Introduction to Curves

The funny box with a diagonal line in Photoshop

Curves in Photoshop is a very powerful tool but is often underexposed to users because they don't understand how to use the command. Keep in mind that while I am introducing Curves in the context of graphic design, the command is not limited to this only. In fact, for example, many Photoshop users use Curves to correct digital photographs.

How Do Curves Work?

Note: this paragraph touches a bit on the technical side of things. If you can't be bothered, skip to the next paragraph which discusses an example.

To understand how Curves work, let us first examine the Curves command window in Photoshop. The hotkey to open the command window is CTRL + M, or you can go to Images > Adjustments > Curves...

From the image, we can see that the Curves window works like a Cartesian plane. This means for a value (or in this case colour) from the x-axis (horizontal), there is a corresponding one on the y-axis (vertical). Basically the curve you can edit acts as a function that tells Photoshop how to change the image.

Learning from Example

Alright, enough technical bla-bla. Let us look at how Curves work in a real life example. To let you see things more clearly, I am going to use a greyscale (black & white) image for this example. You'll see why pretty soon.


With the Curves window opened, if you move your mouse to the canvas, it would turn into an eyedropper. If you click on the canvas with the eyedropper, a circle will appear on the curve in the Curves window indicating which part of the curve the current colour selection belongs to.

Therefore, if you click on the black square with the eyedropper, a circle will appear right at the bottom left of the curve. If you click on the white square, the circle will appear on the upper left. If you click on the 50% grey square, a circle will appear right on the centre of the curve. If you drag the eyedropper from the black to the white on the gradient, the circle will slowly slide down the curve, corresponding to your colour selection.

If we open the Curves window (still using the squares and gradient image) and changed it to something like the image below:


All the black will be changed to white, and the white to black, like this:


In simple words, the black and white are inverted. Note that the left point on the curve aligns with black on the horizontal gradient and with white on the vertical gradient. This also applies to the right point.

To see how this knowledge can be applied to our graphic design, click on to the next page.

Applying the Curves Technique

To demonstrate the power of curves, we shall make a simple metal bar from a gradient with just the Curves command. For this purpose, we are going to start with this simple white-to-black gradient:



Of course, the rationale of white facing the top is that our light source is from above. Next, fire up the Curves command by pressing CTRL + M. Edit the curve to resemble this (of course, without the colour-coded lines and text):



As a result, our gradient will start from pure white to a darker hue (colour-coded blue line), and then to a lighter hue (colour-coded green line), and then to black. This is the resultant image that I got:



I hope that with the detailed explanation above, you will be able to grasp the idea of how Curves work in Photoshop.

Why Use Curves?

However, have you ever thought why the heck are we using curves when we can easily define a gradient to achieve the same result? This is because gradients can only be used in a linear direction (i.e. in a straight line).

For example, what if I wanted to apply the same metallic effect to this image?



Obviously, gradients can't be used to achieve this easily. However, if we applied the same Curve to this particular image, we would get this:



Of course, this doesn't look so convincing as a metal block, but a little bit of tweaking can fix that. My objective is to show you the advantage that Curves have over gradients.

In short, Curves can be applied to irregular shapes, something that gradients cannot do. So, start experimenting with curves today! I am sure you can improve your techniques in graphic designing.

A Short Reminder

Before ending this tutorial, I would like to stress that Curves are not limited to the use mentioned in this tutorial only. Tons of people use curves as well to sharpen digital photos or for colour correction. For more information, visit the following Curve resources:

• Intro to the Photoshop Curves command @ thegoldenmean.com
• Photoshop Curves: Stepping Up From Levels - Photo Tips ...
• And of course a Google search