lowpass improvement ?

Software darktable
1

Following the Harry Durgin tutorial about the interesting properties of lowpass & highpass, I’ve noticed that, if it is easy to increase the overall contrast, it’s sometime more difficult to increase local constrast in both low and high lights at the same time.
To solve this Harry exports his image after high lights improvement for example, then tweaks the low lights and recomposes the 2 images.
Basically the lowpass module amplifies (contrast) the difference between image & blurred image around the image middle grey but doesn’t give the possibility to chose the middle point of this amplification.
I’ve made a try intoducing in lowpass module a fulcrum contrast slider (by default at 50, Lab L) letting the user set where he wants to make the amplification happen.

Image before lowpass.

image
image1045×573 372 KB

Adding 2 lowpass instances, one for low (fulcrum=15) and the other for high lights (fulcrum=75).
image
image1030×531 379 KB

You can see on the histogram that each pic has enlarged, but the overall contrast of the image has not changed that much.

What do you think ?

2

Sounds like a good idea! Best to propose a PR and GitHub for others to play with that and see how it works on more pictures.

3

I think this use case overlaps exactly with what shadows/highlights module does (which is exactly a masked lowpass filter). For this, you can even use local-contrast and lower down the midtone range or the equalizer. I don’t get why people love to hurt themselves with low-pass and its workarounds when you have specialized modules that have the workarounds built-in already.

Now that I think of it, a contrast fulcrum in equalizer would be great.

1 Like
4

Yes, I think than some other modules based on blurred image difference could also have this contrast fulcrum added.