A tone curve is a function used to modify the relationship between the input and output. (Basically their brightness values)
The horizontal(X) axis represents the input(original pixels) and the vertical(Y) axis represents the output(edited pixels). The line(curve) shows the relationship between these.
To understand how the input value is changed to get the output value for any pixel, draw a vertical line from an input value till it intersects the curve at a point. From that point, draw a horizontal line till it touches the output axis and that will give you the output value.
By default, the curve is a straight line that makes an angle of 45° to the X-axis. This means that all input and output values are equal.
(in a typical 8-bit per channel image, the values will vary from 0-255 (total 256 values). So any value on the input line, say 79 will correspond to 79 on the output line via the straight line curve).
If the curve is overall raised, for the same input, the output is increased. If the curve is lowered, for the same input, the output is decreased. The following images should make this clear: (click to enlarge)
As you can see, input and output levels are equal in the original state. I have drawn the 3 coloured(Blue, Red, Green) lines to represent pixels of specific brightness levels.{The output values in the following are not 100% accurate, they are only as accurate as I could approximate from the ordinate(output axis) location. They should be within 10% of the actual value, but that is not important. The trend is important here.}
If we raise the curve:
You can see that pixels that originally had a brightness value of 191 now have a brightness value of 230. So also those that had 127 and 63 now have 195 and 230. Therefore the brightness of the overall image has increased as you can see.
Now if we bring the curve down from its original position:
Pixels that had brightness values 63, 127 and 191 now have values 41, 73, 131 respectively. You would expect the image to darken, which it has, as you can see.
Sometimes, editors use what is called the 'S-curve" to increase the contrast in the midtone areas. The curve is slightly raised between the midtones and highlights and slightly dropped between the midtones and shadows. So brighter areas in the midtones become brighter(without affecting the values in the highlights too much) while darker areas in the midtones become darker(without darkening the shadows much). This give the 'contrasty' look. The S-Curve should not be overdone though.
Here is an example of the S-Curve:
There is also an inverted-S-Curve, which is the opposite(lower the curve a bit b/w highlights n mids, raise b/w mids and shadows). It decreases the contrast, but I have hardly ever used it.
The 'curves' tool in Photoshop works in the same manner.
The horizontal(X) axis represents the input(original pixels) and the vertical(Y) axis represents the output(edited pixels). The line(curve) shows the relationship between these.
To understand how the input value is changed to get the output value for any pixel, draw a vertical line from an input value till it intersects the curve at a point. From that point, draw a horizontal line till it touches the output axis and that will give you the output value.
By default, the curve is a straight line that makes an angle of 45° to the X-axis. This means that all input and output values are equal.
(in a typical 8-bit per channel image, the values will vary from 0-255 (total 256 values). So any value on the input line, say 79 will correspond to 79 on the output line via the straight line curve).
If the curve is overall raised, for the same input, the output is increased. If the curve is lowered, for the same input, the output is decreased. The following images should make this clear: (click to enlarge)
As you can see, input and output levels are equal in the original state. I have drawn the 3 coloured(Blue, Red, Green) lines to represent pixels of specific brightness levels.{The output values in the following are not 100% accurate, they are only as accurate as I could approximate from the ordinate(output axis) location. They should be within 10% of the actual value, but that is not important. The trend is important here.}
If we raise the curve:
You can see that pixels that originally had a brightness value of 191 now have a brightness value of 230. So also those that had 127 and 63 now have 195 and 230. Therefore the brightness of the overall image has increased as you can see.
Now if we bring the curve down from its original position:
Pixels that had brightness values 63, 127 and 191 now have values 41, 73, 131 respectively. You would expect the image to darken, which it has, as you can see.
Sometimes, editors use what is called the 'S-curve" to increase the contrast in the midtone areas. The curve is slightly raised between the midtones and highlights and slightly dropped between the midtones and shadows. So brighter areas in the midtones become brighter(without affecting the values in the highlights too much) while darker areas in the midtones become darker(without darkening the shadows much). This give the 'contrasty' look. The S-Curve should not be overdone though.
Here is an example of the S-Curve:
There is also an inverted-S-Curve, which is the opposite(lower the curve a bit b/w highlights n mids, raise b/w mids and shadows). It decreases the contrast, but I have hardly ever used it.
The 'curves' tool in Photoshop works in the same manner.
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