The Curve Tool
Imaging - Imaging
The curve tool is one of the most powerful yet underestimated tools - not overly loved by many because it is delicate to handle and unforgiving to usage errors.

In its default RGB presentation (shown to the left below) the "curve" is a line with fix points to the lower left and the upper right. To the left and bottom of the curve canvas you can find gradient bars ranging from deep black to bright white. The bar to the bottom represents the image as is (input) whereas the left bar shows the output range. The "curve" defines the relationship between input and output. If we match the two end points of the line with the gray value of the gradient bars it becomes obvious that the lower left point represent the black point of the image whereas the upper right point is the white point. The line/curve represents the transition between the two.

If you move your mouse pointer on the line you can click and drag the line off its center position and move it to either direction. Doing so you will change the image. If you release the curve you´ve created an anchor. You can repeat the procedure and introduce more anchors. You can move all anchors including both black and white point. The tool will create a smooth curve touching all anchors (-> therefore "curve" tool).

The default diagonal line will create an image identical to the original one. If you move a point off its center position its output luminance will change compared to its original value (see the sample point in the right image below).

The sample illustration below shows a sample transition to an "S" formed curve which we´ll discuss later:



Similar to the histogram we can divide the diagram into three luminance classes:



Dos and Don´ts ...

For the vast majority of applications there´re three things you shouldn´t do with the curve tool ....
  • Squeezing a tonal range
    If you tune a local part of the curve to a degree where it is almost horizontal you´ll lose (virtually) all contrast in this section.
    You can do so ...
    a) by moving the white and/or black points towards the horizontal center cutting off highlights (towards white) and/or shadows (towards black). In other words: you´re potentially loosing dynamic range by doing so. This resembles a little bit to setting a new white and/or black point in the histogram tool but it´s far less accurate because the curve tool will give you no representation of the actual distribution of the tonal range. You may running into a clipping situation without knowing about it.
    b) by creating a curve with very flat horizontal turning point you loose most local contrast in that range.


    You may notice that a rather broad tonal range on the input side (on lower gradient bar marked in red) is squeezed to a tiny range or point on the output side (left gradient bar).

  • Bloating tones
  • It's also possible to do just the opposite of squeezing contrast - you can tune a part of the curve to a very flat vertical layout where a tiny input portion will be bloated out on the output side thus generating a broad range with very poor tonality/smoothness.

  • Reducing contrast
    by moving the black- and/or white point on the Y-axis. Usually that´s not a good idea though because you´ll loose deep blacks and/or bright whites - the tonal range will be compressed into the new reduced representation.




As a consequence you should have set up both black and white points appropriately BEFORE using the curve tool.

Naturally there may be situations where is makes sense to use the curve tool as mentioned above but for most mortals this is meaningless.

Ok, I guess understanding the usage principal is easy. Now let´s have a deeper look at the concept ...

Brightness

Changing image brightness is a pretty easy task - just push the line off "away" from the shadows by dragging the line into the upper triangle. By doing so you increase the relative share of bright image regions.


Typically you want to do so for the mid tones which means to create a new anchor at the center point of the curve (initially a line here). For a roughly symmetrical effect (similar degree of brightening for dark and bright image portions) you drag this anchor towards the upper left hand corner.
Naturally it works the other way round if you want to lower the mid tone brightness.

Move mouse cursor over the buttons below this illustration to change image brightness

You may notice that the inner portion of the histogram changes accordingly. However, both white and black point remain static. In other words: you will not run into a clipping situation using the curve tool - you will NOT change the dynamic RANGE but simply the brightness.

Dependent on the position of your anchor you can also recover highlights or shadows - more on this later ...

Contrast

Image contrast is also represented by the curve. In its initial position the line/curve represents the given contrast "as is" - the tool will not show you whether the absolute contrast is harsh or smooth.

If we want to increase the contrast of an image we need to increase the gradient of the curve (and vice versa). How can we do that ?
Not thinking about the consequences we could move the black point straight to the right and the white point to the left - this will easily increase the gradient of the curve. HOWEVER, as a result we will loose all image data in the very bright and very dark image portions - this is mentioned in the "Dos and Don´ts above. So that´s no good.
 
As an alternative we leave both black and white point at their initial positions and create two new anchors - one near the lower left (A) and one near the upper right (B). By moving (A) downwards (towards shadows) and (B) upwards (towards highlights) we create a  curve similar to an abstract "S". In the sample illustration below you may notice that the center portion of this "S" has a steeper gradient thus a higher contrast -  we increased mid tone contrast and decreased shadow and highlight contrast. Visually the latter are far less significant though. Nonetheless we didn´t really destroy much of the tonality here which is also visible in the histogram - specifically there´s no clipping when increasing contrast using the "S".

This may sound very theoretical again so I encourage your to have a look at the illustration below - it´s not all that difficult.

Move mouse cursor over the buttons below the illustration to change image contrast


Those veterans from the analog era may recognize the "S" curve from film data sheets. Image sensors produce a linear relationship between the real world and the tonal range of an image whereas film behaves like an "S" curve. If you want your digital image to look similar to film you may simply apply the S curve in the curve tool - if you´re shooting in JPEG mode the camera will already apply this modification to some degree for you but in RAW mode you´ve to do that yourself.

Highlight-/Shadow-Recovery

Sometimes the dynamic range of a scene is quite extreme resulting in blown out highlights or deep dark shadows. If you worked in RAW mode there´s a good chance that you can rescue your image (due to the vastly superior dynamic range compared to JPEG).

If you understood changing image brightness you should already have the craftsmanship to recover the effected image portions. Recovering highlights means nothing else than reducing brightness within the LOCAL highlight portion of the curve and vice versa for shadows.

The only problem is that you don't really want to affect mid tone contrast as well as deep blacks and whites. You can "secure" these areas by introducing supporting anchors here minimizing significant luminance changes here.

Move mouse cursor over the buttons below the illustration

You may notice that the sky and cloud portion gets slightly darker whereas the shadows recover a little - it's actually quite a bit more obvious in the full scale image. The change is also visible in the histogram where local parts at the extreme ends move towards the mid tones.


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