Curves are combined like photoshop layers: bottom to top! This program handles curves on the composite *and* the individual RGB channels if you are making subtle color corrections, for instance.
Click 'Choose files' to bring up file selection dialog, or select, drag, and drop files onto button with mouse.
Select files in order by holding down ⌘ (Mac) or Ctrl (Windows) key and selecting files in order with mouse. If you screw up the ordering, you can use your mouse to grab the handle on the left of the curve file and move it to the correct position in the stack of curves. This needs to be in the same order as in your photoshop layer adjustments because curve math is not commutative, e.g. A * B * C ≠ A * C * B!
Press composite curves button below to combine curves in order specified.
You will see the graph drawn with the input curves along with the combined curve output. To save this output, click the 'Download .acv' button. If you are using Google Chrome, a file save dialog will appear. If you are on Mac OS Safari, the file will be given a generic name and will be downloaded to your Downloads folder. In general, any of these web apps on my site will work better in Chrome than on Safari, because of the gutter bumpers that Apple insists on putting in the entire bowling alley.
If you are using this utility to do something involving digital negatives, be careful about the order of the curves. In many cases when making digital negatives, the first step is to limit the output with a curve that caps the maximum black to say 40%. If you are using a curve to do this, any subsequent measurements and correction curves created for those measurements need to be first in the processing order, followed by the limiting curve. This utility provides a good way to visualize what is going on. Try changing the order of the curves, and click the composite button to see the differences in the final curve output!
This program uses its own spline-fitting algorithm, so the resulting curve will in some cases not be exactly like the combined effect in photoshop. In all the extreme examples I have tested though, it is generally within 1-2% and you will not be able to detect any differences with your human meat-world eyeballs.