by a 256×1 pixel regular gradient. Apply your curves on this file and resave it as a new curve. Stunning idea, and it works!

This is how I use this method for curve merging.
Let’s say you have two curves: curve_01.acv and curve_02.acv where curve_01.acv is your first charthrob curve and curve_02.acv is a second pass charthrob curve for finetuning.

1. open any file in photoshop and open the curves palette. You see a straight line curve.
2. click on the pencil button next to the curve button. You enter manual curve editing mode. Save this straight curve as a .amp file. Name it merge_curves.amp for example. (if you don’t click on the pencil you get a .acv instead of a .amp file and that doesn’t do the trick)
3. rename this file to merge_curves.raw making it a photoshop raw file
4. open this raw file in photoshop
5. photoshop shows a dialog asking for the image dimensions and some more details
6. in this dialog put width: 256 pixels, height: 1 pixel, channels count: 1, depth: 8 bits, header size: 0.
7. you just created a regular 256×1 pixel grayscale gradient representing all the data in a straight curve. Save (a copy of) this file for later use

8. in comes the magic!
9. tada…..

10. apply curve_01 to this file
11. apply curve_02 to the allready curved file (doing exactly as you would do with your real file)
12. flatten, if needed, and save as: my_merged_curves.amp
13. open the image you want to curve
14. apply my_merged_curves.amp et voila!
15. if you want a regular .acv file click on the curve button next to the pencil button in the curves palette and save the curve again.

Many thanks to Martin Evening and Russel Williams for pointing this out.

It looks somewhat complicated but with an allready saved raw file it is a snap.
This method can also be used to flip a curve.



  1. Beste Kees,

    Ik probeer al een aatal maal je tutorial te gebruiken voor het samen stellen van 2 curven. het lukt mij tot punt 12. ik heb dus twee curven in de RAW file maar ik kan dat niet opslaan als een .amp file in CS3. ik kan er wel gewoon amp achtertikken natuurlijk maar kan dan dit niet meer als curve gebruiken. kan je me een tip geven?


    remko de graaff

    — Remko    29 12 2009 - 13:19    #
  2. Je slaat het bestand als photoshop raw bestand op en geeft het een nieuwe naam met de extensie .amp. Dat bestand kun je vervolgens in het curvepalet ‘laden’ en weer veranderen in een .acv bestand.

    Kees Brandenburg    29 12 2009 - 14:53    #
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