using curves

Discussions, questions, comments and suggestions regarding Capture One PRO, Capture One PRO(For Sony), Capture One DB and Capture One Express(For Sony) 11.x for Mac

using curves

Postby NNN636286928999838177 » Thu Nov 22, 2018 2:42 am

I've been using C1 for about a year but have not explored curves - until now. I've watched several videos about using curves in C1 and, in the videos, it works like I imagined. When I use it, though, I get unexpected results.

I have a fence in my picture that is backlit and I want to increase the exposure of the fence some. The first thing I try is adjusting the Shadows slider. This affects all of the shadows, not just the fence. My next step would usually be to mask the fence and adjust the shadows or contrast that way.

I thought maybe Curves might be let me adjust the shadows on the fence without masking them by using the Curves tool. I thought I could just select the dark part of the fence using "PIck Luma Curve Point" and that dragging the control point it creates on the curve upward to lighten the shadows on the fence. When I do that, though, instead of just moving the area of the curve around the control point upwards, it seems to bend the entire curve upwards in a sort of bell shape centered around the midtones. Tbe point on the curve I selected shows "input 24 output 24". When I move that point upwards the entire curve changes. The area over the midtones and highlights increases more and faster than the selected point, if that makes sense.

I don't understand why I drag the selected point on the curve upwards but the curve around that point doesn't change much but the midtones and highlights parts of the curve increase drastically. Can anyone explain the rational behind this behavior?
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Re: using curves

Postby NNN636735230673236717 » Thu Nov 22, 2018 6:40 am

Wow, that question could fill a book…

Take a look at my illustration:

Image

Think of the ‘curve’ control as a flexible/elastic mirror with special properties.
The red line (the line at 45°) represents the mirror. The bottom (green) of the control represents the input values. The right side (blue) represents the output values. The red line (the mirror) is initially anchored at only two points: bottom-left (X: 0 in = 0 out) and top-right (Y: 255 in = 255 out). If no other control points are applied to the curve (it is straight line) then the control is what I call in a linear mode: in other words what goes in, comes out (0=0, 24=A=24, 127=C=127, etc…). If you select point A and move it to point B you will create a curve X->B->Y (the dashed red line). The shape of that curve is a special case of Bezier curve (with via point(s)). Imagine shooting an arrow from point X via point B and hitting a target at point Y. It is trajectory, not a straight line. That trajectory will now dictate how the input will be translated into output. Imagine having input with value 127: if the curve only has 2 control points (X & Y) then the output will be 127 (C) because the line is straight, but if we have additional control point (B) that value of 127 becomes 192 (D) due to the shape of that curve dictated by that additional control point B. In reality there are no control points C & D, they are for reference and clarity only. I hope that answers your question: why adjusting in one area affects other areas.

Just to add something here: You can have many control point. By adding more control points you can effectively immobilize the curve and control only very small range. Put control points at 64, 128 and 192 and then move the 64 one up or down and you will mainly control the range between 0 and 128. More control points: smaller the effected range.

Image

But too many control point in wrong places can induce oscillation:

Image

BTW. The way you described the problem (the fence, etc.) I would only attempt it with masking.
__________
Rob
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