Vibrance

[Solido]

Hi!

I'm looking for this feature but not sure about the path to travel,
I've asked for it in libvips first so it can bubble up here.

In other case here's a interesting link on a method relying on HSV
https://stackoverflow.com/a/71744334/5506671

Best regards

[kipcole9]

Yes, saw your topic there and was about to suggest the answer probably lies in converting to hsv colourspace and adjusting the s band with an appropriate tone curve. I'll be very interested to see what John suggests, and I'll happily work up a prototype for image.

[kipcole9]

This discussion is also somewhat related.

[kipcole9]

vibrance (not including compensation for skin tones) is something like this:

{:ok, vibranced} = 
  Image.with_colorspace(image, :hsv, fn hsv ->
    # Apply a tone curve to the saturation band. See the docs for the options
    {:ok, mapped} = Image.apply_tone_curve(hsv[1])
    # Rejoin the bands to create a new :hsv image
    Image.join_bands([hsv[0], mapped, hsv[2]])
  end)

The defaults for Image.apply_tone_curve/2 are quite mild so you might look at options that fit specific image requirements.

I'm not sure that it's straight forward - or even appropriate - to derive an individual value for driving vibrance but I'm open to suggestions. If you find a combination of parameters to Image.apply_tone_curve/2 that you think best represents vibrance then I can consider adding it as a distinct function.

[Solido]

It's working well!
I've tuned it a little as I'm learning
Can you join the discussion in Vips too?

[kipcole9]

I've implemented Image.vibrance/3. Would you consider giving that a test run before I release it? Just configure {:image, github: "elixir-image/image"} in your mix.exs.

[Solido]

Hi Cole, the effect is there yet very subtle. At force 1 a threshold of 60 seems more aligned but maybe after skintone (aka gold tone) preservation it will behave differently. 70 is too much right now.

Image.vibrance!(image, -0.8) => error, guard on negative number.

It's really great to have this kind of effect in Image, it's more about the fine tuning.

Thanks a lot!

[kipcole9]

The next thing I plan to do is look at skin tone masking. Basically it will use Image.chroma_mask/2 - I just need to work out how to best select skin tones. Which are of course a wide range. I'm going to use this research as a guide.

[Solido]

After more reading I've found out that every photo editing software in fact deal with skin tones preservation but also sunset colors! My interpretation is that we instantly invalidate pictures with non aligned face color since we're so sensitive to people around us.

The skin color detection algo would be a fantastic integration. If possible it would be empowering to be able to pass this color range as a parameter. Photo editor does not offers that but from an artistic point of view it make a lot of sense to be able to work around a specific color spectrum.

Question: can we use a list of chroma masks instead of just one?

[kipcole9]

Chroma masking, including skin tone masking, will be separate from vibrance. I'm pretty sure there are other reasons for it. Basically you can apply a mask to any image. Therefore the approach will be, like a lot of image processing, to compose the vibrance image with the mask and the base image.

I'm not trying to replace Lightroom or photoshop - just provide composable image processing primatives at one level higher than libvips.

[kipcole9]

I've changed the default threshold to 60 as you suggest. And the vibrance now is adjusted to be -1.0 to +1.0. Both changes are now on the main branch so mix deps.update image should do it.

[Solido]

Maybe we can also expose h3 = 6.0 * h1 / h2 as vibrance.options parameter to fine tune the curve?

[kipcole9]

I think it might be more appropriate to add a parameter that takes any logistic function with a default being the one that John created. That way anyone can create their own function.

[Solido]

If possible yes! maybe a standardized way to create such curves that would align with lut or histogram. At the end of the day, the more we can define curves the more control we've everywhere.

[Solido]

To clarify I would rather have a formula that convert to img and then pass this img to align with other methods in the framework than having a different approaches that take code or img. Looking for a common representation.