Affinity Photo curves settings

[Mujabad]

When I' m using the Curves diagram in AP, I would like to only lighten or darken a specific brightness in the image. When pinpointing a certain spot on the curves line and trying to lighten the deep black shadows below that value, I also see the mid lights are changing (probably relative to and compensating for what I' m trying to do).

Let’ s just say I want to lighten below a certain black point (maybe 0.6 or any other value). I don’ t know how to fill in any numbers (like maybe 0.6 as base /or any other number). It’ s also impossible to get a point fixed on the curves line/trying to adjust only the blacks that are below that point, without moving the whole line. 

Is there a way to move the black point up to a certain value (that could be a base value for a fine art photo paper), without affecting mid- and higher lights?

Thank you very much!

[smadell]

Good morning, @Mujabad. I also use DxO PhotoLab (as you do) and I noticed your question on the DxO forum. I followed it here, and thought I would give you a hand. There are a number of ways to do what you're after inside of Affinity Photo. I am going to use a black & white gradient as a base layer.

The easiest way to lighten up a black point is to use the Levels adjustment. Add a Levels adjustment and bring the "Output Black Level" slider up to the desired point. This will set your black point there. Given the math, I think (not 100% sure) that other pixels will adjust themselves accordingly.

You can do something similar in Curves, and this gives you more leeway to leave the other pixels less affected or unaffected. However, since the Curves adjustment uses a Bezier curve, raising one point will sometimes have a compensatory change on other points. The solution to this is to place multiple points on the Curves line, and these additional points are "anchors" which will prevent too much change in the curve other than in the area you want.

In the example below, the red arrows represent raising the black point (notice that the leftmost point has been set "up" along the left border, and this can be dialed in precisely using the Y coordinate at the bottom). The green arrows are the anchor points which have the effect of preventing too much change along the rest of the curve.

The best way of placing an adjustment so that it ONLY affects a specific lightness range in your image is to use Blend Options (which is similar to Blend-If in Photoshop, although a hell of a lot more powerful). You can access it from the Layers panel, or directly from an adjustment panel.

In the last example (below) I am putting a Recolor adjustment onto the gradient (simply because it's easy to see) and then clicking the Gear icon inside of the adjustment panel. The Blend Options panel opens up, and the settings I have used limit the adjustment so that it only allows the Recolor adjustment to be visible when the underlying pixels (the gradient itself) have luminosity values of 25% or less.

Affinity Photo is a super-powerful program, and it is undoubtedly capable of doing what you're trying to accomplish.

[thomaso]

1 hour ago, Mujabad said:

I would like to only lighten or darken a specific brightness in the image. When pinpointing a certain spot on the curves line and trying to lighten the deep black shadows below that value, I also see the mid lights are changing

Another option aside levels / curves, for shadows & highlights only and without the option to directly type the desired value for black:
In the Info panel set a column to your preferred mode, e.g. Grayscale + place a marker at a darkest image spot. Then use a Shadows/Highlights adjustment for the intensity of black with the Strenght slider + optionally the Range slider to widen the affected colour values.

[Lisbon]

(Lots of good suggestions)

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2 hours ago, Mujabad said:

Let’ s just say I want to lighten below a certain black point (maybe 0.6 or any other value).

Look at the Min/Max option inside curves.

Just for demonstration purposes, im going to darken below 0.6.
Its easier to see the effect this way.

[thomaso]

1 hour ago, smadell said:

bring the "Output Black Level" slider up to the desired point. This will set your black point there. Given the math, I think (not 100% sure) that other pixels will adjust themselves accordingly.

What makes you doubt? To me it appears obvious (not only in your two screenshots before|after) that the Levels slider must affect the entire range and rather compresses all values – unless the gamma slider gets used additionally, with the disadvantage of affecting a large mid range again, while both sliders modify the contrast of the entire image.

[smadell]

I said I was not 100% sure because, although it certainly seems that way, I don’t know the precise math that underlies the adjustment. Even your own post uses the phrase “it appears obvious” (emphasis mine) which tells me that you are not “100%” sure, either! Nevertheless I agree, in a practical sense, that setting Output Black affects all pixels and not just the blacks below that point. I just tend to be cautious about stating things as absolutes.

[Mujabad]

@smadell / Lisbon and thomaso.

Thank you very much for all this advice. I have tried several ideas and I' m not sure yet. Lighten black point by using Levels adjustment only doesn' t seem to be  good idea as it still effects all other brightness settings (mid tones/highlight). Using "anchors" in Curves looks promising. Not sure yet how to use numbers effectively there (in my first example bp of 0,6 below which I would then want some detail instead of pure black). Blending could be interesting as well (although I' ll have to get used to that workflow). Apart form the fact that I would like to lighten the values below 0,6 in my example (instead of darken it), I like the idea of min/max option inside curves. Pretty simple and effective. Worth more tries.

I use DXO PL for more than 90% of all my images, and pretty satisfied. There are a few limitations that I don' t like at all (like this, but also no 10/30bit colour support and soft proofing). I used Affinity Photo only every now and then (mostly for HDR or panoramic or maybe an odd effect). So, not so knowledgeable here. The sheer number of possibilities inside AP and the much less intuitive workflow frightened me a little (even after I bought the Workbook a couple of years back). ll probably use it more from now on though. 

It looks to me, that there are enough possibilities inside Affinity Photo to experiment with.

Thank you very much again!